Effect of different strontium precursors on the growth process and optical properties of SrWO4 microcrystals J. C. Sczancoski1 • W. Avansi2 • M. G. S. Costa3 • M. Siu Li4 • V. R. Mastelaro4 • R. S. Santos5 • E. Longo1 • L. S. Cavalcante5 Received: 31 May 2015 / Accepted: 22 August 2015 / Published online: 3 September 2015 � Springer Science+Business Media New York 2015 Abstract In this paper, an experimental study was per- formed on the effect induced by different strontium precur- sors in the growth processes and optical properties of strontium tungstate (SrWO4) microcrystals synthesized by the co-precipitation method. The structural behavior was analyzed by means of X-ray diffractions, Rietveld refine- ments, Fourier transform (FT)-Raman, and FT-infrared spectroscopies. X-ray absorption near-edge structure spectra performed at the W-L1 and L3 edges revealed the first coordination shell around the tungsten atoms is composed of four oxygens, i.e., existence of tetrahedral [WO4] clusters inside the SrWO4 structure. Field emission scanning electron microscopy (FE-SEM) images showed the presence of pitch and longleaf pine cone-like SrWO4microcrystals formost of the strontium precursors employed in the synthesis. Based on these FE-SEM images, a hypothetical crystal growth mechanism was proposed to explain the origin of these microcrystals. The optical properties were investigated by ultraviolet–visible spectroscopy and photoluminescence (PL) measurements at room temperature. The different optical band gap values found for thismaterial, depending on the type of strontium precursor, were correlated with the existence of intermediary energy levels within the forbidden region. Finally, PL profiles were associated to the degree of distortions in tetrahedral [WO4] clusters. Introduction Strontium tungstate (SrWO4) is an important material belonging to the scheelite class with excellent optical properties, especially for technological applications in solid- state lasers and stimulated Raman scattering [1, 2]. Basi- cally, this tungstate has attracted the attention of the scien- tific community and technological areas because of its interesting physicochemical properties, mainly including blue-green phosphors [3], photocatalytic activity for degra- dation of organic dyes [4, 5], cathodoluminescence [6], thermal expansion [7, 8], luminescence [9, 10], and so on. Over the last few years, pure and doped SrWO4 phase has been normally formed by solid-state reaction [11, 12], Czochralski crystal growth [13, 14], and flux evaporation [15]. These preparation methods usually require complex experimental procedures, sophisticated equipments, and rigorous synthesis conditions. Also, there is the probability of formation of deleterious phases, polydisperse particle size distribution, and uncontrolled morphology. Thus, new synthesis methods as electrochemical [16], molten salt [17], sonochemical [18], mechanically assisted solution reaction [19], chemical solution [20], pulsed laser [21], microemulsion [22], solvothermal-mediated microemul- sion [23], hydrothermal [24], microwave irradiation [25], and microwave-hydrothermal [26] have been developed with the intention of minimizing these drawbacks. & L. S. Cavalcante laeciosc@bol.com.br 1 CDMF-Universidade Estadual Paulista, P.O. Box 355, Araraquara, SP 14801-907, Brazil 2 DF-UFSCar-Universidade Federal de São Carlos, P.O. Box 676, São Carlos, SP 13565-905, Brazil 3 Instituto Federal do Maranhão, Quı́mica, São Luı́s, MA 65025-001, Brazil 4 IFSC-Instituto de Fı́sica de São Carlos, São Carlos, SP 13560-250, Brazil 5 PPGQ-GERATEC-Universidade Estadual do Piauı́, João Cabral, N. 2231, P.O. Box 381, Teresina, PI 64002-150, Brazil 123 J Mater Sci (2015) 50:8089–8103 DOI 10.1007/s10853-015-9377-2 http://crossmark.crossref.org/dialog/?doi=10.1007/s10853-015-9377-2&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1007/s10853-015-9377-2&domain=pdf In materials science, there is a particular interest in the development of simple synthetic routes with efficient control on the particle shapes and sizes at micro/nanoscale [27]. For example, Thongtem et al. [28] reported the syn- thesis of SrWO4 nanoparticles using a microwave-assisted solvothermal route. In this study, these researchers ana- lyzed the influence of pH condition, microwave power, and synthesis times in the formation of these nanoparticles. In recent years, several researchers have employed SrWO4 crystals as host matrix for the incorporation of trivalent rare earths (Eu3?, Er3?, Yb3?, and Tb3?) [29] in order to apply in light emission diodes, anode material for lithium-ion batteries [30], and catalyst to removal of toxic metal lead (II) from water [31]. On the other hand, scien- tific studies on the formation of SrWO4 microcrystals by the co-precipitation method with different strontium pre- cursor salts have not found in the literature. Therefore, in this paper was analyzed the effect of dif- ferent strontium precursors on the growth processes and optical properties of SrWO4 microcrystals synthesized by the co-precipitation method at room temperature. These microcrystals were characterized by X-ray diffraction (XRD), Rietveld refinement, X-ray absorption near-edge structure (XANES), Fourier transform Raman (FT-Raman), Fourier transform infrared (FT-IR), ultraviolet–visible (UV–Vis) absorption spectroscopy, photoluminescence (PL) measurements, and field emission scanning electron microscopy (FE-SEM). A plausible growth mechanism for the formation of SrWO4 microcrystals was proposed. Finally, PL properties were analyzed in terms of distortions in [WO4] clusters, according with effect induced by the strontium precursor in the synthesis of SrWO4 microcrystals. Experimental details Synthesis of SrWO4 microcrystals SrWO4 microcrystals were synthesized according to the following experimental procedure: 5 9 10-3 mols of tungstic acid [H2WO4] (99 % purity, Aldrich), 5 9 10-3 mols of different strontium salts: strontium acetate [Sr(CH3CO2)2] (99.5 % purity, Aldrich), strontium nitrate [Sr(NO3)]2 (99.5 % purity, Aldrich), and strontium chlo- ride hexahydrate [SrCl2�6H2O] (99.5 % purity, Aldrich) were dissolved in 100 mL of deionized water. In the pre- cipitation reaction, Sr2? cations are electron pair acceptors (Lewis acid), while the WO2� 4 anions are electron pair donors (Lewis base). The reaction between these two species in solution at room temperature with different strontium salts resulted in the formation of SrWO4 microcrystals, as shown in following equations: (1, 2) strontium acetate, (3, 4) strontium nitrate, and (5, 6) strontium chloride hexahydrate. H2WO2 sð Þ þ SrðCH3CO2Þ2 sð Þ �! H2O Sr2þaqð Þ þWO2� 4 aqð Þ þ 2Hþ aqð Þ þ 2CH3CO � 2 aqð Þ ð1Þ Sr2þaqð Þ þWO2� 4 aqð Þ þ 2Hþ aqð Þ þ 2CH3CO � 2 aqð Þ ! SrWO4 sð Þ þ 2Hþ aqð Þ þ 2CH3CO � 2 aqð Þ ð2Þ H2WO2 sð Þ þ SrðNO3Þ2 sð Þ �! H2O Sr2þaqð Þ þWO2� 4 aqð Þ þ 2Hþ aqð Þ þ 2NO� 3 aqð Þ ð3Þ Sr2þaqð