Structural refinement, Raman spectroscopy, optical and electrical properties of (Ba12xSrx)MoO4 ceramics S. K. Ghosh1 • S. K. Rout1 • A. Tiwari1 • P. Yadav1 • J. C. Sczancoski2 • M. G. R. Filho3 • L. S. Cavalcante3 Received: 3 May 2015 / Accepted: 14 July 2015 / Published online: 19 July 2015 � Springer Science+Business Media New York 2015 Abstract In this paper, structural refinement, Raman spectroscopy, optical and electrical properties of barium strontium molybdate [(Ba1-xSrx)MoO4] ceramics with different (x) contents (x = 0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6; 0.7; 0.8; 0.9; and 1) were synthesized by the solid state reaction method. These ceramics were structurally char- acterized by X-ray diffraction (XRD), Rietveld refinement, and micro-Raman spectroscopy. The shape of the grains for these ceramics was observed by means of scanning elec- tron microscopy (SEM) images. The optical properties were investigated using ultraviolet–visible (UV–Vis) absorption spectroscopy and photoluminescence (PL) measurements. The dielectric and ferroelectric properties were analyzed by permittivity (er), loss tangent (tan d) and polarization versus electric field (P–E) hysteresis loop. XRD patterns, Rietveld refinement, and micro-Raman spectra showed that all ceramics are monophasic with a scheelite-type tetragonal structure. A decreased of lattice parameters and unit cell volume was observed with the increase of Sr2? ions into BaMoO4 lattice. Rietveld data were employed to model the [BaO8], [SrO8] and [MoO4] clusters in the tetragonal lattices. The SEM images indicate that increased x content promotes a decrease in the grain size and modifications in the shape. UV–Vis spectra indi- cated a decrease in the optical band gap values with an increase in x content in the (Ba1-xSrx)MoO4 ceramics. PL emissions exhibit a non-linear behavior to increase or decrease with the increase of Sr2? ions in the tetragonal lattices, when excited by a wavelength of 350 nm. The P–E decreases along with slim hysteresis loop towards higher Sr2? ions concentration. These effects are correlated with decrease in lattice parameters and c/a ratio in this tetragonal lattice. The microwave dielectric constant and quality factor were measured using the method proposed by Hakki–Coleman. Temperature coefficient and quality factor of these materials were measured by vector network analyzer. 1 Introduction Recently, extensive research has been conducted on alka- line molybdate and tungstates materials because these materials have been widely employed in several industrial applications, such as: optoelectrical devices, microwave dielectric devices, gas sensor, photo-catalyst, amplifier and electrochromic devices [1–8]. These alkaline materials can be classified into two different classes; the first has a wolframite-type monoclinic structure and second has a scheelite-type tetragonal structure both with a general formula: ABO4, where: (A = Ba2?, Sr2?, Ca2?, Mg2?, Zn2? and Pb2? ions) and (B = Mo6? and W6?) [9, 10]. Depending on the electronic density and size of A2? ionic radius ions, the molybdates and tungstates can display one of two types of structure [11]. The larger Ba2? and Sr2? cations are coordinated to eight oxygen (O) atoms in solid in tetragonal lattice which is composed of deltahedral & S. K. Rout skrout@bitmesra.ac.in & L. S. Cavalcante laeciosc@bol.com.br 1 Electroceramics Laboratory, Department of Physics, Birla Institute of Technology, Mesra, Ranchi, India 2 LIEC-IQ-Universidade Estadual Paulista, Araraquara, São Paulo 14801-907, Brazil 3 PPGQ-GERATEC-Universidade Estadual do Piauı́, João Cabral, N. 2231, P.O. Box 381, Teresina, PI 64002-150, Brazil 123 J Mater Sci: Mater Electron (2015) 26:8319–8335 DOI 10.1007/s10854-015-3498-x http://crossmark.crossref.org/dialog/?doi=10.1007/s10854-015-3498-x&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1007/s10854-015-3498-x&domain=pdf [BaO8]/[SrO8] clusters and tetrahedral [MoO4] clusters, while the smaller Mg2? and Zn2? cations are coordinated to six O atoms in solid in tetragonal lattice which is composed both by octahedral [MgO6]/[ZnO6] and [MoO6] clusters [12, 13]. Moreover, in AWO4 and/or AMoO4 materials with scheelite-type tetragonal structure have a body centered tetragonal scheelite structure, with four formula units per primitive cell (Z = 4) [14]. These molybdates are good host materials for other alkaline and rare earth elements and have been used to improve many physical properties, such as: photoluminescence (PL), optical band gap, ferroelectric, dielectric, and photocatalysis [15–19]. These electronic properties are related to the presence of order–disorder and/ or distortion at medium range into tetragonal lattice [20]. Depending on the nature of the substituted ions into tetrag- onal lattice in the host matrix of scheelite materials enhances the intensity of PL spectra and shift towards the shorter wavelength region at room temperature [21, 22]. The exis- tence of localized energy levels in between conduction and valence band has a significant effect on the values of the band gap [23]. The thermodynamics and enthalpies of formation of tungstates and molybdate materials depends on the ionic radii, pH and alkaline earth metals which is highest in BaWO4 and BaMoO4 compounds compare to other alkaline metals followed by strontium molybdate SrMoO4(SMO), CaMoO4 (CMO) etc., its kinetics of formation governs the transport of matter during heating process [24–26]. More- over, different synthesis method and heating process, such as: solid state reaction route [27–30], precipitation [31–34], hydrothermal conventional [35, [36], polymeric