lable at ScienceDirect Journal of Alloys and Compounds 695 (2017) 3094e3103 Contents lists avai Journal of Alloys and Compounds journal homepage: http: / /www.elsevier .com/locate/ ja lcom Photoluminescent properties of ZrO2: Tm3þ, Tb3þ, Eu3þ powdersdA combined experimental and theoretical study L.X. Lovisa a, *, J. Andr�es b, L. Gracia b, M.S. Li c, C.A. Paskocimas a, M.R.D. Bomio a, V.D. Araujo d, E. Longo e, F.V. Motta a a DEMAT, CT, UFRN, Av. Sen. Salgado Filho, 3000, CEP 59072-970 Natal, RN, Brazil b Departament de Química Física i Analítica, Universitat Jaume I, Campus del Riu Sec, Castell�o E-12071, Spain c IFSC, USP, Av. Trabalhador S~ao Carlense, 400, CEP 13566-590 S~ao Carlos, SP, Brazil d UFRPE, Av. Rua Dom Manoel de Medeiros, CEP52171-900 Recife, PE, Brazil e LIEC, IQ, UNESP, Rua Francisco Degni s/n, CEP 14801-907 Araraquara, SP, Brazil a r t i c l e i n f o Article history: Received 12 May 2016 Received in revised form 14 November 2016 Accepted 23 November 2016 Available online 25 November 2016 Keywords: ZrO2:RE Photoluminescence DFT calculations White LEDs * Corresponding author. E-mail address: lovisaengmat@ig.com.br (L.X. Lovi http://dx.doi.org/10.1016/j.jallcom.2016.11.341 0925-8388/© 2016 Elsevier B.V. All rights reserved. a b s t r a c t Rare-earth (RE) element-based materials for optical applications have received increasing attention owing to the emission properties of RE ions, which render these materials suitable for use in color displays, lasers, and solid-state lighting. In the present work, ZrO2:RE (RE ¼ Tm3þ, Tb3þ, and Eu3þ) powders were obtained via complex polymerization, and characterized by means of X-ray diffraction (XRD), Raman spectroscopy, UVevisible absorption spectroscopy, and photoluminescence measure- ments. The XRD patterns and Raman spectra revealed the tetragonal phase of ZrO2 co-doped with up to 4 mol.% RE3þ and stabilization of the cubic phase, for up to 8 mol.% RE3þ. In addition, the photo- luminescence measurements revealed simultaneous emissions in the blue (477 nm), green (496.02 nm and 548.32 nm), and red-orange (597.16 nm and 617.54 nm) regions. These emissions result from the Tm3þ, Tb 3þ, and Eu3þ ions, respectively. Energy transfers, such as 1G4 levels (Tm3þ) / 5D4 (Tb3þ) and 5D4 levels (Tb3þ) / 5D0 (Eu3þ), occurred during the emission process. Calculations based on density functional theory (DFT) were performed, to complement the experimental data. The results revealed that structural order/disorder effects were generated in the cubic and tetragonal ZrO2 phases in the ZrO2:Eu3þ powders, and changes in the electronic structure were manifested as a decrease in the band gap values. The chromaticity coordinates of all the samples were determined from the PL spectrum. The coordinates, x¼ 0.34 and y ¼ 0.34, of the ZrO2:8%RE sample corresponded to a point located in the white region of the CIE diagram and color correlated temperature (CCT) was found to be 5181 K. More importantly, the present results indicate that ZrO2:RE powders constitute promising photoluminescent materials for use in new lighting devices. © 2016 Elsevier B.V. All rights reserved. 1. Introduction The synthesis and characterization of rare earth (RE)-doped nanomaterials have been extensively investigated. These materials have excellent properties [1,2], such as narrow emission band- widths (<10 nm), long luminescence lifetime, high photostability, and low toxicity [3,4]. In recent years, these materials have received considerable attention owing to their use in several important applications, such as multiplexed imaging and sensing, bioassays, sa). and multiplex biodetection [5e14]. The development of lumines- cent materials, which are more efficient than those currently available, is extremely challenging. Moreover, the use of a suitable host material for the RE ions is essential for achieving high effi- ciency [15]. Zirconium dioxide or zirconia (ZrO2) and ZrO2-based materials constitute an appropriate host for RE, owing to a unique combi- nation of different properties. These include: high refractive index, large optical band gap, low optical loss, and high transparency in the visible and near-infrared regions, good chemical stability [16,17], and lower phonon frequency (~470 cm�1) than other matrices such as Y2O3 (~597 cm�1) and TiO2 (~700 cm�1) [18]. As such, the luminescence efficiency of active