Þ þWO2� 4 aqð Þ þ 2Hþ aqð Þ þ 2NO� 3 aqð Þ ! SrWO4 sð Þ þ 2Hþ aqð Þ þ 2NO� 3 aqð Þ ð4Þ H2WO2 sð Þ þ SrCl2 � 6H2O sð Þ �! H2O Sr2þaqð Þ þWO2� 4 aqð Þ þ 2Hþ aqð Þ þ 2Cl�aqð Þ þ 6H2O ð5Þ Sr2þaqð Þ þWO2� 4 aqð Þ þ 2Hþ aqð Þ þ 2Cl�aqð Þ þ 6H2O ! SrWO4 sð Þ þ 2Hþ aqð Þ þ 2Cl�aqð Þ þ 6H2O: ð6Þ In order to increase the ionization rate of H2WO4, the pH solution was adjusted up to 10 by the addition of 6 mL of ammonium hydroxide [NH4OH] (30 % in NH3, Mallinckrodt). Thereafter, these aqueous solutions were stirred for 30 min at room temperature. After the co-pre- cipitation reaction is completed, these systems were washed and centrifuged several times with deionized water to neutralize the pH (&7). Finally, the collected precipi- tates were dried in a lab oven at 60 �C for some hours. Characterizations of SrWO4 microcrystals The synthesized microcrystals were structurally character- ized by X-ray diffraction (XRD) using a DMax/2500PC diffractometer (Rigaku, Japan). XRD patterns were obtained with Cu-Ka radiation in the 2h range from 10� to 90�, using a scanning speed of 2�/min. For Rietveld rou- tines were adopted a 2h range from 10� to 110� with a scanning speed of 0.02�/s. The electronic and local atomic structures around tungsten (W) atoms were checked by X-ray absorption near-edge structure (XANES). W L1,3- edge XANES spectra were measured at the National Syn- chrotron Light Laboratory (LNLS) in Brazil, using the D04BXAFS1 beam line. XANES data were collected in transmission mode at room temperature with the samples deposited on polymeric membranes. These spectra were recorded for each sample using an energy step of 1.0 eV, before and after the edge, and 0.7 eV near the edge for W-L1 and L3 edge, respectively. The tungsten oxide (WO3) 8090 J Mater Sci (2015) 50:8089–8103 123 (C99 %) purchase from Aldrich was employed as refer- ence compound in these measurements. FT-Raman spectra were recorded by means of a RFS100 spectrophotometer (Bruker, Germany) equipped with a Nd:YAG laser (k = 1064 nm), operating at 100 mW. FT-IR spectra were performed in the range from 50 to 1200 cm-1, using an Equinox 55 spectrometer (Bruker, Germany) in transmit- tance mode. The morphologies were analyzed using a Supra 35-VP FE-SEM (Carl Zeiss, Germany) operated at 15 kV. UV–Vis absorption were taken using a Cary 5G spectrophotometer (Varian, USA) in diffuse reflection mode. PL spectra were obtained with a Monospec 27 monochromator (Thermal Jarrel Ash, USA) coupled to a R446 photomultiplier (Hamamatsu Photonics, Japan). A krypton ion laser (Coherent Innova 200K, USA) (k = 350 nm) was used as excitation source. The incident laser beam power on the samples was kept at 14 mW. UV– Vis and PL measurements were taken three times for each sample to ensure the reliability of the results. In our study, all experimental measurements were performed at room temperature. Results and discussion X-ray diffraction and Rietveld refinement analyses Figure 1 shows the XRD patterns of SrWO4 microcrystals co-precipitated with different strontium precursors. The diffraction peaks can be used to evaluate the structural order at long range or periodicity of the material. In our case, the intense and sharp peaks of SrWO4 crystals co-precipitated at room temperature indicate a good crystallinity or periodicity at long range [32]. In Fig. 1a–c, XRD patterns provided the SrWO4 microcrystals synthe- sized with Sr(NO3)2, SrCl2�6H2O, and Sr(CH3CO2)2 pre- cursors have single phase, which were perfectly assigned to the scheelite-type tetragonal structure with space group I41/a (ICSD card no. 155793) [33]. Figure 2a–c illustrates the Rietveld refinement plots of SrWO4 microcrystals co-precipitated with different stron- tium precursors. The Rietveld refinement is a method in which the profile intensities obtained from step-scanning measurements of solid samples allow to estimate an approximate structural model for the real structure [34]. In our study, the Rietveld refinements were performed using the general structure analysis system (GSAS) software package with the EXPGUI graphical interface [35]. The refined parameters were scale factor, background, shift lattice constants, pro- file half-width parameters (u, v, w), isotropic thermal parameters, strain anisotropy factor, atomic functional positions, bond lengths, and bond angles. The background was corrected using the Chebyshev polynomials of the first kind [36]. The diffraction peak profiles were better fitted by the Thompson-Cox-Hastings pseudo-Voigt (pVTCH) function [37] and asymmetry function described by Finger et al. [38]. The strain anisotropy was corrected by the phenomenological model described by Stephens [39]. The theoretical diffraction pattern was taken from ICSD card no. 155793 [33], which is based on the SrWO4 phase with scheelite-type tetragonal structure and space group I41/a. The Rietveld refinements of SrWO4 microcrystals are shown in Fig. 2a–c, which are in good agreement with XRD results illustrated in Fig. 1. The obtained results from Rietveld refinements are listed in Table 1. In this table, the fitting parameters (Rwp, Rp, RBragg, v 2, and S) indicate a good agreement between refined and observed XRD patterns for SrWO4 phase. The lattice parameter values, unit cell volume, atomic positions, and bond angles confirmed the SrWO4 phase has a tetragonal structure. All refinements reported in our study are in good agreement with those previously published [40, 41]. Structural representation of SrWO4 crystals Figure 3 illustrates the SrWO4 structure modeled through the Diamond Crystal and Molecular Structure Visualiza- tion software [42], using the lattice parameters and atomic positions obtained from Rietveld refinements as input data. SrWO4 crystallizes in a tetragonal structure with space group I41/a [43]. Chen et al. [44] described the tetragonal SrWO4 structure constituted of four molecules per unit cell (Z = 4). Jia et al. [45] explained the A-sites related to strontium (Sr) atoms present point symmetry (S4). Thus, SrWO4 structure exhibits the Sr atoms bonded to eight Fig. 1 XRD patterns of SrWO4 microcrystals synthesized with a SrNO3, b SrCl2�6H2O, and c Sr(CH3CO2)2. The vertical lines indicate the respective positions and intensities found in ICSD card no. 155793 J Mater Sci (2015) 50:8089–8103 8091 123 oxygens ([SrO8] clusters), forming snub-disphenoid poly- hedra (8 vertices, 12 faces, and 18 edges) and point-group symmetry (D2d). On the other hand, tungsten (W) atoms are coordinated to four oxygens ([WO4] clusters) with tetra- hedral geometry (4 vertices, 4 faces, and 6 edges) and point-group symmetry (Td). The tetrahedral [WO4] clusters are slightly distorted inside the structure, as consequence of O–W–O bond angles (a = 106.1� and b = 116.43�) as shown in Fig. 3. XANES spectroscopy analyses Figure 4a shows W L1-edge XANES spectra, (b) area of the W L1-edge peak, and (c) W L3-edge XANES spectra of SrWO4 microcrystals co-precipitated with different stron- tium precursors, respectively. According to the literature [46–48], W L1-edge XANES spectrum is a powerful tool to provide any information on the coordination environment (tetrahedral, square-based pyramid, octahedral, etc.), oxidation state, and local geometry of tungsten atoms. A closer examination in Fig. 4a revealed the XANES spectra of SrWO4 micro- crystals present intense and narrow W-L1 pre-edge absorption peaks (X) at around 12108 eV. This particular behavior is related to the existence of distorted tetrahedral [WO4] clusters (Inset in Fig. 4a) [49]. On the other hand, the spectrum of WO3 (standard sample) exhibited a slight shoulder in this same energy region. Kuzmin and Purans [50] reported the W-L1 pre-edge absorption peak (X) is originated by the electronic transitions from 2s(W) to 5d(W) ? 