precursor method [37, 38], microwave-hydrothermal [39, 40] and micro-emulsion method [41, 42] has been used to prepared the BaMoO4 and SrMoO4materials. It is being observed that these synthesis techniques are responsible for evolution for various morphological grains and hence changes the physi- cal and chemical properties of these ceramics materials. A few papers have been reported in literature for ostentation of (Ba1-xSrx)MoO4 solution solids in form of thin films [43–45] and powders [46, 47]. Therefore, in this paper, we report on the effective way to improve the electronic properties of (Ba1-xSrx)MoO4 ceramics synthe- sized by the solid state reactions. The difference in ionic sizes of the doping elements manifested itself the overall structure in the form of Goldschmidt’s tolerance factor (t) is found to be important parameters in controlling the microwave dielectric, ferroelectric hysteresis loop, switching of polarization, shifting of Raman active modes and bending and stretching of metal-oxide polyhedra bonds in micro-Raman spectra. For t[ 1, Mo-site has much room, resulting in increase of damping of the second mode involving the B-site vibration. Damping could be mini- mized by changing the ionic size to fulfill t * 1. Based on this concept, we choose smaller sized Sr2? ions in place of higher radii Ba2? ions at A-site so as to reduce the t values. The physical properties of this scheelite materials are coupled with the unit cell dimension, crystal symmetry, as well the nature of the band structure. Finally, in work, we explain with more details their optical, microstructural, dielectric and ferroelectric properties. 2 Experimental procedure 2.1 Synthesis of (Ba12xSrx)MoO4 ceramics The (Ba1-xSrx)MoO4 ceramics with (x = 0; 0.1; 0.2, 0.3; 0.4; 0.5; 0.6; 0.7, 0.8; 0.9, and 1) were prepared using solid state reaction method. High purity chemical barium car- bonate [BaCO3] (99 % Merck India Ltd.), strontium car- bonate [SrCO3] (99 %, Himedia Chemicals) and molybdenum oxide [MoO3] (99 %, Alfa Aesar) has been used. The stoichiometrically calculated reagents are thor- oughly mixed in the liquid medium using agate mortar and pestle for 6 h. Then the dried ceramics at different (x) compositions were heat treated at 850 �C for 4 h and 1000 �C for 2 h with intermediate grinding. The solid solution reaction occurs according to Eq. (1) below: 1� xð ÞBaCO3ðsÞ þ 1MoO3ðsÞ þ xð ÞSrCO3ðsÞ ! Ba1�xSrxð ÞMoO4ðsÞ þ 2CO2ðgÞ ð1Þ These ceramics were structurally characterized as pre- sented in the following section. 2.2 Characterizations Structural characterization of all these ceramics were done by X-ray diffractions (XRD) patterns using a Philips diffractometer model PW-1830 with Cu–Ka radiation (k = 1.5406 Å) in the 2h range from 20� to 75� in the normal routine with a scanning velocity of 0.02� and from 20� to 80� with a scanning velocity of 1�/min in the Rietveld routine (both with 4 h of measurement). The samples calcined at 1000 �C and above showed single phase character without any evidence of any secondary phase. Thus, 1000 �C is considered as the optimized cal- cinations temperature for all the compositions. The cal- cined monophasic molybdate powder was mixed with polyvinyl alcohol as a binder and pressed uniaxially under pressure of 80 kg/cm2 to form disk shape pellet of 10 mm diameter and 1.5 mm thickness. These pellets were sin- tered at 1050 �C for 2 h to provide maximum shrinkage and compactness. Structural refinement was carried out for all the compositions x = 0.00–0.10 using the Rietveld’s refinement program EXPGUI interface to the program GSAS (General Structural Analysis System). Raman 8320 J Mater Sci: Mater Electron (2015) 26:8319–8335 123 spectroscopic studies were done by using (in Via, Reishaw, UK) an excitation wavelength of 514 nm. The Fourier transform infra red (FTIR) spectra were recorded at room temperature by the standard KBr pellet technique using a FTIR Spectrophotometer (Spectrum 1000, Perkin Elmer, Japan). The diffuse reflectance spectra of the pure and doped ceramics powder samples was taken at room tem- perature using double beam UV–Visible spectrometer (Lambda 35, Winlab V6.0, Perkin-Elmer, USA). PL mea- surements were made with a Monospec 27 monochromator (Thermal Jarrel Ash, USA) coupled to an R446 photo- multiplier (Hamamatsu Photonics, Japan). A krypton ion laser (Coherent Innova 90K, USA) (k = 350 nm) was used as the excitation source; its maximum output power was maintained at 500 mW. Microstructure characterization was performed using a scanning electron microscope (SEM, JEOL-6330F, JEOL, Japan). Room temperature dielectric constant and dielectric loss were measure by using Alpha high resolution dielectric impedance analyzer (Novacontrol, Gmbh, Germany) at a frequency range 1 Hz–1 GHz. That especially optimized for dielectric materials with low loss factor over a broad frequency range. Before conducting any impedance measurements a calibration was carried out using an internal calibration. Ferroelectric hysteresis loop and switching polarization were measured on 10 mm diameter and 1.5 mm thickness silver coated samples at different applied voltage using precision high voltage amplifier interface (Radiant Tec. Inc., Model no. HVA 100611-792, USA). Microwave dielectric measurement are performed by using a Vector Network Analyzer (N5230A, Agilent Technologies, USA) in a TE01d mode of cylindrical shaped of BaMoO4, (Ba1-xSrx)MoO4 and SrMoO4 materials. 