ions may be improved mailto:lovisaengmat@ig.com.br http://crossmark.crossref.org/dialog/?doi=10.1016/j.jallcom.2016.11.341&domain=pdf www.sciencedirect.com/science/journal/09258388 http://www.elsevier.com/locate/jalcom http://dx.doi.org/10.1016/j.jallcom.2016.11.341 http://dx.doi.org/10.1016/j.jallcom.2016.11.341 http://dx.doi.org/10.1016/j.jallcom.2016.11.341 L.X. Lovisa et al. / Journal of Alloys and Compounds 695 (2017) 3094e3103 3095 via incorporation into the ZrO2 matrix [19]. RE ions can enhance the emission of photoluminescent mate- rials, and the corresponding energy transfer (ET) process occurs between an ion donor (D) and an ion acceptor (A). During the ET process, the energy of D (which is in an excited electronic state) is transferred to A. Specific conditions must be fulfilled in order to realize this mechanism. These include: (i) the emission band of D is partially superimposed on the absorption band of A, and (ii) the distance (R) between D and A must be sufficiently short, since the energy transfer efficiency is proportional to 1/R6, to enable inter- action of the dipoleedipole emission bands of the material [20]. These unique properties have led to the widespread use of RE ions in optical devices. Direct excitation of Eu3þ ions is a relatively inefficient process, owing to the forbidden nature of the 4f transitions. However, Eu- doped inorganic materials may exhibit efficient luminescence emissions upon ultraviolet excitation. Thesematerials also exhibit a large Stokes shift, sharp emission spectrum, and have a long life- time, high chemical/photochemical stability, low toxicity, and reduced photobleaching, owing to shielding of the 4f electrons [21]. As such, Quan et al. [22] obtained spherical ZrO2:Eu3þ particles by using a spray drying process followed by a post annealing treat- ment. Gedanken et al. [23] used a sonochemical method for the europium-oxide doping of ZrO2 nanoparticles. Furthermore, Tiwari et al. [24] determined the effect of varying Eu3þ concentration on the photo- and thermoluminescence of ZrO2 nanophosphors. Some RE ion-doped ZrO2 materials, such as ZrO2:Eu [25], ZrO2:Tb [26], and ZrO2:Tm [27], have interesting properties. Vidya et al. [28] investigated the color-tunable photoluminescence photocatalytic activities and phase transformation of a ZrO2:Tb3þ nanophosphor. Mari et al. [29] determined the photoluminescence properties of Tb3þ in ZrO2 zirconia host matrices (prepared via combustion synthesis) at different calcination temperatures. Shang et al. [30] examined the process of energy transfer be- tween Tm3þ and Ho3þ excited by a UV nanocrystal-LaOF laser and classified the interaction between the ions as a quadrupole- quadrupole type of interaction. Joshi [31] examined the energy transfer from Tb3þ and Eu3þ in zinc phosphate glasses and concluded that these ions undergo mainly dipole-dipole in- teractions. Moreover, the efficiency of this transfer was highest at a concentration of 8.6 mol% Eu3þ. The emissions of Tm3þ, Eu3þ, and Tb3 þ ions fall within the blue, red, and green regions, respectively, of the visible spectrum [32e34]. Particles that have tunable emis- sion colors are obtained from a combination of lanthanide ions in a host material. The first-ever generation of white light from the simultaneous emission of blue, green, and red, under UV excitation, was obtained for borate-based glasses co-doped with Ce3þ, Tb3þ, and Mn3þ [35]. Furthermore, the spectra of Tm3þ, Tb3þ, and Sm3þ co-doped silicate glass, which was excited in the near-UV region, exhibited bands corresponding to blue, green, and orange-red emissions [36]. This paper can be considered a prolongation of previously work (CITA) in which the main focus is the investigation of the photo- luminescent properties of the particles of ZrO2 co-doped with Tm3þ, Tb3þ, and Eu3þ, by using the polymerization method [37e40]. The use of the complex polymerization method in the research materials is widespread because it presents advantages such as good homogeneous distribution of different metal ions along the polymer formed, facilitating control stoichiometric. Other positive aspects of the method are the low temperature synthesis, obtaining nanometric particles and reproducibility. At this time, we will examine the effect of the concentration of RE (mol%) in the discussed property. In addition, first-principle calculations