Article pubs.acs.org/IC
Nonlinear Composition-Dependent Optical Spectroscopy of Ba2xSr2−2xV2O7 Hongwei Fang, Xiantao Wei, Shaoshuai Zhou, Yonghu Chen, Changkui Duan, and Min Yin* Key Laboratory of Strongly-Coupled Quantum Matter Physics, Chinese Academy of Sciences, School of Physical Sciences, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui Province 230026, P. R. China ABSTRACT: In general, adjusting the composition of a fluorescent material is an effective way to tune its luminescent properties such as peak energy and bandwidth. In most solid-solutions, the emission peak shifts linearly with the materials’ composition, which is referred to as Vegard’s Law. However, we found extraordinary variations in our samples Ba2xSr2−2xV2O7, that is, both the excitation and emission peaks show nonlinear dependence on the composition x, and the same is true for the spectral bandwidths. The nonlinearities are not due to structural anomaly, as all the samples are confirmed to be solid-solutions by X-ray diffraction measurements. To explain these phenomena, we proposed a model by considering the disorder of Ba2+ and Sr2+ distributions in solidsolutions and the changes of configurations between the ground and excited electronic states. This novel phenomenon could be applied to further exploit new fluorescent materials.
1. INTRODUCTION
2. EXPERIMENTAL SECTION
As common host for inorganic luminescent materials, vanadates have drawn much attention due to their excellent electronic, optical, and chemical characteristics resulting mainly from their 3d electrons.1−3 The charge-transfer transition involving d electrons plays a significant role in photoluminescence because the oxygen-to-metal charge-transfer absorption bands are usually quite broad and intense in many inorganic compounds such as vanadates, molybdates, and tungstates.4−6 Compared to common vanadate such as YVO4, alkaline-earth metal vanadates such as barium vanadates and strontium vanadates M2V2O7 (M = Ba, Sr) are more intriguing because of their strong selfactivated broadband emission and, when doped with rare earth ions such as Eu3+, their potential in spectral converter for WLEDs.7−9 Furthermore, tunable photoluminescence based on structural evolution has been reported in some solid-solutions such as nanocrystals ZnxCd1−xSe, oxyfluoride Sr3−xBaxAlO4F, and clinopyroxene (Na1−xCax) (Sc1−xMgx)Si2O6 phosphors, which suggests that solid-solution can be utilized to develop new fluorescent materials, because controllable luminescence can be achieved by simply adjusting the relative constituents of the solid-solution.10−12 In this work, we studied the luminescence properties for the vanadate solid-solution Ba2xSr2−2xV2O7 (BSVO). Different from the common linear composition dependence in most solid-solutions, we observed that the peak energy and bandwidth in the excitation bands vary as a concave function of x, while those in the emission bands are convex. These novel spectroscopic characteristics are in contrast with the linear composition dependence commonly observed in solid-solution. A model was proposed to explain those phenomena.
2.1. Materials and Synthesis. The samples Ba2xSr2−2xV2O7 (x = 0, 0.16, 0.25, 0.5, 0.75, and 1) were all synthesized via traditional hightemperature solid-state reaction method. The constituent oxides and carbonates consisted of BaCO3 (analytical reagent (AR) grade), SrCO3 (AR), and V2O5 (AR). Stoichiometric molar ratio of the raw materials were thoroughly mixed, fully ground, and prefired at 450 °C for 6 h. Subsequently after ground again, the obtained samples were recalcined at 1000 °C for 8 h. The crystal structures were analyzed by a MXPAHF rotating anode X-ray diffractometer (Cu Kα radiation). Photoluminescence excitation (PLE) and emission (PL) spectra were recorded on a HITACHI 850 fluorescence spectrometer, which utilized a 150 W Xe lamp as its excitation source. 2.2. Characterization of BSVO. The X-ray diffraction (XRD) measurements were performed for all our samples, and the results are plotted in Figure 1a. The XRD patterns for pure SVO and BVO fit well with their respective standard cards, which demonstrates that single-phase phosphors were obtained. Meanwhile, the XRD patterns of Ba2xSr2−2xV2O7 (x = 0.16, 0.25, 0.5, 0.75) phosphor samples are shown in Figure 1a (curves b−e). The XRD patterns of BSVO are similar to those of BVO and SVO samples, but with linear shift of peak positions with variation of x (Figure 1b), confirming the formation of solid solutions. The lattice parameters for all the samples of the Ba2xSr2−2xV2O7 series were obtained from the XRD data. As presented in Figure 2, all of the lattice constants a, b, and c show approximately linear contraction from x = 0.5 to x = 1, that is, with the increase of Sr content.