3 Results and discussion 3.1 XRD analyses Figure 1a, b illustrates the XRD pattern of (Ba1-xSrx) MoO4 ceramics from (x = 0 to 0.5) and from (x = 0.6 to 1) heat treated at 1000 �C for 2 h, respectively. All diffraction peaks indicate that (Ba1-xSrx)MoO4 ceramics with different composition are monophasic nature indexed to the scheelite-type tetragonal structure with space group I41/a, which is in agreement with the inorganic crystal structure database (ICSD) card No. 50281 for BaMoO4 and 173120 for SrMoO4 and, respectively [48, 49]. The sharp and well defined diffraction peaks indicated a high degree of crystallinity and structurally long range ordered system. The diffraction peaks are found shifted to higher 2h position with an increase of Sr2? ion concentration. According to the Bragg’s law (k = 2dsinh) this shift of 2h position occur when there is a reduction in unit cell lattice parameters and hence decrease in cell volume. 3.2 Rietveld refinements analyses Rietveld refinement program [50] has been used to calcu- late the lattice parameters with the structural refinement plots as shown Fig. 2a–f for some (x) compositions of (Ba1-xSrx)MoO4 ceramics forming solution solids. The background was modeled by 16 terms linear inter- polation function. The peak profile were described by a pseudo-Voigt function, profile half-width parameters (u, v, Fig. 1 XRD patterns of (Ba1-xSrx)MoO4 ceramics with x in the range from a 0 to 0.5 and b 0.6 to 1 heat treated at 1000 �C. The vertical lines indicate the position and relative intensity of the ICSD card No. 50821 and No. 173120 for BaMoO4 and SrMoO4 phases, respectively J Mater Sci: Mater Electron (2015) 26:8319–8335 8321 123 Fig. 2 Rietveld refinement plots of selected (Ba1-xSrx)MoO4 ceramics for some concentrations with: a x = 0, b x = 0.2, c x = 0.3, d x = 0.5, e x = 0.8, and f x = 1 8322 J Mater Sci: Mater Electron (2015) 26:8319–8335 123 w), isotopic displacement parameters, preferred orientation factor, occupancy and atomic functional position, Lor- entzian peak broadening factor, and microstrain effect of crystallites where subsequently refined. The structural refinement was performed between the experimental and observed XRD pattern and yields acceptable reliable factor. Details regarding the Rietveld refinements are listed in Table 1. In Table 1 are presents the Rietveld refinement data for (Ba1-xSrx)MoO4 ceramics at different (x) compositions. Some variation in the atomic position of oxygen’s atoms was observed, while barium, strontium and molybdenum atoms have fixed atomic position (Table 1). Hence these variations in the atomic positions of the oxygen’s atoms can leads to the formation of different types of distortions in (O–Ba–O), (O–Sr–O), and (O–Mo–O) bonds and sub- sequently produce different levels of distortion in delta- hedral [BaO8]/[SrO8] clusters and tetrahedral [MoO4] clusters in the lattice [51]. 3.3 Lattice parameters and unit-cell volume analyses The substitution of Sr2? ions at A-site reduces the lattice parameter and unit cell volume of (Ba1-xSrx)MoO4 ceramics as illustrated in Fig. 3 and inset Fig. 3, respectively. A subsequent lattice strain has been observed due to Sr2? ion substitution in the host lattice. This strain gener- ated when two adjacent grain come in contact during their growth. Induce lattice strain is calculated by using the relation in Eq. (2): s ¼ Dd d ¼ dd � dp dp ð2Þ where, dd and dp are lattice spacing of substituted (Ba1-x Srx)MoO4 ceramics and pure BaMoO4 ceramics, respec- tively. When dd\ dp, strain is negative suggesting the compressive strain in the materials were listed in Table 1. The values of compressive strain increased with Sr2? ions concentration, which indicated the higher diffusion and kinetics of Sr2? ions in matrix host BaMoO4 lattice. In addition, the average grain size is also calculated by using the Scherrer’s formulae: D ¼ 0:9k BcoshB where, D is the grain size, k is the X-ray wavelength, B is full width half maxima (FWHM) at peak position 26.5� (112) and hB is the Bragg’s diffraction angle [52]. 3.4 Representation of the BaMoO4, (Ba0.5Sr0.5)MoO4 and SrMoO4 unit cells Figure 4a–c illustrate the schematic representation of unit cell tetragonal structure for a selected concentration of (Ba1-xSrx)MoO4 ceramics with (x = 0, 0.5 and 1). Standardization of crystal structure and fractional coordinate was modeled by visualization for electronic and Table 1 Lattice parameters, unit cell volume, statistical parameters of quality obtained by Rietveld refinement and strain lattice for the (Ba1-xSrx)MoO4 ceramics at different x concentration synthesized by solid state method (Ba1-xSrx)MoO4 Lattice parameters (Å) Unit cell volume (Å3) Rwp Rp v2 S a = b c x = 0 5.56 12.77 395.41 0.340 0.267 1.22 – x = 0.1 5.55 12.69 390.88 0.031 0.017 1.82 -0.004 x = 0.2 5.52 12.60 383.92 0.018 0.009 2.10 -0.008 x = 0.3 5.51 12.55 381.01 0.016 0.008 1.95 -0.009 x = 0.4 5.50 12.47 377.21 0.010 0.006 1.66 -0.014 x = 0.5 5.48 12.41 372.76 0.005 0.002 2.50 -0.016 x = 0.6 5.47 12.34 364.15 0.002 0.001 2.00 -0.020 x = 0.7 5.45 12.26 364.15 0.005 0.004 1.25 -0.024 x = 0.8 5.43 12.18 359.12 0.006 0.003 2.00 -0.028 x = 0.9 5.41 12.10 354.14 0.007 0.003 2.33 -0.031 x = 1 5.40 12.04 351.08 0.013 0.004 2.71 -0.033 Fig. 3 Change in lattice parameters (a = b = c) [Å] and (inset) unit cell volume (a 9 b9 c) [Å3] for (Ba1-xSrx)MoO4 ceramics with different x concentrations J Mater Sci: Mater Electron (2015) 26:8319–8335 8323 123 structural analysis (VESTA) program version 3.2.1 [53, 54] using the lattice parameters and atomic coordinate obtained from Rietveld refinement data presented in Tables 1 and 2. In Table 2 the as it can be to observe some variations in the atomic positions related to oxygen atoms were observed, while the barium, strontium and molybdenum atoms have fixed atomic positions. These results indicate