were performed in order to explain the structural and electronic changes induced by the doping of ZrO2:Eu. The energy transfer processes between Tm3þ and Tb3þ and between Tb3þ and Eu3þ, were also discussed. X-ray diffraction (XRD), Raman spectroscopy, UVevisible reflectance spectroscopy, and photoluminescence (PL) measure- ments were used to characterize the samples. Moreover, a chro- maticity diagram was determined from emission spectra data, in order to verify the efficiency of these materials during the emission of white light. The remainder of this paper is organized as follows: Section 2 describes the synthesis method, characterization techniques, and computational details; Sections 3 and 4 present the results and conclusions, respectively. 2. Experimental section 2.1. Synthesis of ZrO2:RE powders The samples were prepared by using a complex polymerization method. During the synthesis, the zirconium citrate was obtained by dissolving zirconium nitrate (Vetec, 99%) in an aqueous citric acid solution, under agitation, at a temperature of ~80 �C. Doping was performed by adding cations of RE to the solution. A europium solution and a thulium solution were prepared by dissolving Eu2O3 (Aldrich, 99.9%) and Tm2O3 (Aldrich, 99.9%), respectively, in nitric acid. Each solution was then separately mixed with the zirconium citrate solution. Terbium nitrate (Aldrich, 99.9%) was subsequently added to the mixed solution. Furthermore, ethylene glycol was added to the solution, under constant stirring, in order to promote polymerization of the citrate, through the polyesterification reac- tion. The molar ratio between citric acid and ethylene glycol used was set to 60/40 (mass ratio). After 4 h, water was completely removed, thereby yielding a translucent resin. Various (1, 2, 4 and 8 mol % of RE) dopant concentrations were considered. This per- centage of RE is on the contribution of all dopants, such as: ZrO2: 1% RE, ZrO2: 2%RE, ZrO2: 4%RE and ZrO2: 8%RE correspond respectively to Zr 0.99O2: 0.0033 Tb 0.0033 Tm 0.0034 Eu, Zr 0.98O2: 0.0066 Tb 0.0066 Tm 0.0068 Eu, Zr 0.96O2: 0.0133 Tb 0.0133 Tm 0.0134 Eu and Zr 0.92O2: 0.0266 Tb 0.0266 Tm 0.0268 Eu. The polymeric resin was heat-treated at 350 �C (10 �C/min) for 4 h, leading to partial decomposition of the poly- meric gel; this resulted in the formation of an expanded resin, which consisted of partially pyrolyzed material. The resulting powders were annealed at 600 �C for 2 h at a heating rate of 10 �C/ min. 2.2. Characterization of ZrO2:RE (RE ¼ Tm3þ, Tb3þ, and Eu3þ) powders The as-synthetized powders were examined by XRD (Shimadzu diffractometer model XRDe7000), using Cu-Ka radiation. In addi- tion, Raman spectrometry (Horiba Jobin-Yvon Raman Labram) was performed at room temperature; an Olympus BX41 TMmicroscope equipped with a 514 nm-wavelength laser, was used as the exci- tation source. UVevis reflectance spectra (Cary model 5G) and PL spectra (Thermal Jarrel-AshMonospec 27 monochromator and Hamatsu R446 photomultiplier) of the ZrO2:RE particles were also obtained. A 350.7 nm-wavelength laser with krypton ions (Coher- entInnova) and an output of ~13.3 mW, was used as the excitation source during the PL measurements; these measurements were all performed at room temperature. To characterize white light resulting from the aforementioned mixing, we calculated the chromaticity coordinates using the spectrum represented in Fig. 3. The chromaticity coordinates of red (the x coordinate), green (the y coordinate) and blue (the z coordinate) were determined according to the system of the International Commission on Illumination given in 1968 [41,42] using the following relationships: Fig. 1. XRD patterns of (a) non-doped ZrO2 and ZrO2:xRE (b) x ¼ 1%, (c) x ¼ 2%, (d) x ¼ 4%, and (e) x ¼ 8%, calcined at 600 �C. L.X. Lovisa et al. / Journal of Alloys and Compounds 695 (2017) 3094e31033096 x ¼ ðXÞ ðX þ Y þ ZÞ y ¼ ðYÞ ðX þ Y þ ZÞ z ¼ ðZÞ ðX þ Y þ ZÞ (1) where parameters X, Y and Z are the following spectral integrals: X ¼ Z xPðlÞdl Y ¼ Z yPðlÞdl Z ¼ Z zPðlÞdl (2) Here P(l) is luminescence spectrum of the samples, that provide, for each within the visible range, the emitted intensity. The func- tion P(l) is determined empirically, the values of l for components x, y and z are 599, 555 and 446 nm, respectively and x, y and z are functions of spectral summarizing. Integrals (2) were calculated through the spectral interval of 350e800 nm. The CCT value was estimated by using McCamy empirical formula [43]. The quality of white light is calculated using McCamy empirical formula in terms of CCT values, which is expressed as: CCT ¼ �449n3 þ 3525n2 � 6823nþ 5520:33 (3) where n ¼ ðx�xeÞ ðy�yeÞ is the inverse slope line, xe ¼ 0.332 and ye ¼ 0.186. 