© XXXX American Chemical Society
3. RESULTS AND DISCUSSION 3.1. Structure Properties. Like most pyro-compounds, BVO and SVO both have dimorphic structures. SVO has a triclinic crystal phase (α-phase) at high temperature with space Received: June 6, 2016
A
DOI: 10.1021/acs.inorgchem.6b01368 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. Schematic crystal structure diagrams of BVO. (a) The structure of BVO unit cell (V5+ ions outside the unit cell are not plotted). (b) The structure of one VO4 dimer (V2O7 group).
Ba2+ or Sr2+ ions, which illustrates explicitly that the bond lengths between V and shared O are longer than other V−O bond lengths in both pure BVO and SVO host. Clearly, VO4 groups do not constitute regular tetrahedrons in BVO and SVO.
Figure 1. Powder XRD patterns for structural analysis of our samples BSVO. (a) The XRD patterns of the samples Ba2xSr2−2xV2O7 (x = 0, 0.16, 0.25, 0.5, 0.75, and 1). The standard patterns JCPD 39−1432 for pure BVO and JCPD 48−145 for pure SVO host are also shown as reference. (b) Enlargement of the XRD patterns in the range from 26.5° to 29.5°.
3.2. LUMINESCENCE PROPERTIES The PLE and PL spectra of BSVO at various x are plotted in Figure 4. The PLE spectrum (the red solid line in Figure 4a) shows that BVO is a self-activated phosphor with strong excitation broadband ranging from 270 to 360 nm and peaking at ∼335 nm. The emission spectrum (the red solid line in Figure 4b) is characterized by a strong blue-green broadband ranging from 430 to 650 nm, with maximum at ∼493 nm. BSVO of all the different compositions have similar spectral shape in both PLE and PL spectra. Figure 4 also shows that, as x decreases from 1 to 0.25, the excitation broadband witnesses a gradual blue shift, and then as x decreases further to 0, it experiences a red shift. Meanwhile, a contrary variation tendency can be observed in the PL spectra (Figure 4b), which displays a gradual red shift first and then blue shift when the composition varies from Ba-rich to Sr-rich. It is noted that the turning point in both spectra is x = 0.25. To obtain the peak energy (PE) and full width at halfmaximum (fwhm) as functions of Sr/(Ba + Sr) ratio in the excitation and emission spectra, we converted the responsecorrected intensity−wavelength spectrum Iλ(λ) into intensity− wavenumber spectrum IE(E) = λ2Iλ(λ). The spectra are plotted in Figure 5. In accordance with the results obtained by Figure 4, a dramatic variation with x can be observed in both the excitation and emission spectra. As PE in excitation maximizes and PE in emission minimizes at the same ratio Sr/(Ba + Sr) = 0.75, we obtained the largest PE difference at this ratio, which was explicitly plotted in Figure 5c. Furthermore, the blue dashed line in Figure 4c shows that the average of the excitation and emission PEs is linearly dependent on Sr/(Ba + Sr) ratio. This phenomenon is commonly observed in semiconductor materials such as quantum dots and nanowires, when constituted by two materials with similar structure in solidsolution, which follows Vegard’s Law.15−18 Figure 5d plots the variation of fwhm with the Sr/(Ba + Sr) ratio, and compared to Figure 5c, a variation tendency similar to that of PE is observed. The luminescence of BVO (or SVO) originated from the deexcitation of the charge transfer transition of the full 2p orbital of O2− to the vacant orbital of V5+. Common VO4 groups in most vanadates (e.g., YVO4 and ScVO4) form regular tetrahedrons, which have Td symmetry. In each VO4 group, different ways of bonding among all V−O bonds will contribute to the energy-level splitting.