that the positions of the oxygen atoms are very variable in the lattice as shown by the X-ray powder diffraction data technique. In Fig. 4a–c, these unit cells, all the molybde- num atoms are coordination by four oxygen atoms which form tetraedral [MoO4] cluster with a symmetry group Td and tetrahedron polyhedra. These tetrahedral clusters were slightly distorted in matrix (Ba1-xSrx)MoO4 lattice as a result this distortion in tetrahedral [MoO4] clusters cause changes in O–Mo–O bond length and bond angle which further modified the energy levels and enhanced structural order–disorder in the host lattice [55]. On the other hand barium and strontium atoms are bonded to eight oxygen atoms and formed deltahedral [BaO8]/[SrO8] clusters with a symmetry group of (D2d). The deltahedral [BaO8] clusters have the same coordination number as the deltahedral [SrO8] clusters in A-site and only different electronic environment which influence the optical and electrical properties in the material [56]. 3.5 Micro-Raman spectroscopy analyses According to the group theory calculation, molybdate with a scheelite-type tetragonal structure contain 26 different vibration modes which are as follows Eq. (3) below [57]: C Ramanð Þþ Infrared½ �f g ¼ 3Ag þ 5Bg þ 5Eg � � þ 5Au þ 3Bu þ 5Eu½ � ð3Þ where, Ag, Bg, and Eg are Raman active vibrational modes. The A and B modes are non-degenerate, whereas the E modes are doubly degenerate. Ag, Bg and Eg are Raman modes that arise from the same motion of clusters in the (Ba1-xSrx)MoO4 lattice. Therefore, 13 Raman-active vibrational modes in (Ba1-xSrx)MoO4 materials are antic- ipated according to Eq. (4) below [58]: CfðRamang ¼ 3Ag þ 5Bg þ 5Eg � � ð4Þ Vibration modes observed in Raman spectra for molybdate are further classified into two groups: internal and external vibrational modes. The internal vibrational modes are related to [MoO4] tetrahedral clusters vibration in the lattice. The external modes are related to phonon lattice vibration which correspond to [BaO8]/[SrO8] deltahedral clusters [47]. In isolated tetrahedral [MoO4] have a cubic symmetry (Td) and their vibration consist of Fig. 4 Schematic representation of tetragonal (Ba1-xSrx)MoO4 unit cells with [BaO8]–[SrO8]–[MoO4] clusters for some concentrations with: a x = 0, b x = 0.5, and c x = 1, respectively 8324 J Mater Sci: Mater Electron (2015) 26:8319–8335 123 four internal modes such as: m1(A1), m2 (E), m3(F2), m4(F2), one free rotational mf.r (F1) modes and one external mext (F2) modes [59]. In scheelite-type tetragonal symmetry [MoO4] clusters is reduced to S4 point symmetry. Figure 5a, b show of (Ba1-xSrx)MoO4 ceramics from (x = 0 to 0.5) and from (x = 0.6 to 1) heat treated at 1000 �C for 2 h, respectively. In order to better understand, the region between 100 and 950 cm-1 was highlighted and ten different vibration modes were identified. The internal modes m1(A1), m2 (E), m3 (F2), m4 (F2), were observed in the range between 888 to 892 cm-1, 846 to 839 cm-1, 797 to 791 cm-1, 383 to 360 cm-1, 368 to 347 cm-1, and 331 to 325 cm-1. The free rotational mode mf.r (F1) was detected in range between 184 and 192 cm-1 and external mext (F2) modes were detected in between 108 and 139 cm-1 frequency range. The results are in agreement with that reported in the previous literature [60]. The Raman spectra vibrational modes are related to (/O–Mo–O?) symmetry stretching of tetrahedral [MoO4] clusters assigned as m1 (A1), sym- metry bending vibration m2 (E), anti-symmetry stretching m3 (F2), anti-symmetry bending vibration m4 (F2) and free rotational mf.r (F1) mode. The external modes mext are cor- respond to the motion of the deltahedral [BaO8]/[SrO8] clusters assigned as a symmetry bending. It is observed that an increase in Sr2? ions concentration in host matrix induced a shift in Raman active mode towards higher fre- quency side. These shifts in Raman modes are depend on the cationic mass of AMoO4 scheelite-type structure [61]. In (Ba1-xSrx)MoO4 ceramics the mode frequency increased with a mass reduction of A2? cation, i.e., substitution of Sr2? for Ba2? cations. This behavior is probably due to lighter metal ion which strongly interacts with cluster group [MoO4] and produces higher force constant and increase the vibrational frequency in Ba1-xSrx)MoO4 ceramics. The electronegativity of Ba2? ions is different from Sr2? ions and might be another factor which influ- ences the shift in Raman active modes. The increase in electro negativity in Sr2? cation results in higher force constant of the stretching vibration between tetrahedral [MoO4] and deltahedral [BaO8]/[SrO8] clusters and their respective [O–Ba–O–Mo–O–Ba–O], [O–Sr–O–Mo–O–Ba– O] and [O–Sr–O–Mo–O–Sr–O] bond strength. As already mentioned earlier, the Rietveld refinement data indicated that the lattice parameters and unit cell volume decrease with the increase in Sr2? ions concentration which indi- cated the reduction of O–Mo–O, O–Ba–O and O–Sr–O bond lengths, is one of the reason to increase the force constant and vibrational frequency in these metallic groups. In addition, other factors which also influence the Raman active behavior, such as: average grain size, different methods of sample preparation and order–disorder corre- lation in host lattice. 