2p(O) orbitals. In other published studies [51], these authors explained that these electronic transitions are Fig. 2 Rietveld refinement plots of SrWO4 microcrystals synthesized with the precursors salts: a SrNO3, b SrCl2�6H2O, and c Sr(CH3CO2)2 8092 J Mater Sci (2015) 50:8089–8103 123 all dipole forbidden in undistorted octahedral [WO6] clusters (inversion center) and allowed in both distorted octahedral [WO6] and tetrahedral [WO4] clusters. More- over, the intensity of the W-L1 pre-edge absorption peak (X) is very sensitive and dependent on the degree of dis- tortion in octahedral [WO6] clusters relate to 5d(W) ? 2p(O) orbitals [52]. We calculate the area of the W-L1 pre-edge peak to evaluate the effect of strontium precursors in the degree of distortion of [WO4] clusters (order–disorder at short range) found in SrWO4 micro- crystals (Fig. 4b). For this purpose was employed the Fig. 3 Schematic representation of tetragonal SrWO4 structure Fig. 4 XANES spectra of SrWO4 microcrystals at the a W-L1 pre- edge, b area of the W-L1 pre-edge peaks, and c XANES spectra at the (a) W-L3 edge. WO3 with monoclinic structure (Sigma-Aldrich 99.9 % purity) was used as standard sample Table 1 Lattice parameters, unit cell volume, site occupancy, and statistical parameters obtained from Rietveld refinements of SrWO4 microcrystals synthesized with different strontium precursors Wyckoff Site x y z .Atoms Strontium 4b -4 0 0.25 0.625 Tungsten 4a -4 0 0.25 0.125 Oxygen 16f 1 0.2395 0.1062 0.0453 §Atoms Strontium 4b -4 0 0.25 0.625 Tungsten 4a -4 0 0.25 0.125 Oxygen 16f 1 0.2465 0.1106 0.0455 “Atoms Strontium 4b -4 0 0.25 0.625 Tungsten 4a -4 0 0.25 0.125 Oxygen 16f 1 0.2497 0.0925 0.0421 Rwp = 6.16 %, Rp = 4.54 %, RBragg = 3.28 %, v2 = 3.9, S = 1.975, a = b = c = 90� (a = b = 5.4277(6) Å e c = 11.9727(5) Å). Rwp = 7.20 %, Rp = 5.37 %, RBragg = 2.78 %, v2 = 5.0, S = 2.236, a = b = c = 90� (a = b = 5.4281(2) Å e c = 11.9762(2) Å). Rwp = 6.95 %, Rp = 5.29 %, RBragg = 1.99 %, v2 = 4.0, S = 2.0, a = b = c = 90� (a = b = 5.4276(3) Å e c = 11.9761(4) Å). . SrWO4 with Sr(CH3CO2)2 precursor, § SrWO4 with SrCl2�6H2O precursor, and “ SrWO4 with Sr(NO3)2 precursor J Mater Sci (2015) 50:8089–8103 8093 123 peakFit program (4.12 version), using the Voigt function [53]. According to the literature [54], the high area of the W-L1 pre-edge peak of SrWO4 microcrystals synthesized with SrNO3 is associated to the strong interaction between tungsten and oxygen atoms. WO3 used as standard sample exhibited a low value for the area of W-L1 pre-edge peak; however, this behavior was ascribed to change of coor- dination in octahedral [WO6] clusters, as a consequence of a split into eg and t2g orbitals by the ligand field theory. However, SrWO4 microcrystals have only tetrahedral [WO4] clusters, which present a split into t2 and e orbitals. XANES spectra at the W L3-edge of SrWO4 micro- crystals are illustrated in Fig. 4c. Over again, WO3 was employed as reference sample in these spectra. The pre- edge peak was located at around 10208 eV for SrWO4 microcrystals and approximately 10210 eV for WO3. SrWO4 microcrystals synthesized with different strontium precursors have their W atoms bonded to four oxygens in a tetrahedral environment ([WO4] clusters), while the WO3 has W atoms bonded to six oxygens in a distorted octa- hedral configuration ([WO6] clusters). As a response to these differences in the coordination number, a slight shift in the respective positions of edge peak (Y) was detected between these samples in XANES spectra (& 10208 eV for SrWO4 microcrystals (.) and &10210 eV for WO3 powder (j)). The origin of this pre-edge peak at the W L3- edge is ascribed to the permitted dipole transition from 2p3/2(W) level to quasi-bound mixed state 5d(W) ? 2p(O) [55]. XANES spectra in our study of SrWO4 microcrystals are in good agreement with those previously published on tungstates with scheelite-type tetragonal structure [56]. FT-Raman and FT-IR spectroscopies analyses According to group theory calculations, SrWO4 micro- crystals are able to present 26 different vibration modes (Eq. (7)) [57]: C Raman½ �þ infraredð Þ ¼ 3Ag þ 5Bg þ 5Eg � � þ 5Au þ 3Bu þ 5Euð Þ; ð7Þ where Ag, Bg, and Eg are Raman-active vibration modes; A and B modes are nondegenerate, while E modes are doubly degenerate. The subscripts [g] and (u) indicate the parity under inversion in centrosymmetric of SrWO4 microcrys- tals. Au and Eu modes correspond to zero frequency of acoustic modes, while the others are optic modes. In addition, Ag, Bg, and Eg modes arise from the same motion of SrWO4 microcrystals. Therefore, it is expected 13 zone- center Raman-active modes for SrWO4 microcrystals, as presented in Eq. (8) [58]: C Raman½ � ¼ 3Ag þ 3Bg þ 5Eg: ð8Þ Degreniers et al. [59] reported the vibrational modes observed in Raman spectra of SrWO4 can be classified into two groups: external and internal modes. The vibrational external modes are related to lattice phonon, which corre- sponds to the motion of [SrO8] clusters and the rigid units. The vibrational internal modes are correspondent to the vibration inside tetrahedron [WO4] clusters, considering the center of mass in stationary state. The isolated [WO4] clusters have a cubic symmetry point (Td) and its vibrations are composed of four internal modes (m1(A1), m2(E1), m3(F2)), and m4(F2)), one free rotation mode (mf.r.