2.3. Computational details First-principle calculations, based on the density functional theory (DFT), were performed by using the Vienna ab initio simu- lation package (VASP). The Kohn-Sham equations were solved by using the Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional [44], and the electron-ion interaction was described via the projector-augmented-wave pseudo potentials. Moreover, the plane-wave expansion was truncated at a cut-off energy of 520 eV, and the Brillouin zones were sampled by using Monkhorst-Pack special k-point grids. Cubic and tetragonal phases of ZrO2, both undoped and doped at 12% of Eu substituted for Zr, were consid- ered. In addition, a 12% of Tb and Tm substitutions were tested. The valence electron density is defined by 12 (4s24p65s24d2) electrons for Zr atoms, 6 (2s22p4) electrons for O atoms and 17 (5s25p66s24f7) electrons for Eu atoms. For Tb and Tm atoms, three f-like electrons are treated as core states and 9 electrons are used as valence states for both. A supercell with 48 atoms was used for both systems, 2 � 2 � 1 and 2 � 2 � 2 for cubic and tetragonal phases, respectively. In the case of 12% doping, two Zr4þ were substituted by two Eu3þ and oxygen vacancy (both near and far from Eu atoms) was included, to maintain the electroneutrality of the cell. In order to obtain a small amount of Eu doping, extremely large supercells must be used, thereby resulting in a high computational cost. The cell parameters and positions of all atoms were allowed to relax, and the conju- gated gradient energy minimization method was used to obtain relaxed systems. This was achieved by setting a threshold value (i.e., 0.01 eV$Å�1) for the forces experienced by each atom. To ensure geometrical and energetic convergence of the cubic and tetragonal ZrO2 structures, a 3 � 3 � 1 Monkhorst-Pack special k- point grid was used. 3. Results and discussion 3.1. XRD characterization XRD patterns of pure ZrO2 and co-doped ZrO2:xRE (x ¼ 1, 2, and 4%) powders are shown in Fig. 1(a)e(d). Diffraction peaks are located at approximate angles of: 30.07�, 35.02�, 50.28�, 59.87�, 62.60�, and 73.66� corresponding to the (101), (110), (112), (201), (103), and (202) planes, respectively, of the ZrO2 tetragonal phase [ICSD 81-1546]. Fig. 1(e) shows the pattern corresponding to ZrO2:8%RE. In this case, peaks occur at ~29.79�, 34.57�, 50.02�, 59.56�, 62.27�, and 73.35�, corresponding to the (111), (200), (220), (311), (222), and (400) planes, respectively, of the cubic phase of ZrO2 [ICSD 81-1551]. This phase is stabilized with increasing amount of bi- or trivalent cations introduced into the ZrO2 structure [45,46]. In addition, the replacement of Zr4þ cations by RE3þ results in the formation of oxygen vacancies. This leads, in turn, to a change in the lattice parameters of the unit cell (c/a / 1) and conse- quently, arrangement of the ions in a cubic structure [46]. The ionic radius of oxygen is large, it becomes difficult to maintain the four O2� ions around an ion Zr4þ with a fluorite structure (cubic), due to large repulsion between the ions O2�. With the introduction of dopant RE3þ replacing Zr4þ, there is the appearance of oxygen va- cancies in order to offset the charges and as result the force of repulsion between the O2� decreases, giving conditions to accom- modate the ions in the cubic structure. The size of the crystallites in the sample was estimated from the Scherrer equation [47,48] and the full-width half-maximum (FWHM) of an observed peak. The average crystallite size (D) of ZrO2:RE powders was determined from the strongest peaks cor- responding to the (101) tetragonal phase and (111) cubic phase. The lattice parameter (a, c), unit-cell volume (V), and crystallite size of the ZrO2:RE samples are listed in Table 1. 