3.6 UV–Vis absorption spectroscopy and optical band gap values analyses The diffuse reflectance spectra (DR spectra) of the (Ba1-xSrx)MoO4 ceramics in the range of 200–800 nm (UV–Vis) diffuse reflectance spectra are shown in Fig. 6a–f and the found optical band values for [(Ba1-xSrx)MoO4] ceramics with different (x) contents are shown in Fig. 6g. Table 2 Atomic coordinates (x, y, z) obtained by Rietveld refinement for selected x concentration of (Ba1-xSrx)MoO4 ceramics materials Atoms (Ba1-xSrx)MoO4 Atomic positions x = 0 x = 0.2 x = 0.4 x = 0.6 x = 0.8 x = 1 Ba (x) 0 0 0 0 0 0 (y) 0.25 0.25 0.25 0.25 0.25 0.25 (z) 0.65 0.65 0.65 0.65 0.65 0.65 Sr (x) 0 0 0 0 0 0 (y) 0.25 0.25 0.25 0.25 0.25 0.25 (z) 0.65 0.65 0.65 0.65 0.65 0.65 Mo (x) 0 0 0 0 0 0 (y) 0.25 0.25 0.25 0.25 0.25 0.25 (z) 0.12 0.12 0.12 0.12 0.12 0.12 O (x) 0.228 0.236 0.226 0.248 0.234 0.246 (y) 0.135 0.389 0.396 0.378 0.392 0.365 (z) 0.049 0.159 0.191 0.190 0.193 0.194 J Mater Sci: Mater Electron (2015) 26:8319–8335 8325 123 The optical band gap energy (Egap) was calculated using the method proposed by Kubelka and Munk [62]. This methodology is based on the transformation of diffuse reflectance measurements to estimate Egap values with good accuracy within the limits of the assumptions when modeled in three dimensions [63]. Particularly, it is useful in limited cases with an infinitely thick sample layer. The Kubelka–Munk Eq. (5) for any wavelength is described as: FðR1Þ � ð1� R1Þ2 2R1 ¼ k s ð5Þ where F(R?) is the Kubelka–Munk function or absolute reflectance of the sample. In our case, magnesium oxide (MgO) was the standard sample used in reflectance mea- surements. R? = Rsample/RMgO, where R? is the reflec- tance when the sample is infinitely thick, 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 [64] can be calculated using the following Eq. (6): ahm ¼ C1ðhm� EgapÞn ð6Þ where a is the linear absorption coefficient of the material, hm is the photon energy, C1 is a proportionality constant, and n is a constant associated with the type of electronic transitions (n = 0.5, 2, 1.5, and 3 for direct allowed, indirect allowed, direct forbidden, and indirect forbidden transitions, respectively). Finally, using the remission function described in Eq. (5) with the term k = 2a and C2 as a proportionality constant, we obtain the modified Kubelka–Munk equation, as indicated in Eq. (7): F R1ð Þhm½ �2¼ C2 hm� Egap � � ð7Þ By finding the F(R?) value from Eq. (7) and plotting [F(R?)hm]2 against hm, the Egap value of the [(Ba1-xSrx) MoO4] ceramics was determined. In the earlier literature [51, 65, 66], it was established that BaMoO4 and SrMoO4 exhibit optical absorption spectra governed by direct tran- sition between valence bands (VB) and conduction bands (CB). This characterization is observed when the electrons at the maximum of valence band transit to minimum of conduction band at the same point in the Brillouin zone [67]. Molybdenum atoms in general present an ideal position at tetrahedron center forming tetrahedral [MoO4] clusters. However, our [(Ba1-xSrx)MoO4] ceramics were prepared by the solid state reactions and the successive milling cycles can provoke several distortions or simulta- neous presence of order–disorder into the lattice. These effects can cause small displacements of Mo atoms of- center of symmetry center tetrahedral [MoO4] clusters [68]. These local disorder effect increase the defects between the [BaO8]–[MoO4]–[BaO8], [BaO8]–[MoO4]–[SrO8] and [SrO8]–[MoO4]–[SrO8] clusters as shown previous in Fig. 4a–c, which generate new electronic energy levels within the band gap [69, 70], reducing the optical band gap as illustrated in Fig. 6a–f. Therefore, the decrease in the band gap values (Egap) values with increase in Sr2? ions concentration are attributed to increase in local lattice distortions and intermediary energy levels within band gap and decrease of local electronic density as indicated in the Fig. 6g. Fig. 5 Micro-Raman spectra of (Ba1-xSrx)MoO4 ceramics with x in the range from a 0 to 0.5 and b 0.6 to 1 heat treated at 1000 �C. The vertical lines indicate the position and relative intensity of down pointing triangle for BaMoO4 and asterisk SrMoO4 phases, respectively cFig. 6 UV–Vis diffuse reflectance spectra of the ceramics for some concentrations with: a x = 0, b x = 0.2, c x = 0.3, d x = 0.5, e x = 0.8, and f x = 1 optical band gap values as a function of the x concentration 8326 J Mater Sci: Mater Electron (2015) 26:8319–8335 123 J Mater Sci: Mater Electron (2015) 26:8319–8335 8327 123 3.7 PL emission analyses The photoluminescence spectra at room temperature for (Ba1-xSrx)MoO4 ceramics at different (x) concentrations heat treated at 1000 �C for 2 h are shown in Fig. 7. The scheelite molybdate compounds are well known for exhibit very good emission luminescence at low and at room temperature [71–78]. The spectrum cover a wide band of range from 400 to 700 nm wavelength of the visible spectra, and the profile of the emission band is typically involvement of multi-phonons and multi-levels process [79]. These levels are related to the numerous kind of defects directly related to degree of structural order– disorder in the system. Several theories and explanation related to promote the emission spectra in barium and strontium molybdate based materials [80–82]. A possible explanation to shift of PL emission spectra can be related to modification on morphology/shape of the grains, John– Teller splitting effect on the [MoO4] tetrahedron, structural defects, charge transfer phenomena between [MoO4]o– [MoO4]d complex clusters and surface defect at medium range [10, 83]. According to Lei et al. [32] the BaMoO4 crystals have a maximum PL emission at green region at 530 emission wavelength. These authors attributed that the particle size, crystalline degree, morphology, and surface defect are important factor to improvement of PL emission. Nogueira et al. [47], have observed that BaMoO4 crystals have a maximum PL emission at green region at 481 emission exited with a same wavelength and explained