(F1)) and one translation mode (F2). On the other hand, the tungsten atoms belonging to tetrahedral [WO4] clusters occupy the 4a Wyckoff positions with point symmetry (S4), while the O atoms occupy the 16f Wyckoff positions with point symmetry (C1). Figure 5 shows the FT-Raman spectra in the range from 50 to 1200 cm-1 of SrWO4 microcrystals co-precipitated with different strontium precursors. As can be observed in Fig. 5, FT-Raman spectra revealed the presence of twelve Raman-active vibration modes. One Bg mode was not detectable because of its low intensity. The literature [56] describes the Raman spec- troscopy can be employed as a structural probe to inves- tigate the degree of structural order–disorder at short range in ABO4 materials. Based on this concept, Raman spectra consisting of sharp, intense, and well-defined vibration bands are commonly verified in solids with local structural order. This phenomenon was verified in all SrWO4 microcrystals obtained at room temperature by the Fig. 5 FT-Raman spectra of SrWO4 microcrystals synthesized with different strontium precursors. Insets show the typical bending and stretching vibrations exhibited by O–Sr–O and O–W–O bonds of [SrO8] and [WO4] clusters, respectively 8094 J Mater Sci (2015) 50:8089–8103 123 co-precipitation method with [Sr(CH3CO2)2], [Sr(NO3)]2, and [SrCl2�6H2O]. Our Raman results for SrWO4 micro- crystals are in good agreement with those previously published [58, 59]. Insets in Fig. 5a illustrate each assignment to Raman-active external and internal modes of SrWO4 microcrystals. The first Raman-active Bg mode (83 cm-1) is related to symmetric bending vibrations of (O–Sr–O) bonds in [SrO8] clusters, the second Raman- active Eg mode (100 cm-1) is assigned to free motion at (x, y, z-axis) of [SrO8] clusters, and the third Raman-active Eg mode (129 cm-1) is ascribed to symmetric stretching vibrations of (/ O / Sr ? O?) bonds in [SrO8] clus- ters. The fourth and fifth Raman-active Ag and Eg modes (188 and 236 cm-1) are classically identified as free rota- tion of tetrahedral [WO4] clusters. The sixth Raman-active Bg mode was not possible to detect it. The seventh and eighth Raman-active Ag/Bg modes are overlapped (335 cm-1), which are designed to asymmetric bending vibrations of (/ O / W/Sr;O;) bonds in [WO4] clusters. The ninth and tenth Raman-active Bg and Eg modes (370 and 380 cm-1, respectively) are assigned to symmetric bending vibrations of [WO4] clusters, and the eleventh and twelfth Raman-active Eg and Bg modes (796 and 835 cm-1, respectively) are related to asymmetric stretching vibrations of (/ O–W ? // O–W ?) bonds in [WO4] clusters. Finally, the thirteenth Raman-active Ag mode (919 cm-1) is referent to symmetric stretching vibrations of (/ O / W ? O ?) bonds in [WO4] clusters. In addition, a com- parison was made with the literature [58, 59] (Table 2). In this table, the slight variations in the typical positions of Raman-active vibration modes are caused by distortions or changes in the length (O–W–O)/(O–Sr–O) bonds, modifications in the interaction forces involving the [WO4]–[SrO8]–[WO4] clusters, and the presence of struc- tural order–disorder in the lattice, in consequence of the preparation methods and their experimental conditions. Figure 6a, b shows FT-IR spectra in the range from 395 to 1000 cm-1 and from 1000 to 4000 cm-1 of SrWO4 microcrystals, respectively. In infrared spectra are expected 13 infrared vibrational modes (5Au ? 3Bu ? 5Eu) for tungstates. However, 1Au and 1Eu are acoustic vibrations, i.e., infrared-inactive modes, while the others 3Bu are forbidden infrared modes. Therefore, only 8 infrared-active vibration modes remain, as presented in Eq. (9) [60]: C infraredð Þ ¼ 4Au þ 4Eu: ð9Þ In our FT-IR spectra illustrated in Fig. 6a, only two of eight IR-active modes were verified. 3Au and 3Eu modes may not have been detected due to limitations imposed by the FT-IR equipment. As was previously described, the tungstates with scheelite-type tetragonal structure have eight stretching and/or bending vibrational modes in their FT-IR spectra. In our spectra were verified no more than two modes (1(Au) and 1(Eu)), which were identified at specific positions in the spectra (Fig. 6a). The first strong absorption band located at around 417 cm-1 is ascribed to Au mode. The strong and broad absorption band related to Eu mode located at 823/844 cm-1 was ascribed to (/ O / W / O /)/(? O ? W ? O?) anti-symmetric stretching vibrations inside the [WO4] clusters. In Fig. 6b was verified other absorption bands in FT-IR spectra due to the presence of carbon dioxide (CO2) and water (H2O) arising from the room atmosphere and humidity. The small band noted at 2500 cm-1 is due to m(C=O) stretching mode. The broad absorption band at 3400 cm-1 corre- sponds to O–H stretching vibrations of adsorbed water on the surface of SrWO4 microcrystals. FE-SEM image analyses Figure 7 illustrates the FE-SEM images at low and high magnifications of SrWO4 microcrystals co-precipitated at room temperature with (a–c) [SrNO3], (d–f) Sr(CH3CO2)2, and (g–i) [SrCl2�6H2O], respectively. Figure 7a reveals the SrWO4 microcrystals obtained with SrNO3 precursor are basically formed of several nanocrystals governed by self-assembled process, resulting in pitch pine cone-like microcrystals [61]. FE-SEM images at high magnification (Fig. 7b) proved the SrWO4 micro- crystals are not dense and faceted structures. This behavior was related to significant amount of SrWO4 nanocrystals that have not completely migrated from crystal surface to internal region of pitch pine cone-like microcrystals. These microcrystals have an average size of approximately 1.8 lm. In Fig. 7c was noted a mutual aggregation between pitch pine cone-like microcrystals in a chemical environ- ment under basic pH conditions, leading to the growth of its lateral edges (longleaf pine cone-like microcrystals). The literature [62] have reported the growth of shuttle-like BaWO4 structures in water and ethanol by means oriented attachment, which is related to assembled from several small primary nanoparticles. These same morphological shapes were identified for SrWO4 microcrystals synthe- sized with Sr(CH3CO2)2 precursor (Fig. 7d–f); however, these microcrystals have an average size of approximately 2.2 lm. In Fig. 7g, we can verify the [SrCl2�6H2O] pre- cursor promoted an incomplete growth and agglomeration of some pitch and longleaf pine cone-like SrWO4 micro- crystals. This behavior can be associated to the influence of chloride ions (Cl-) in aqueous solution, during the inter- action process between Sr2? and WO2� 4 ions and formation of first nanocrystals. This particular characteristic of Cl- ions can inhibit the crystal growth process via self- assembly, resulting in defects and irregularities in the pitch and longleaf pine cone-like SrWO4 microcrystals (Fig. 7h, J Mater Sci (2015) 50:8089–8103 8095 123 i). Particularly, these microcrystals have an average size of approximately 1.6 lm. Growth mechanism of SrWO4 crystals Figure 8a–e illustrates a hypothetical growth mechanism of SrWO4 microcrystals formed by the co-precipitation route at room temperature with different strontium precursors. Figure 8a shows the initial synthesis stage of SrWO4 microcrystals by the co-precipitation reaction, which involves the solubilization in water of H2WO4 and different strontium precursors, SrNO3, Sr(CH3CO2)2, and SrCl2 �6H2O. The resulting solutions were placed in ultrasonic bath for 30 min to accelerate the co-precipitation rate. In the sequence, 6 mL of NH4OH was added in each system to intensify the ionization rate