3.2. Raman characterization ZrO2 polymorphism may lead to inaccurate results when the crystalline phases of ZrO2 are identified only via XRD. Das et al. [45] attributed inaccuracies in XRD identification of tetragonal and cu- bic phases, to the low angular resolution (0.03�) of the equipment used; this resolution resulted in an overlap of the peaks Table 1 Value of structural parameters of ZrO2:xRE (0e8 mol%) nanophosphors. Parameters ZrO2 ZrO2:1%RE ZrO2:2%RE ZrO2:4%RE ZrO2:8%RE Crystal System Tetragonal Tetragonal Tetragonal Tetragonal Cubic Space group P42/nmc P42/nmc P42/nmc P42/nmc Fm-3m 2q 30.0266 29.9272 29.8368 30.1498 29.8368 FWHM (rad) 0.3149 0.7872 0.3149 0.9446 0.3936 Lattice parameters (Å) a 3.59756 3.61069 3.60521 3.62814 5.15833 c 5.19462 5.20999 5.19164 5.14336 e Unit cell volume/formula unit (Å3) 67.2312 67.9231 67.4786 67.7040 137.2545 Crystallite size (nm) 14.06 14.49 14.79 14.62 12.78 Fig. 3. UVevisible absorption spectra for particles: undoped ZrO2 and ZrO2:x% RE (x ¼ 1, 2, 4, and 8% mol). L.X. Lovisa et al. / Journal of Alloys and Compounds 695 (2017) 3094e3103 3097 corresponding to these phases. Compared to XRD, Raman spec- troscopy can more accurately distinguish among the crystalline phases of ZrO2 [49]. The band positions, intensities, and shapes can be determined from the Raman spectra. In fact, as shown in Fig. 2 and Table 2, each structure exhibits certain characteristics that correspond to specific locations in the spectra. The bands that occur at 142, 257, 314, 461, 609, and 627 cm�1 in the spectra shown in Fig. 2(a), (b), and (c) are attributed to the vi- bration modes of tetragonal ZrO2 [49e51]. The spectrum (d) of the cubic phase of ZrO2 (fluorite) is characterized by a broad band that occurs at ~605 cm�1 [49]. The active modes in the Raman spectra (corresponding to each crystal) and the c/a ratio of the lattice pa- rameters of tetragonal zirconia, are shown in Table 2. Theoretical calculations of the Raman-active modes of pure tetragonal ZrO2 yield values of 149.4, 294.2, 301.5, 453.6, 611.5, and 650.9 cm�1 for the Eg, A1g, B1g, Eg, B1g, and Eg modes, respectively. In the case of pure cubic ZrO2, a unique mode, which has T2g symmetry, occurs at a wavenumber of 600.7 cm�1. These values concur with previously obtained experimental data. Table 2 Active Raman modes, space group, and c/a lattice parameter ratio for the zirconia polymorphs. Crystal system Space group Active raman modes c/a Tetragonal D4h A1g þ 2B1g þ 3Eg >1 Cubic Oh T2g ¼1 3.3. UVevisible spectroscopy analysis The band gap energies of the ZrO2:RE nanoparticles were esti- mated from the respective diffuse-reflectance spectra, by plotting the square of the KubelkaeMunk function (i.e., F(R)2) as a function of the energy (in eV). The values were determined by extrapolating the linear part of the curve to F(R)2 ¼ 0, as shown in Fig. 3. The ratio between the molar absorption coefficient (k) and scattering coef- ficient (s) is estimated from reflectance data using the Kubel- kaeMunk relation [52] in equation (4): Fig. 2. Raman spectrum of ZrO2:xRE, (a) x ¼ 1%, (b) x ¼ 2%, (c) x ¼ 4%, and (d) x ¼ 8%. FðRÞ ¼ k s ¼ ð1� RÞ2 2R (4) where R is the percentage of reflected light. The incident photon energy (hn) and the optical band gap energy (Eg) are related to the transformed KubelkaeMunk function, [F(R) hn]p ¼ A (hn - Eg), where Eg is the band gap energy, A is a constant depending on the transition probability and p is the power index that is related to the optical absorption process. p equals to 1/2 or 2 for an indirect or a direct allowed transition, respectively. The Eg values are shown in Fig. 3. Intermediate levels of energy in the band gap region result from the structural defects in ZrO2 [53]. For example, oxygen vacancies, the type of structural defect that occurs in the present case, are generated in order to compensate for the Zr4þ ions replaced by RE3þ ions. Eg values of 5.21, 5.09, 5.06, 4.97, and 4.92 eV are obtained for the undoped, 1, 2, 4, and 8 mol.