that the effects of structural order–disorder before and after arrival of the photon that could contribute significantly to PL emissions. Recently, Jena et al. [84] has not observed broadband issue for BaMoO4 crystals an attributed this behavior to large size of cation ionic radius, the greater is the parabola offset value is the probability of non-radiative electron relaxation from the exited state. We believe, that this behavior can be related to vibrational and vibronic relaxation related redistribution of energy due to high energy used in the excitation process (k = 256 nm). The PL emission spectra of (Ba1-xSrx)MoO4 ceramics with (x = 0) is centered at 411 nm wavelength (Fig. 7). We attributed that our pure BaMoO4 ceramics have a particular PL emission in blue region due to presence of electronic levels characteristic of barium as 5d orbitals empty in the conduction band. The replacement of Ba2? ions by Sr2? ions in (Ba1-xSrx)MoO4 ceramics with (x = 0.1, 0.2, 0.3, and 0.5) promotes a great shift of maximum PL emission to green region at (520, 523, 516, and 516 nm), respectively. However, we have noted only that (Ba1-xSrx)MoO4 ceramics with (x = 0,4) exhibits the highest emission intensity of all ceramics and occurs a shift to the blue region. We explained this behavior due to shape of grains and simultaneous presence of polyhedrons and minor grains at higher grains at interface, which will be discussed on. Earlier it has been reported by Wei et al. [85] the blue PL spectra for Ba0.5Sr0.5MoO4 powders is related to might be the John–Teller effect degenerate the excited states of essentially [MoO4] complex anion clusters of the slightly distorted tetragonal symmetry in BaMoO4. With increase in Sr2? ion concentration in (Ba1-xSrx)MoO4 ceramics enhanced charge-transfer mechanism in [MoO4] clusters which shift the emission spectra towards green region. These green PL emission spectra were related to asym- metric distorted tetrahedral [MoO4] clusters, distorted deltahedral [BaO8] and distorted [SrO8] clusters. These distorted clusters favor the population of intermediate levels within the band gap and stimulate the greenish emission. The small peak in red region in certain concen- tration may be due to some defect present in the bulk ceramics materials which leads to certain degree of disor- der in the lattice. This type of disorder is common in molybdate [MoO4] clusters intercalated to Ba/Sr atoms is responsible for small peak in red region. Finally, we have observed that the (Ba1-xSrx)MoO4 ceramics with (x B 1) presents a tendency to a decrease in intensity of PL emis- sion (Fig. 7). 3.8 SEM image analyses Figure 8a–f illustrates the microstructural images of some composition for (Ba1-xSrx)MoO4 ceramics (x = 0.0, 0.4, Fig. 7 PL emission spectra at room temperature for (Ba1-xSrx)MoO4 ceramics with different x concentrations 8328 J Mater Sci: Mater Electron (2015) 26:8319–8335 123 0.6, 0.7, 0.8, and 1.0). The images indicated the presences of octahedral like are shown in Fig. 8a–f. The particular shape is the nature characteristic mor- phology for scheelite types molybdate systems had been reported recently in many literatures [86, 87]. At interme- diate concentration x B 0.6 of Sr2? ion the number of octahedral grains and its sizes were increased, which indi- cated the inter diffusion start between the cations (Ba2?/ Sr2?) and [MoO4] 2- anions clusters leads to aggregates the grains and evolution of larger grains. Further increase of Sr2? ion concentration in the matrix (above x = 0.7) retards the growth process and formation of irregular grains. In Fig. 8e, f for (Ba1-xSrx)MoO4 ceramics with (x = 0.8 and 1.0) appearance of smaller grains with irregular shapes along with octahedral grain size indicated that unlike BaMoO4 microstructure strontium doped BaMoO4 had higher self assembly growth process between grains because of the higher surface energy. The EDX spectra for some composition (x = 0.0, 0.6 and 1.0) to prove the chemical compositional are presented in the Fig. 8 (inset). Fig. 8 SEM images for some concentrations with: a x = 0, b x = 0.4, c x = 0.6, d x = 0.7, e x = 0.8, and f x = 1. EDX spectra (inset) of BaMoO4, Ba0.4Sr0.6MoO4 and SrMoO4 compositions J Mater Sci: Mater Electron (2015) 26:8319–8335 8329 123 3.9 Dielectric and ferroelectric properties analyses Figure 9 shows the frequency dependence dielectric con- stant e0 (main) and dielectric loss tan d (inset Fig. 9) at room temperature for some composition for (Ba1-xSrx) MoO4 ceramics (x = 0.0, 0.2, 0.4, 0.8, and 1.0), both fol- lows inverse dependence of frequency, normally followed by all ferroelectric materials. Room temperature ferroelectric hysteresis loop analysis are recorded on sintered (Ba1-xSrx)MoO4 ceramics to understand the nature of dielectric strength, switching polarization, coercive field and their correlation with order– disorder in the lattice is shown in Fig. 9. The dielectric constant e’ of materials have four different polarization contributions: space charge polarization, dipolar polariza- tion, ionic polarization and electronic polarization [88]. At lower frequency (1 Hz–1 kHz) and at intermediate fre- quency (1–100 kHz) ranges space charge polarization and dipolar polarization are dominated, respectively. Response of ionic and electronic polarizations is more than 100 kHz and more than 1 GHz respectively. It is observed that with increasing frequency the dielectric loss decreases sharply in frequency range (1 Hz–1 kHz) and after 10 kHz it is almost constant. This behavior mostly depends on space charge and dipole polarization effect in the systems. Earlier it was explained that in these