of H2WO4 in the aqueous solution. In this case, Sr2? cations are electron pair acceptors (Lewis acid), which are arising from strontium precursors with distinct solubility product constant [Ksp] in water at room temperature, such as [SrNO3] ? Ksp = 70.9 g/100 mL; [SrCl2�6H2O] ? Ksp = 53.8 g/100 mL, and Sr(CH3CO2)2 ? Ksp = 36.9 g/100 mL. The WO2� 4 anions are electron pair donors (Lewis base), which are arising from H2WO4 in water. The reaction at pH 10 involving these two species (Sr2? / WO2� 4 ions) results in a covalent bond. The covalent bond occurs due to Lewis acid to occupy the lowest molecular orbital (LUMO), which interacts with the highest molecular orbital (HOMO) of the Lewis base. In Fig. 8b is illustrated the interaction process between Sr2? and WO2� 4 ions. Before these inter- actions occur, the solvation energy of water molecules promotes a rapid dissociation of the salts involved in the reaction, so that the Sr2? and WO2� 4 ions are rapidly sol- vated by the water molecules. The partial negative charge of water molecules is electrostatically attracted by Sr2? ions, while the other positive charge is attracted by WO2� 4 ions [63]. However, there is a strong electrostatic attraction between Sr2? and WO2� 4 ions, resulting in the formation of first SrWO4 precipitates or nucleation seeds. After this interaction, instantaneously occurs the formation of first nucleation seeds (Fig. 8c). These nuclei interact with other, forming the SrWO4 nanocrystals (pine nut-like nanocrys- tals), which are able to grow via self-assembly mechanism (Fig. 8d). Basically, in this type of growth mechanism, there is a spontaneous and mutual aggregation between nanocrystals by means of uncountable collision events (particle–particle interactions) followed by the coalescence of SrWO4 nanocrystals. The growth and agglomeration of these nanocrystals promotes the origin of complex super- structures or large SrWO4 microcrystals. It is important to highlight that the average yield (%) results for the forma- tion of nanoparticles of each strontium precursor are cor- related with their solubility product constants at 28 �C/ 100 mL in water. On the other hand, when SrNO3, Sr(CH3CO2)2, and SrCl2�6H2O precursors were used in the co-precipitation synthesis, there is the formation of pitch and longleaf pine cone-like SrWO4 microcrystals (Fig. 8d). In principle, we report, based on our FE-SEM images, that longleaf pine cone-like SrWO4 microcrystals are originated by the aggregation of several pitch pine cone-like micro- crystals (Fig. 8e). The formation and growth of these microcrystals are in agreement with the morphological aspects of other tungstates published in the literature [63]. UV–Vis diffuse reflectance spectroscopy analyses The optical band gap energy (Egap) values were calculated by the Kubelka–Munk equation [64], which is based on the transformation of diffuse reflectance measurements to esti- mate Egap values with good accuracy [65]. Particularly, it is used in limited cases of infinitely thick samples. The Kubelka–Munk equation for any wavelength is described by Table 2 Comparative results between our experimental Raman-active modes of SrWO4 synthesized with different strontium precursors ([Sr(NO3)]2 “, [SrCl2�6H2O]', and [Sr(CH3CO2)2] §) with those published in the literature Lattice mode symmetry (C6 4h) “ ' § [58] [59] Assignments Bg 83 83 83 – 75 text Eg 100 100 100 – 102 Eg 129 129 129 – 133 Bg – – – – – Ag 188 188 188 187 190 tr.f.(F1) Eg 236 236 236 – 238 Ag 335 335 335 334 336 t2(E) Bg 335 335 335 – 336 Bg 370 370 370 373 374 t4(F2) Eg 380 380 380 – – Eg 796 796 796 791 800 t3(F2) Bg 835 835 835 831 837 Ag 919 919 919 912 921 t1(A1) 8096 J Mater Sci (2015) 50:8089–8103 123 K S ¼ 1� R1ð Þ2 2R1 � F R1ð Þ; ð10Þ where F(R?) is the Kubelka–Munk function or absolute reflectance of the sample. In our case, magnesium oxide (MgO) was adopted as standard sample in reflectance measurements; R? = Rsample/RMgO (R? is the reflectance), K is the molar absorption coefficient, and S is the scattering coefficient. In a parabolic band structure, the optical band gap and absorption coefficient of semiconductor oxides [66] can be calculated by the following equation: ahm ¼ C1ðhm� EgapÞn; ð11Þ where a is the linear absorption coefficient of the material, hm is the photon energy, C1 is a proportionality constant, Egap is the optical band gap, and n is a constant associated with different types of electronic transitions (n = 1/2 for direct allowed, n = 2 for indirect allowed, n = 1.5 for direct forbidden, and n = 3 for indirect forbidden). According to the theoretical calculations reported in the literature [67], scheelite (ABO4) crystals exhibit an optical absorption spectrum governed by direct electronic transi- tions. In this phenomenon, after the electronic absorption, the electrons located in minimum energy states in the conduction band (CB) are able to go back to maximum energy states of the valence band (VB) in the same points in the Brillouin zone [68]. Based on this information, Egap values of our SrWO4 microcrystals were calculated using n = 1/2 in Eq. 11. Finally, using the diffuse reflectance function described in Eq. 10 with K = 2a, we obtain the modified Kubelka–Munk equation as indicated in Eq. (12): ½F R1ð ÞhmÞ2 ¼ C2 hm� Egap � � : ð12Þ Therefore, finding the F(R?) value from Eq. (12) and plotting a graph of [F(R?)hm]2 versus hm, Egap values of all SrWO4 microcrystals can be calculated extrapolating the linear portion of UV–Vis curves. Figure 9a–c shows UV–Vis spectra of SrWO4 micro- crystals co-precipitated at room temperature with different strontium precursors. In this figure, there is slight changes in Egap values according to the type of strontium precursor employed in the synthesis of SrWO4 microcrystals. The exponential optical absorption edge and Egap are controlled by the degree of structural order–disorder in the lattice [69]. Therefore, the lowestEgap (4.84 eV) was detected for SrWO4microcrystals obtained with SrNO3 precursor, suggesting the existence of uncountable intermediary energy levels within the forbidden band gap, as a consequence of a high defect density in the lattice, such as distortions in O–W–O or O–Sr–O bonds, oxygen vacancies, cracks, pores, dislocations, grain bound- aries, etc. SrWO4 microcrystals prepared with SrCl2�6H2O and Sr(CH3CO2)2 precursors exhibited Egap of 4.87 and 5.0 eV, respectively (Fig. 9b, c). These results suggest a lower concentration of structural defects in these micro- crystals in relation to those obtainedwith SrNO3 precursor. It is important to highlight that the types as well as the con- tribution single or joint of these energy levels within the band gap in these microcrystals can be achieved only by means of theoretical calculations. Fig. 6 FT-IR spectra in the range from: a 395 to 1000 cm-1 and b 1000 to 4000 cm-1 of SrWO4 microcrystals synthesized with different strontium