% RE-doped materials, respectively. In addition, the calculated values of the cell parameters concur with the experimentally determined results (a ¼ 5.127 Å for the cubic phase, and a ¼ 3.630 Å and c ¼ 5.264 Å for the tetragonal phase). The results of the theoretical calculations indicate that, in both the cubic and tetragonal structures, 12% of Eu produces a local distortion that is both centered on the dopant, and located near the oxygen vacancy (Vo). The geometry of doped ZrO2 with Euþ3 L.X. Lovisa et al. / Journal of Alloys and Compounds 695 (2017) 3094e31033098 showing the coordination polyhedra of the cubic and tetragonal phases is shown in Fig. 4. The vacancy corresponding to an O atom missing from the structure was examined by taking into account the proximity of the Eu atoms. For both phases, large distances between the oxygen vacancy and the Eu atoms, constitute more favorable arrangements than other configurations. Consequently, some Zr atoms are seven- coordinated in the case of the cubic phase and are also neighbored by an Eu atom in the case of the tetragonal phase (see Table 3). The difference between the formation energies of the cubic and tetragonal phases (DEC-T) of ZrO2 and Zr0.88Eu0.12O1.94 are 0.84 eV and �0.34 eV, respectively. These results indicate that the incor- poration of Eu into the ZrO2 structure increases the already higher stability of the cubic phase, relative to that of the tetragonal phase. This explains the preferential Eu doping of the cubic phase of the films. We used a supercell model in which the cubic and tetragonal phases are each assigned 48 atoms, to determine the effect of Eu incorporation into the ZrO2 lattice, on the electronic structure. The total and projected density of states (DOS) of the atoms and orbitals of the pure and doped cubic phase are shown in Fig. 5; the results corresponding to the pure and doped tetragonal phase are shown in Fig. 6. The top of the VB and the bottom of the conduction band (CB) are composed mainly of O 2p levels and Zr 4d levels, respectively. Furthermore, Eg values of 3.21 eV and 3.83 eV were calculated for the respective undoped cubic and undoped tetragonal phases. These values are both lower than their experimentally determined counterparts. However, compared to the former (3.21 eV), the latter (3.83 eV) is closer to Eg of the pure ZrO2 (5.21 eV), investigated in this work. A comparison of the electronic structures shown in Figs. 5(b) and 6(b) reveals that Eu doping leads to a systematic decrease in Eg and an increase in the density of electronic states inside the gap. Moreover, the effect of Tb and Tm incorporation into the ZrO2 lattice has been also explored, and the total and projected density of states on atoms for doped Tb and Tm, in cubic and tetragonal phases, are shown in Fig. S1 of Supplementary Information. Therefore, the theoretical calculations indicate that these states occur in the forbidden zone of energy, owing to the Fig. 4. Geometry of doped ZrO2 with Eu3þ showing the coordi presence of Eu, Tb and Tm transition metals and O vacancies in the ZrO2 lattice. 3.4. PL studies The optical properties of Eu dopants in various host materials, have been characterized [54e62]. Owing to the hypersensitivity of the 5D0 / 7F2 transition, Eu ions can be used to monitor morphological changes in the host material, which are induced by external stimuli [63e66]. Fig. 7 shows the PL spectra of both the undoped and co-doped ZrO2. The band in the emission spectrum of undoped ZrO2 ranges from 376 nm to 648 nmwith a peak centered at 460 nm, as shown in Fig. 7. This is attributed to the (O2�) p/ d (Zr4þ)-type transition [53], which results from a sequence of non-radiative relaxations of localized electrons in the CB; this is followed by band recombina- tionwithin the band gap, and subsequent decrease in energy of the electrons when they move to the VB [67]. Factors such as the par- ticle size and morphology, crystallinity, and the method of syn- thesis [68,69] may influence the photoluminescent properties of the zirconia. The PL spectra of ZrO2:RE (RE ¼ Tm3þ, Tb3þ, and Eu3þ) powders exhibit characteristics of each dopant-ion emission. For example, in the case of excitation at 350 nm, the emission peak at 477 nm is attributed to Tm3þ, which is associated with the 1G4 / 3H6 tran- sition [70,71]. The 5D4 / 7F6 and 5D4 / 7F5 transitions occur at wavelengths of 496.02 nm and 548.32 nm, respectively, and are associated with the emission of Tb3þ [72,73]. In addition, the Eu3þ, 5D0/ 7F1, and 5D0/ 7F2 transitions occur at respective wavelengths of 597.16 nm and 617.54 nm [74,75]. The interference of the host has a more significant effect on the photoluminescent behavior of ZrO2:8%RE than on the behavior of ZrO2. This effect is manifested as the occurrence of a broad band at wavelengths ranging from 380 nm to 480 nm, and results from the structural defects in ZrO2. The order-disorder effects in the coordination of some Zr and doped atoms, verified by theoretical