materials the formation of defects and structural distortion in the lattice promotes by electronic density difference between the deltahedral [BaO8]/[SrO8] clusters, which cause a dielectric loss. In pure BaMoO4 ceramics the defects and distortion are less compare to Sr2? ions modified (Ba1-xSrx)MoO4 ceramics. Therefore, the Sr2? ions enhanced the structural disorder among different cations and anions clusters. These cations and anions in presence of oxygen vacancies transfer charge between clusters and slightly enhanced the conductivity in the scheelite materials. The most common feature in ferroic-type of materials is the presence of the hysteresis loop due to spontaneous switching of domains with respect to applied electric field. The bulk symmetry of the materials plays an important role to determine the shape and size of the hysteresis loop. Typically the hysteresis loop through which the charac- teristic parameters such as saturation polarization (Ps), remnant polarization (Pr), coercive field (Ec) can be determined. In reality the shape of the hysteresis loop depends on number of factors such as thickness of the sample, material composition, presence of disorder in the sample, thermal treatment, grain size, mechanical stress and so on [89, 90]. Their effects on the material properties could well be reflected through the hysteresis loop. If the direction of spontaneous polarization in the material is random or distributed in such a way that the net micro- scopic polarization is zero such materials will not exhibit any ferroelectric hysteresis effect, which requires at least non-centrosymmetric (except cubic) symmetry of the material. This concept highlighted the existence of non- zero polarization or ferroelectric hysteresis in this scheel- ite-type tetragonal structure. The Fig. 10a–d illustrate the macroscopic polarization (P) state for (Ba1-xSrx)MoO4 ceramics (x = 0.0, 0.2, 0.4, 0.8, and 1.0) induced by applying electric field (E = 5, 15, 25 and 35 kV) at room temperature which is increased gradually by increasing the electric field strength. In these particular material the P–E hysteresis loops is more complex as conductivity is coexisting with normal ferroelectric which normally deteriorates the ferroelectric- ity if the conductivity is large [91]. According to the Fig. 10a, b at intermediate concentration (x B 0.6) the conductivity are dominated effect which transform hys- teresis loop into round type of loop and so sign of satura- tion polarization are observed in these materials. The presence of conductivity in hysteresis loop through another way that is a large gap between the starting and ending points of the applied electric field. Large discrepancy between starting and ending electric field is so called ‘‘gap’’ is high at intermediate concentration might be due to higher disorder at cation and anion sites and the existence of free charge carrier in the matrix. In Fig. 10c, d can be observed a slim hysteresis loop to high concentration of Sr2? ions in (Ba1-xSrx)MoO4 ceramics and for pure SrMoO4 ceramics, which indicates a increase in lattice symmetry of bulk material. As mention in the earlier sec- tion that shrinkage of unit cell volume with strontium ion and modified the c/a ratio in the lattice, hence reduces the tetragonal symmetry. In addition the grain size had an Fig. 9 Frequency dependence of dielectric constant (e0) and loss factor (tan d, inset) of (Ba1-xSrx)MoO4 ceramics for some concen- trations with: a x = 0, b x = 0.4, c x = 0.6, d x = 0.8, and e x = 1 8330 J Mater Sci: Mater Electron (2015) 26:8319–8335 123 effect on the shape of hysteresis loop. When grain size is large the area of the loop is large as compare to the smaller grain. The competitive interaction between reduce lattice symmetry and grain sizes had a major cause for narrow hysteresis loop in higher concentration of strontium ion. The microwave dielectric constant for (Ba1-xSrx)MoO4 ceramics were measured by Hakki–Coleman method [92] at a respective frequency range vary in between 9 and 12 GHz. Permittivity (er), temperature coefficient resonant frequency (sf), quality factor (Q 9 f) and loss tangent (tan d) values of the sintered pellets were measured using TE01d resonance mode of dielectric resonator. The values of selected concentration are listed in Table 3. In this table, we can note that with increase in the Sr2? ion the values of the dielectric constant are decreases. According to the Shannon [93] additive rule, the substitu- tion of lower polarizability a(Sr2?) = 4.24 Å3 ion in place of higher polarizability a(Ba2?) = 6.40 Å3, the dielectric constant should decrease which is observed in our present study. As already mention Hakki–Coleman TE01d reso- nance mode method has been found for suitable for real dielectric constant is given by Eq. (8): er ¼ 1þ c pDf1 � � a21 þ b21 � � ð8Þ where, c is the velocity of the light, a1 is given by chart mode and b1 is obtained from resonance frequency (f1) and the simple dimension. The temperature coefficient of dielectric resonator were measured using temperature controlled hot plate in the temperature range of 25–75 �C using the following Eq. (9): se ¼ 1 f � � Df DT � � ð9Þ where, (Df/DT) is the resonant frequency change with respect to temperature. The dielectric constant is also Fig. 10 P–E hysteresis loop of (Ba1-xSrx)MoO4 ceramics for some concentrations with: a x = 0, b x = 0.4, c x = 0.6, d x = 0.8, and e x = 1 J Mater Sci: Mater Electron (2015) 26:8319–8335 8331 123 depends on the molar volume, secondary phases, density and ionic polarizability [94] of the compounds. Structural and microstructural analyses have no evidence of