precursors. The vertical lines dashed indicate the positions of IR-active modes J Mater Sci (2015) 50:8089–8103 8097 123 PL emission analyses Figure 10 illustrates PL spectra recorded at room temper- ature of SrWO4 microcrystals co-precipitated at room temperature with different strontium precursors. These PL spectra present a broad band covering a large part of visible electromagnetic spectra (from 380 to 600 nm), indicating a contribution of several energy states within the band gap. These states are related to the numerous types of defects directly associated to the degree of structural order–disorder in the lattice. Topological disorder is a type of disorder associated with glassy and amorphous solid structures, in which the structure cannot be defined in terms of a periodic lattice. According to the literature [70–72], the blue-green PL emissions of SrWO4 are caused by the surface and structural defects, particle Fig. 7 FE-SEM images at low and high resolution of SrWO4 microcrystals synthesized with a–c SrNO3, d–f Sr(CH3CO2)2, and g–i SrCl2�6H2O 8098 J Mater Sci (2015) 50:8089–8103 123 shapes, narrow particle size distribution, Jahn–Teller splitting effect in [WO2� 4 ] tetrahedron, preferred orienta- tion, etc. Based on our Rietveld refinements and UV–Vis spectra, there is the possibility of electronic transitions involving single clusters ([WO4] or [SrO8]) or from cluster to cluster ([WO4]–[SrO8] and/or [SrO8]–[SrO8]) in the PL response of SrWO4 microcrystals. Basically, PL properties of solids can be influenced by the concentration of different intrinsic (bulk/surface) and extrinsic defects (structural order–disorder), which are responsible for the modifica- tions in the number of intermediary energy states within the band gap. These structural defects are able to promote a symmetry break, causing a polarization in the SrWO4 structure by the electronic charge transfer from ordered (o) to disordered (d) clusters (formation of e–h• pairs). Hence, we presume the existence of four types of complex clusters, in which the first and second are more distorted (d) and assigned as [WO4]d and [SrO4]d clusters, respectively. The third and fourth are considered less distorted or ordered (o) and designed as [WO4]o and [SrO4]o clusters. In this pro- posed mechanism, the possible charge transfers between ½WO4�xd–½SrO8�xo, ½SrO8�xd–½WO4�xo, ½WO4�xd–½SrO8�xo– ½WO4�xo; and ½SrO8�xd–½WO4�xo–½SrO8�xo clusters are shown in the following equations below: Fig. 8 Proposed growth mechanism of SrWO4 microcrystals synthesized with different strontium precursors [SrNO3, SrCl2�6H2O, and Sr(CH3CO2)2] in a aqueous solution; b electrostatic attraction between Sr2? and WO2� 4 ions in solution; c formation and interaction of first nucleation seeds, resulting in SrWO4 nanocrystals; d growth stage of nanocrystals (pine nut-like nanocrystals) via self-assembly mechanism, forming pitch cone-like microcrystals; and e mutual aggregation of several pitch pine cone-like microcrystals, resulting in longleaf pine cone- like microcrystals J Mater Sci (2015) 50:8089–8103 8099 123 �!hm Excitation¼k¼350 nmð Þ½WO4�xd �! e0 ½SrO8�xo ! ½WO4��d�½SrO8�0o ð13Þ ½SrO8�xd �! e0 ½WO4�xo ! ½SrO8��d�½WO4�0o ð14Þ ½WO4�xd ! e0 ½SrO8�xo�½WO4�xo !½WO4��d�½SrO8�0o �½WO4�xo !½WO4��d�½SrO8�xo�½WO4�0o ð15Þ SrO8½ �xd! e0 WO4½ �xo� SrO8�xo! � � SrO8��d�½WO4�0o � SrO8�xo! � � SrO8��d�½WO4�xo�½SrO8�0o �!hm0 Emission6¼k 6¼350nmð Þ : ð16Þ In Eqs. (13–16), the cluster-to-cluster charge transfer (CCCT) in a crystal containing more than one type of cluster is characterized by excitations involving electronic transitions from one cluster to another cluster. During the excitation process at room temperature, the electrons situ- ated at lower intermediary energy levels (O 2p orbitals) absorb the photon energies (hm) (350 nm & 3.54 eV). As consequence of this phenomenon, the energetic electrons are promoted to higher intermediary energy levels (W 5d orbitals) located near the CB. When the electrons fall back to lower energy states, the energies arising from these electronic transitions are converted in photons (hm0). In this case, the several photons (hm0) originated by the partici- pation of different energy states during the electronic transitions are responsible for the broad PL spectra. In addition, PL profiles of SrWO4 microcrystals exhibit dif- ferent emissions intensities, depending on the type of Fig. 9 UV–Vis spectra of SrWO4 microcrystals synthesized with different strontium precursors 8100 J Mater Sci (2015) 50:8089–8103 123 strontium precursor (Fig. 10). Moreover, insets in Fig. 10 illustrate different O–W–O bond angles (a and b), sug- gesting that the tetrahedral [WO4] clusters are distorted in the lattice. These bond angles were estimated using the lattice parameters and atomic positions obtained from Rietveld refinements as input data in the crystal and molecular structure visualization software [42]. These distortions are key factors for the origin of intermediary energy states within the forbidden region, influencing in the PL response of SrWO4 microcrystals. Conclusion In summary, SrWO4 microcrystals were synthesize by the co-precipitation method at room temperature with different strontium precursors [SrNO3, SrCl2�6H2O, and Sr(CH3- CO2)2]. XRD patterns and Rietveld refinements revealed the SrWO4 microcrystals have a scheelite-type tetragonal structure with a good degree of crystallinity (ordered at long range), in which the [WO4] clusters are slightly dis- torted in the lattice. Independent of the type of strontium precursor, FT-Raman spectra showed the SrWO4 micro- crystals have well-defined vibrational bands, suggesting a structurally ordered matrix at short range. FT-IR spectra detected the IR-active modes of typical anti-symmetric stretching vibrations of tetrahedral [WO4] clusters. According to the XANES spectra, all samples obtained in this work have their W atoms bonded to four oxygens ([WO4] clusters). The calculated area of the W L1-edge peaks indicated the SrWO4 microcrystals synthesized with SrNO3 have a strong interaction between O–W–O bonds in relation to other precursors. In morphological terms, SrWO4 is composed of pitch and longleaf pine cone-like microcrystals, when the SrNO3, SrCl2�6H2O, and Sr(CH3- CO2)2 precursors were used in the co-precipitation syn- thesis. Comparing the Egap values, the results demonstrated the type of strontium precursor used in the formation of SrWO4 microcrystals is able to originate distinct quantities and/or distributions of intermediary energy states within the band gap. The lowest Egap was detected for SrWO4 microcrystals obtained with SrNO3 precursor, suggesting a structure with a high concentration of structural defects. The differences in PL profiles, especially in terms of intensity, were related to electronic transitions in distorted [WO4] clusters and/or from cluster to cluster ([WO4]? [SrO8] and/or [SrO8]?