calculations, can result in the pro- duction of new levels between the valence and the conduction bands, which favor the PL emission properties. As Fig. 7(c) shows, the emission intensity of the Eu3þ ions increases with increasing nation polyhedral: a) cubic phase and b) tetragonal phase. Table 3 Distances in A for the coordination polyhedra for doped ZrO2 with Eu3þ for the cubic and tetragonal phases. Cubic Tetragonal Symbol Ligands Faces Dmin Dmax Symbol Ligands Faces Dmin Dmax Eu1 8 12 2.281 2.471 Eu1 7 8 2.286 2.368 Eu2 8 10 2.308 2.459 Eu2 8 11 2.331 2.512 Zr1 8 12 2.071 2.611 Zr1 7 10 2.044 2.394 Zr2 8 12 2.061 2.568 Zr2 8 12 2.081 2.48 Zr3 7 9 2.079 2.184 Zr3 8 12 2.063 2.617 Zr4 8 10 2.091 2.265 Zr4 8 12 2.055 2.501 Zr5 8 12 2.066 2.487 Zr5 8 12 2.108 2.376 Zr6 7 10 2.054 2.218 Zr6 8 12 2.141 2.269 Zr7 8 12 2.141 2.347 Zr7 8 12 2.118 2.391 Zr8 8 12 2.11 2.326 Zr8 7 10 2.071 2.297 Zr9 7 9 2.006 2.303 Zr9 8 12 2.127 2.44 Zr10 7 10 2.041 2.267 Zr10 8 12 2.098 2.685 Zr11 8 12 2.131 2.268 Zr11 8 12 2.118 2.391 Zr12 7 10 2.089 2.206 Zr12 7 10 2.071 2.297 Zr13 7 9 2.058 2.219 Zr13 8 12 2.127 2.44 Zr14 7 10 2.069 2.261 Zr14 8 12 2.098 2.685 Fig. 5. Total and projected density of states on atoms and orbitals for the pure (a) and doped cubic phase (b). L.X. Lovisa et al. / Journal of Alloys and Compounds 695 (2017) 3094e3103 3099 concentration of Eu3þ, reaches a maximum at 4 mol.% RE, and de- creases thereafter (owing to the quenching effect) [76]. The critical quenching concentration of Eu3þ is defined as the concentration at which the emission intensity begins to decrease. Similarly, the critical distance, corresponding to the critical quenching concen- tration, is defined as the average distance between the nearest Eu3þ ions, at which energy transfer processes occur. 3.5. Energy transfer in ZrO2:RE (RE ¼ Tm3þ, Tb3þ, and Eu3þ) powders A schematic energy level diagram illustrating Tm3þ, Tb3þ, and Eu3þ absorption, non-radiative relaxation, and processes leading to blue, green, and red emissions is shown in Fig. 8. The energy level 5D4 of Tb3þ is very close to the energy level of Tm3þ 1G4 as seen in the energy diagram (Fig. 8). This setting energy levels contributes to efficient energy transfer process (ET1) between Tm3þ ions and Tb3þ [77]. The 1G4 level of Tm3þ is completely filled by the charge car- riers (electrons) from the excitation process (l ¼ 350 nm). The increased concentration of Tm3þ promotes the increase of the in- tensity of the transition 1G4 / 3H6, this increased intensity of Tm3þ also acts as a source for transporting energy for the sublevel 5D4 of Tb3þ, this effect is realized by increasing the intensity of transition 5D4 / 7F5 (550 nm) Tb3þ, shown in Fig. 7. The Energy transfer (ET2) between Tb3þ and Eu3þ has been extensively studied, in order to understand the photoluminescent behavior [78]. In fact, the luminescence intensities of various rare- earth ions can be enhanced or quenched by the energy transfer Fig. 6. Total and projected density of states on atoms and orbitals for the pure (a) and doped tetragonal phase (b). Fig. 7. Photoluminescence emission spectra of (a) ZrO2:xRE (x ¼ 1e8 mol%), (b) undoped ZrO2, (c) quenching effect in the transition 5D0 / 7F1 (Eu3þ) at 617.54 nm. L.X. Lovisa et al. / Journal of Alloys and Compounds 695 (2017) 3094e31033100 from other co-doped rare-earth ions [79e81]. ET2 between Tb3þ and Eu3þ may occur in hosts, such as tungstates, zeolite-Y, yttria, porous silicon, borate, hydrate, and molybdates [82e85]. The probability of this transfer is proportional to R�6 (R: average dis- tance between Tb3þ and Eu3þ), and hence the efficiency of the ET2 process increases gradually with increasing Eu3þ-doping concentration. Furthermore, R decreases with increasing Eu3þ concentration and therefore, the energy transfer efficiency of Tb3þ / Eu3þ increases. Owing to the quenching effect, this behavior is not unique to the ZrO2:8% RE, as shown in Fig. 7(c). An analysis of the results depicted in Fig. 8 renders that elec- trons on Tb3þ ions are promoted from the ground state (4f8) to the excited state (4f75d), by 350.7-nm UV light. These electrons then relax to the lowest excited state 5D4, by means of a multi-phonon Fig. 8. Schematic diagram of the Tm3þ, Tb3þ, and Eu3þ energy levels and the processes leading to blue, green, and red emission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 