any secondary phases in these materials. Bulk density and porosity correction were applied to calculate the exact values of dielectric constant as shown in Eq. (10): er ¼ eobs 1þ 1:5Pð Þ; and P ¼ 1� q qth � � ð10Þ where, P is the porosity of the materials. Slightly random variation in quality factor (Q 9 f) [95] and temperature coefficient of resonance frequency (sf) with x content in the compounds. This variation is most probably due to porosity in the samples and the above porosity correction is not applied to the values of the quality factor and sf. Generally the values of sf are depends on coefficient of thermal expansion (a1) and temperature coefficient of dielectric constant (se) of the sample as given by following Eq. (11) below: sf ¼ �a1 � 1 2 se ð11Þ The se has strong effect with lattice energy and unit cell volume of the materials. The combined effect of all the above mention parameters are determined the electrical properties of the ceramic materials over a wide microwave frequency range which is very much similar to the earlier reported values of dielectric constant in other wolframite and/or scheelite-types of structure [96–98]. 4 Conclusion In summary, we have obtained with success monophasic (Ba1-xSrx)MoO4 ceramics with different (x) compositions at 1000 �C by the solid state reactions method. XRD pat- terns and Micro-Raman spectra indicate that all (Ba1-x Srx)MoO4 ceramics are ordered at long and short range with a scheelite-type tetragonal structure. Rietveld refine- ment data show that all ceramics obtained which form a solid solution were perfect and occurred with a decrease in lattice parameters and unit-cell volume following the increase of (x) in the lattice. Moreover, these data were employed to model [BaO8], [SrO8] and [MoO4] clusters by using lattice parameters and atomic positions. Raman-ac- tive modes reveal typical symmetric stretching and bending vibrations of tetrahedral [MoO4] clusters and deltahedral [BaO8]/[SrO8] clusters. UV–Vis diffuse reflectance spectra indicate that the substitution of Ba2? by Sr2? ions promotes a decrease in optical band gap values due to the appearance of intermediary energy levels within the band gap. PL spectra presented a broad-band profile typical of a system in which relaxation occurs by several paths involving the participation of numerous states within the band gap of the material. We suggested that these states are related to the defects associated to symmetry breaks between the delta- hedral [BaO8]/[SrO8] and [MoO4] clusters and surface defects at medium range. The interplay between these clusters and defects generates a specific PL emission color. SEM images indicated that the increase of the Sr2? ions promotes leads a decrease in average grain size and pro- motes a change to near-spherical grains. Moreover, the appearance of poly-disperse grains lead to both size and shape due to inhomogeneous grain growth. Evolution of P–E hysteresis loop at room temperature highlighted the ferroelectric behavior in these materials. The shape and sizes of the loop are correlated with change in lattice parameters and c/a ratio in the system. Slim hysteresis loop at higher strontium concentration indicated the decrease in tetragonal symmetry. In particular at intermediate con- centration hysteresis loop is more complex as conductivity is coexisting with normal ferroelectric. Dipole interaction is dominating effect at intermediate frequency range which controls the value of dielectric constant and dielectric loss in these materials. The microwave dielectric properties such as dielectric constant, dielectric loss and quality factor and temperature coefficient of dielectric constant are cor- related with change in molar volume, lattice constant and contraction of unit cell volume in these materials. Acknowledgments Indian authors gratefully acknowledge the finan- cial support from DST Fast Track project (F. No. SB/FT/CS-044/2013) Govt. of India. The Brazilian authors acknowledge the financial support of agencies: CNPq (304531/2013-8) and FAPESP (2012/14004-5). Table 3 Dielectric constant (er), molar volume (Vm), quality factor (Q 9 f), temperature coefficient of resonance frequency (sf) and dielectric loss (tan d) for selected x concentration of (Ba1-xSrx)MoO4 ceramics materials (Ba1-xSrx)MoO4 er Vm (Å3) sf (ppm/�C) Q 9 f (GHz) tan d (910-3) x = 0.1 11.544 97.72 -37.45 2819.69 3.59 x = 0.2 11.785 95.98 -24.78 3212.81 3.18 x = 0.5 11.511 93.16 -24.67 2259.04 4.56 x = 0.7 11.280 91.03 -24.82 3218.59 3.16 x = 1 11.263 87.77 -18.60 2633.37 3.87 8332 J Mater Sci: Mater Electron (2015) 26:8319–8335 123 References 1. G. Davidson, Spectroscopic Properties of Inorganic and Organometallic Compounds (Royal Society of Chemistry, Great Britain, 1998), pp. 1–303 2. L.S. Cavalcante, F.M.C. Batista, M.A.P. Almeida, A.C. Rabelo, I.C. Nogueira, N.C. Batista, J.A. Varela, M.R.M.C. Santos, E. Longo, M. 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Mater. 47, 313–316 (2011) J Mater Sci: Mater Electron (2015) 26:8319–8335 8335 123 Structural refinement, Raman spectroscopy, optical and electrical properties of (Ba1minusxSrx)MoO4 ceramics Abstract Introduction Experimental procedure Synthesis of (Ba1minusxSrx)MoO4 ceramics Characterizations Results and discussion XRD analyses Rietveld refinements analyses Lattice parameters and unit-cell volume analyses Representation of the BaMoO4, (Ba0.5Sr0.5)MoO4 and SrMoO4 unit cells Micro-Raman spectroscopy analyses UV--Vis absorption spectroscopy and optical band gap values analyses PL emission analyses SEM image analyses Dielectric and ferroelectric properties analyses Conclusion Acknowledgments References