[SrO8]). Acknowledgements The authors acknowledge the financial support of the following Brazilian research funding institutions: the FAPESP (2012/14004-5; 2013/07296-2), CNPq (304531/2013-8; 479644/ 2012-8), National Laboratory of Synchrotron Light (D04B-XAFS1- 11883), and CAPES. References 1. Fan L, Fan YX, Duan YH, Wang Q, Wang HT, Jia GH, Tu CY (2009) Continuous-wave intracavity Raman laser at 1179.5 nm with SrWO4 Raman crystal in diode-end-pumped Nd:YVO4 laser. Appl Phys B 94:553–557 2. Wang A, Wang C, Jia G (2009) Recent advances in strontium tungstate scheelite material. Front Chem China 5:61–70 3. Liao J, Qiu B, Wen H, Chen J, You W, Liu L (2009) Synthesis process and luminescence properties of Tm3? in AWO4 (A = Ca, Sr, Ba) blue phosphors. J Alloys Compd 487:758–762 4. Shan Z, Wang Y, Ding H, Huang F (2009) Structure-dependent photocatalytic activities of MWO4 (M = Ca, Sr, Ba). J Mol Catal A 302:54–58 5. Cavalcante LS, Sczancoski JC, Batista NC, Longo E, Varela JA, Orlandi MO (2013) Growth mechanism and photocatalytic properties of SrWO4 microcrystals synthesized by injection of ions into a hot aqueous solution. Adv Powder Technol 24:344–353 6. Lou Z, Cocivera M (2002) Cathodoluminescence of CaWO4 and SrWO4 thin films prepared by spray pyrolysis. Mater Res Bull 37:1573–1582 7. Fan JD, Zhang HJ, Wang JY, Jiang MH, Boughton RI, Ran DG, Sun SQ, Xia HR (2006) Growth and thermal properties of SrWO4 single crystal. J Appl Phys 100:063513–063518 8. Fan YX, Liu Y, Duan Y, Wang Q, Fan L, Wang HT, Jia GH, Tu CY (2008) High-efficiency eye-safe intracavity Raman laser at 1531 nm with SrWO4 crystal. Appl Phys B 93:327–330 9. Porto SL, Longo E, Pizani PS, Boschi TM, Simões LGP, Lima SJG, Ferreira JM, Soledade LEB, Espinosa JWM, Cássia-Santos MR, Maurera MAMA, Paskocimas CA, Santos IMG, Souza AG (2008) Photoluminescence in the CaxSr1-xWO4 system at room temperature. J Solid State Chem 181:1876–1881 10. Tian G, Sun S (2011) Additive induced morphology changes of nano-crystalline SrWO4. Cryst Res Technol 46:389–392 11. Dong FJ, Hu ZA, Zheng WP, Wen GW, Ji WY (2006) Synthesis of polycrystalline materials of SrWO4 and growth of its single crystal. Front Chem China 1:264–267 12. Yang S, Sun J (2004) Synthesis and properties of Tb3?-doped CaxSr1-xWO4. J Rare Earths 22:331–333 Fig. 10 PL spectra of SrWO4 microcrystals synthesized with differ- ent precursors salts J Mater Sci (2015) 50:8089–8103 8101 123 13. Caprez A, Meyer P, Mikhail P, Hulliger J (1997) New host- lattices for hyperfine optical hole burning: materials of low nuclear spin moment. Mater Res Bull 32:1045–1054 14. Errandonea D, Pellicer-Porres J, Manjón FJ, Segura A, Ferrer- Roca C, Kumar RS, Tschauner O, Rodrıguez-Hernandez P, Lopez-Solano J, Radescu S, Mujica A, Munoz A, Aquilanti G (2005) High-pressure structural study of the scheelite tungstates CaWO4 and SrWO4. Phys Rev B 72:174106–174119 15. Patel AR, Arora SK (1974) Crystal growth of BaWO4 and SrWO4 by flux evaporation. J Cryst Growth 23:95–100 16. Chen L, Gao Y (2009) Electro-deposition of luminescent molybdate and tungstate thin films via a cell route. Mater Chem Phys 116:242–246 17. Jiang X, Ma J, Yao Y, Sun Y, Liu Z, Ren Y, Liu J, Lin B (2009) Low-temperature synthesis of SrWO4 nano-particles by a molten salt method. Ceram Int 35:3525–3528 18. Thongtem T, Phuruangrat A, Thongtem S (2008) Characteriza- tion of MeWO4 (Me = Ba, Sr and Ca) nanocrystallines prepared by sonochemical method. Appl Surf Sci 254:7581–7585 19. Rangappa D, Fujiwara T, Watanabe T, Yoshimura M (2008) Preparation of Ba1-xSrxWO4 and Ba1-xCaxWO4 films on tung- sten plate by mechanically assisted solution reaction at room temperature. Mater Chem Phys 109:217–223 20. Maurera MAMA, Souza AG, Soledade LEB, Pontes FM, Longo E, Leite ER, Varela JA (2004) Microstructural and optical characterization of CaWO4 and SrWO4 thin films prepared by a chemical solution method. Mater Lett 58:727–732 21. Huang JY, Jia QX (2003) Structural properties of SrWO4 films synthesized by pulsed-laser deposition. Thin Solid Films 444:95–98 22. Chen Z, Gong Q, Zhu J, Yuan YP, Qian LW, Qian XF (2009) Controllable synthesis of hierarchical nanostructures of CaWO4 and SrWO4 via a facile low temperature route. Mater Res Bull 44:45–50 23. Sun L, Guo Q, Wu X, Luo S, Pan W, Huang K, Lu J, Ren L, Cao M, Hu C (2007) Synthesis and photoluminescent properties of strontium tungstate nanostructures. J Phys Chem C 111:532–537 24. Ciminelli VST, Dias A (2000) Theoretical predictions and experimental results of the hydrothermal processing of strontium tungstates. Ferroelectrics 241:271–278 25. Thongtem T, Phuruangrat A, Thongtem S (2010) Microwave- assisted synthesis and characterization of SrMoO4 and SrWO4 nanocrystals. J Nanopart Res 12:2287–2294 26. Sczancoski JC, Cavalcante LS, Joya MR, Espinosa JWM, Pizani PS, Varela JA, Longo E (2009) Synthesis, growth process and photoluminescence properties of SrWO4 powders. J Colloid Interface Sci 330:227–236 27. Ryu EK, Huh YD (2008) Morphology-controlled synthesis of SrWO4 crystals. Mater Lett 62:3081–3083 28. Thongtem T, Phuruangrat A, Thongtem S (2008) Preparation and characterization of nanocrystalline SrWO4 using cyclic micro- wave radiation. Curr Appl Phys 8:189–197 29. Pereira PFS, Nogueira IC, Longo E, Nassar EJ, Rosa ILV, Cavalcante LS (2015) Rietveld refinement and optical properties of SrWO4:Eu 3? powders prepared by the non-hydrolytic sol-gel method. J Rare Earths 33:113–128 30. Zhang L, Bai Q, Wang L, Zhang A, Zhang Y, Xing Y (2014) Synthesis and electrochemical properties of SrWO4/graphene composite as anode material for lithium-ion batteries. Funct Mater Lett 7:1450010–1450013 31. Sharma JC, Vijay A, Bhardwaj S (2013) Photocatalytic activity of a novel compound SrWO4: removal of toxic metal lead (II) from water. World Appl Sci J 23:208–212 32. Gouveia AF, Sczancoski JC, Ferrer MM, Lima AS, Santos MRMC, Li MS, Santos RS, Longo E, Cavalcante LS (2014) Experimental and theoretical investigations of electronic struc- ture and photoluminescence properties of b-Ag2MoO4 micro- crystals. Inorg Chem 53:5589–5599 33. Rodriguez-Hernandez P, Lopez-Solano J, Radescu S, Mujica A, Munoz A, Errandonea D, Pellicer-Porres J, Segura A, Ferrer- Roca C, Manjón FJ, Kumar RS, Tschauner O, Aquilanti G (2006) Theoretical and experimental study of CaWO4 and SrWO4 under pressure. J Phys Chem Solids 67:2164–2171 34. Rietveld HM (1969) A profile refinement method for nuclear and magnetic structures. J Appl Cryst 2:65–71 35. Larson AC, Von Dreele RB (1994) General structure analysis system (GSAS), Los Alamos National Laboratory Report LAUR. 86:748–768 36. Chebyshev PL (1854) Théorie des mécanismes connus sous le nom de parallélogrammes. Mémoires des Savants trangers prsents—Acadmie de Saint-Ptersbourg. 