4 Chromaticity coordinates and correlated temperature color for ZrO2:xRE (x: 1, 2, 4 and 8 mol%). Sample x y CCT (K) Color ZrO2: 1% Eu 0.41 0.36 3.174 Yellow ZrO2: 2% Eu 0.50 0.37 1.988 Orange ZrO2: 4% Eu 0.43 0.38 2.917 Yellow ZrO2: 8% Eu 0.34 0.34 5.181 White L.X. Lovisa et al. / Journal of Alloys and Compounds 695 (2017) 3094e3103 3101 relaxation process. The electrons may return to the ground state, thereby resulting in Tb3þ emissions (5D4 / 7F6, 5, 4). Alternatively, their excitation energymay be transferred from the 5D4 (Tb3þ) level to the higher excited energy levels of Eu3þ (4f 6) through cross relaxation; these levels then relax to the 5D0 (Eu3þ) level, thereby resulting in red-orange emissions (5D0 / 7F0, 1, 2). The 5D4 / 7F6, 5, 4, 3 emissions of Tb3þ overlap with the 7F0, 1 / 5D0, 1, 2 absorptions of Eu3þ and hence, the energy transfer from Tb3þ to Eu3þ is, in general, very efficient. Fig. 9 shows the CIE coordinates of ZrO2:xRE (1e8 mol.%), while Table 4 lists the CIE coordinate values and CCT values for samples ZrO2:xRE (1e8 mol.%). We obtained white light emission from a single component, by co-doping the ZrO2 host with Tm3þ, Tb3þ, and Eu3þ ions. Under the excitation of UV light, a full-color emission is obtained, resulting from the simultaneous blue, green, and red Fig. 9. CIE chromaticity diagram for ZrO2:xRE (x ¼ 1, 2, 4, and 8 mol%). emission of the Tm3þ, Tb3þ, and Eu3þ ions. It is observed in Fig. 7, for the sample ZrO2: 8% RE, the photoluminescent behavior of ZrO2 host was very significant. The presence of oxygen vacancies (V0) in the matrix is responsible for the emergence of broadband emission at around 450 nm [86]. The oxygen vacancy always leads to for- mation of energy levels within the band gap. When ZrO2 is excited by a photon, the electrons are trapped by V0 and centers are created (F) [87]. Then recombination centers (F) with the holes (hþ) creates the transmitter excited states. From these states originate transi- tions which decay to a state with lower energy level. The band of blue emission from the ZrO2 contributes along with the specific emission of rare earth on white emission as shown in Fig. 8. A single-compositionwhite-emitting phosphor is therefore obtained. In fact, this white emission occurs independent of the excitation, depends on the doping concentration of the rare-earth ions, and is obtained by blending the aforementioned simultaneous emissions. The emissions are characterized by the colors emitted from each sample. This characteristic is defined by chromaticity coordinates x and y. 4. Conclusions ZrO2:RE powders were successfully obtained via complex polymerization. The phase (i.e., tetragonal) comprising the ZrO2:xRE (x: 1, 2, and 4 mol.%) samples was identified via XRD analysis, whereas the cubic phase, stabilized in ZrO2:8%RE, was identified via Raman spectroscopy. The structural and electronic effects, resulting from Eu in both the cubic and tetragonal ZrO2:Eu3þ phases, were explained by calculating (using DFT) the relevant energies. The photoluminescence emission spectra reveal transitions of the type: 1G4 / 3H6 (477 nm), 5D4 / 7F5,6 (496.02 nm and 548.32 nm), and 5D0/ 7F1,2 (597.16 nm and 617.54 nm) from Tm3þ, Tb3þ, and Eu3þ, respectively. An inter-level energy transfer, 5D4 (Tb3þ) / 5D1 (Eu 3þ), also occurred. In addi- tion, according to the CIE diagram, the CIE coordinates (x: 0.34 and y: 0.34) calculated for ZrO2: 8%RE, correspond to a point that lies in the white region. The results of this work suggest that these ma- terials have significant potential for use in the field of light-emitting diodes. Acknowledgment The authors gratefully acknowledge the financial support of the Brazilian governmental research funding agencies CAPES, CNPq 402127/2013-7, FAPESP 2013/07296-2 and INCTMN 2008/57872-1. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.11.341. References [1] F. 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Introduction 2. Experimental section 2.1. Synthesis of ZrO2:RE powders 2.2. Characterization of ZrO2:RE (RE = Tm3+, Tb3+, and Eu3+) powders 2.3. Computational details 3. Results and discussion 3.1. XRD characterization 3.2. Raman characterization 3.3. UV–visible spectroscopy analysis 3.4. PL studies 3.5. Energy transfer in ZrO2:RE (RE = Tm3+, Tb3+, and Eu3+) powders 4. Conclusions Acknowledgment Appendix A. Supplementary data References