7:539–586 37. Thompson P, Cox DE, Hastings JB (1987) Rietveld refinement of Debye-Scherrer synchrotron X-ray data from Al2O3. J Appl Cryst 20:79–83 38. Finger LW, Cox DE, Jephcoat AP (1994) A correction for powder diffraction peak asymmetry due to axial divergence. J Appl Cryst 27:892–900 39. Stephens PW (1999) Phenomenological model of anisotropic peak broadening in powder diffraction. J Appl Crystallogr 32:281–289 40. Hallaoui A, Taoufyq A, Arab M, Bakiz B, Benlhachemi A, Bazzi L, Villain S, Valmalettea JC, Guinnetona F, Gavarri JR (2015) Influence of chemical substitution on the photoluminescence of Sr(1–x)PbxWO4 solid solution. J Solid State Chem 227:186–195 41. Pereira PFS, de Moura AP, Nogueira IC, Lima MVS, Longo E, de Sousa Filho PC, Serra OA, Nassar EJ, Rosa ILV (2012) Study of the annealing temperature effect on the structural and lumines- cent properties of SrWO4: Eu phosphors prepared by a non-hy- drolytic sol-gel process. J Alloys Compd 526:11–21 42. http://www.crystalimpact.com/diamond/ 43. Li Q, Jia C (2014) Synthesis and characteristics of SrWO4:Sm 3? nanofiber phosphors by electrospinning method. Nanosci Nan- otechnol Lett 6:1014–1017 44. Chen Y, Wu QS, Ding YP (2007) Oil/water interface synthesis and optical property of strontium tungstate nanorods. Nano 2:195–199 45. Jia G, Wang C, Xu S (2010) Local site symmetry determination of scheelite-type structures by Eu3? spectroscopy. J Phys Chem C 114:17905–17913 46. Krayzman V, Levin I, Woicik JC, Yoder D, Fischer DA (2006) Effects of local atomic order on the pre-edge structure in the Ti-K X-ray absorption spectra of perovskite CaTi1-xZrxO3. Phys Rev B 74:224104–224110 47. Gracia L, Longo VM, Cavalcante LS, Beltran A, Avansi W, Li MS, Mastelaro VR, Varela JA, Longo E, Andres J (2011) Pres- ence of excited electronic state in CaWO4 crystals provoked by a tetrahedral distortion: an experimental and theoretical investiga- tion. J Appl Phys 110:043501–043511 48. Cavalcante LS, Almeida MAP, Avansi W Jr, Tranquilin RL, Longo E, Batista NC, Mastelaro VR, Li MS (2013) Cluster coordination and photoluminescence properties of a-Ag2WO4 microcrystals. Inorg Chem 51:10675–10687 49. Basu S, Naidu BS, Viswanadh B, Sudarsan V, Jha SN, Bhat- tacharyya D, Vatsa RK (2014) Nature of WO4 tetrahedra in blue light emitting CaWO4 probed through the EXAFS technique. RSC Adv 4:15606–15612 50. Kuzmin A, Purans J (2001) Local atomic and electronic structure of tungsten ions in AWO4 crystals of scheelite and wolframite types. Radiat Meas 33:583–586 51. Poirier GL, Cassanjes FC, Messaddeq Y, Ribeiro SJL, Michalowicz A, Poulain M (2005) Local order around tungsten atoms in tungstate fluorophosphates glasses by X-ray absorption spectroscopy. J Non-Cryst Solids 351:3644–3648 52. Montanari B, Barbosa AJ, Ribeiro SJL, Messaddeq Y, Poirier G, Li MS (2008) Structural study of thin films prepared from 8102 J Mater Sci (2015) 50:8089–8103 123 http://www.crystalimpact.com/diamond/ tungstate glass matrix by Raman and X-ray absorption spec- troscopy. Appl Surf Sci 254:5552–5556 53. http://www.sigmaplot.com/products/peakfit/peakfit.php 54. Daviero-Minaud S, Rolle A, Kongmark C, Vannier RN (2009) Local environment in Ba2In2-xWxO5?3x/2 oxide ion conductors. J Solid State Chem 182:289–294 55. Ko JYP, Hu Y, Armelao L, Sham TK (1990) XANES and XEOL studies of Eudoped calcium tungstate in silica synthesized by sol- gel method. J Phys 190:012078–012081 56. Gonçalves RF, Cavalcante LS, Nogueira IC, Longo E, Godinho MJ, Sczancoski JC, Mastelaro VR, Pinatti IM, Rosa ILV, Mar- ques APA (2015) Rietveld refinement, cluster modelling, growth mechanism and photoluminescence properties of CaWO4:Eu 3? microcrystals. CrystEngComm 17:1654–1666 57. Porto SP, Scott JF (1967) Raman spectra of CaWO4, SrWO4, CaMoO4, and SrMoO4. Phys Rev 157:716–719 58. Wannapop S, Thongtem T, Thongtem S (2011) Characterization of SrWO4-PVAand SrWO4 spiders’ webs synthesized by elec- trospinning. Ceram Inter 37:3499–3507 59. Degreniers S, Jandl S, Carlone C (1984) Temperature dependence of the Raman active phonons in CaWO4, SrWO4 and BaWO4. J Phys Chem Solids 45:1105–1109 60. Golubović A, Gajić R, Dohčević-Mitrović Z, Nikolić S (2006) Nd induced changes in IR spectra of CaWO4 single crystals. J Alloys Compd 415:16–22 61. Gong Q, Qian X, Ma X, Zhu Z (2006) Large-scale fabrication of novel hierarchical 3D CaMoO4 and SrMoO4 mesocrystals via a microemulsion-mediated route. Cryst Growth Des 6:1821–1825 62. Yin Y, Yang F, Yang Y, Gan Z, Qin Z, Gao S, Zhou B, Li X (2011) Controlled synthesis of BaWO4 hierarchical nanostruc- tures by exploiting oriented attachment in the solution of H2O and C2H5OH. Superlatt Microstruct 49:599–607 63. Tian Y, Chen B, Yu H, Hua R, Li X, Sun J, Cheng L, Zhong H, Zhang J, Zheng Y, Yu T, Huang L (2011) Controllable synthesis and luminescent properties of three-dimensional nanostructured CaWO4:Tb 3? microspheres. J Colloid Interface Sci 360:586–592 64. Kubelka P, Munk F (1931) Ein Beitrag zur optik der far- banstriche. Zeit Für Tech Physik 12:593–601 65. Morales AE, Mora ES (2007) Use of diffuse reflectance spec- troscopy for optical characterization of un-supported nanostruc- tures. U Pal Rev Mex Fis S 53:18–22 66. Smith RA (1978) Semiconductors, 2nd edn. Cambridge Univer- sity Press, London 67. Zhang Y, Holzwarth NAW, Williams RT (1998) Electronic band structures of the scheelite materials CaMoO4, CaWO4, PbMoO4, and PbWO4. Phys Rev B 57:12738–12750 68. Lacomba-Perales R, Ruiz-Fuertes J, Errandonea D, Martı́nez- Garcı́a D, Segura A (2008) Optical absorption of divalent metal tungstates: correlation between the band-gap energy and the cation ionic radius. Eur Phys Lett 83:37002–37006 69. Longo VM, Orhan E, Cavalcante LS, Porto SL, Espinosa JWM, Varela JA, Longo E (2007) Understanding the origin of photo- luminescence in disordered Ca0.60Sr0.40WO4: an experimental and first-principles study. Chem Phys 334:180–188 70. Tian G, Sheng N, Qiu X (2014) Structure and photoluminescence properties of SrWO4 3D microspheres synthesized by the sur- factant-assisted hydrothermal method. Cryst Res Technol 49:360–365 71. Ling CS (2014) Cyclic Microwave-assisted metathetic synthesis and spectroscopic properties of SPION/SrWO4:Er3?, Yb3? composites. Asian J Chem 26:1848–1852 72. Sun X, Sun X, Li X, He J, Wang B (2014) Molten salt synthesis, characterization, and luminescence of SrWO4, SrWO4:Tb 3? and SrWO4:Eu 3? powders. J Mater Sci 25:2320–2324 J Mater Sci (2015) 50:8089–8103 8103 123 http://www.sigmaplot.com/products/peakfit/peakfit.php Effect of different strontium precursors on the growth process and optical properties of SrWO4 microcrystals Abstract Introduction Experimental details Synthesis of SrWO4 microcrystals Characterizations of SrWO4 microcrystals Results and discussion X-ray diffraction and Rietveld refinement analyses Structural representation of SrWO4 crystals XANES spectroscopy analyses FT-Raman and FT-IR spectroscopies analyses FE-SEM image analyses Growth mechanism of SrWO4 crystals UV--Vis diffuse reflectance spectroscopy analyses PL emission analyses Conclusion Acknowledgements References