Figure 2. Lattice parameters as a function of x in Ba2xSr2−2xV2O7 at various x. These values were refined in the P1̅ space group from the data of Figure 1. All lattice constants a, b, c, and the cell volume witness an approximately linear contraction when Sr content increases in the solid-solution.
group P1̅ (No. 2) and a tetragonal crystal phase (β-phase) at low temperature, with space group P41 (No. 76). The transition temperature between these two crystal phases is ∼645 °C.13 BVO exists as a triclinic structure (α-phase) at high sintering temperature, while orthogonal crystal phase (β-phase) is stable at low sintering temperature. By using a sintering temperature of 1000 °C, all the samples we obtained are triclinic. The structural diagram of BVO is plotted in Figure 3. Four V2O7 groups and four different kinds of Ba site occupations can be seen in each unit cell. Different from the common VO4 group in some vanadates (e.g., YVO4 and ScVO4), the VO4 group in BVO (or SVO) is not isolated but forms dimers by sharing a common oxygen atom.14 Table 1 shows the bond lengths and bond angles for four different kinds of V−O bonds in each VO4 group surrounded by four different kinds of site occupations for B
DOI: 10.1021/acs.inorgchem.6b01368 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Table 1. Bond Lengths Ri (i = 1, 2, 3, and 4) (Å) and the Angles θi (i = 1, 2, 3, and 4) between the V−O Bonds and the Approximate C3v Axis of Four Different VO4 Groups Vi (i = 1, 2, 3, and 4) in a Unit Cell BVO R1 R2 R3 R4 θ1 θ2 θ3 θ4
SVO
V1
V2
V3
V4
1.818 1.692 1.686 1.663 0 111.24 106.96 109.79
1.824 1.705 1.666 1.688 0 108.75 111.62 115.20
1.785 1.667 1.678 1.700 0 111.73 106.74 111.07
1.833 1.700 1.698 1.669 0 108.26 111.18 115.31
R1 R2 R3 R4 θ1 θ2 θ3 θ4
V1
V2
V3
V4
1.819 1.663 1.683 1.705 0 110.75 105.09 111.12
1.810 1.660 1.695 1.676 0 111.23 108.57 113.51
1.815 1.669 1.668 1.711 0 105.09 112.31 110.19
1.822 1.687 1.696 1.657 0 110.71 107.96 113.56
excited states 1T1,2 and 3T1,2 after considering different total spins.19 Given the transition selection rules, only electric-dipole transition 1A1−1T2 is effective. However, since VO4 dimers (without inversion center) replace regular VO4 group, Td symmetry in isolated VO4 group would be reduced to approximate C3v symmetry for each of the VO4 in the V2O7 cluster, further lifting the degeneracy.9 Similar to the case with Td symmetry, the excited states separate into spin singlets with high energies and spin triplets with low energies because of the Coulomb exchange interaction, and the excitation occurs from the ground state to the spin singlets, while the states involved in the emission are spin triplets due to the nonradiative relaxation between excited spin singlets and spin triplets. The schematic diagram is depicted in Figure 6. Figure 4. Normalized PLE (a) and PL (b) spectra of Ba2xSr2−2xV2O7 at various x. The black dotted lines at x = 0.16 were added because of the intense variation between x = 0 and x = 0.25. The brown dotted lines display the variation tendency of excitation and emission peak shifts.
Figure 6. Schematic energy diagram for VO4 group. (a) The case with strict Td symmetry (such as in YVO4). (b) The case of approximate C3v symmetry in pure BVO and SVO host. (c) Broadening of the energy distribution in solid-solution BSVO.
The difference between the excitation and emission PEs is related to the averaged Coulomb exchange interaction between spin singlets and spin triplets of the excited states. While the exchange interactions for BVO and SVO are very close to each other due to similarity of V−O bond lengths of VO4 dimers in two hosts, it can be broadened in solid-solutions due to inhomogeneity, whose average strength may increase due to its nonlinear dependence on bond length. The linear dependence of the average of excitation and emission PEs on composition together with the opposite deviation of excitation and emission PEs from the average hints for the increase of Jex for solidsolutions. The dependence of energies for the ground and various localized excited electronic states on the displacement of coordinated ions/atoms is usually described by the configuration coordinate model. In our case, the deviation of configuration between the lowest spin triplet excited state and the spin singlet ground state governs the fwhm in the
Figure 5. Variation of PE and fwhm with Sr/(Ba + Sr) ratio in excitation and emission spectra. (a) Solid and hollow triangles show PE and the energy at half-maximum with various Sr/(Ba + Sr) ratio in the excitation, respectively. (b) Solid and hollow rectangles show PE and the energy at half-maximum with various Sr/(Ba + Sr) ratio in the emission, respectively. (c) Solid circles show the average of PE between the excitation and emission with various Sr/(Ba + Sr) ratio. (d) The black line with hollow rectangles and the red line with hollow triangles are fwhm of the emission and excitation bands, respectively. The hollow circles are the averages of the two fwhm values for the excitation and emission bands.
Under Td symmetry, the nonbonding t1 produced by O2− and e produced by V5+ give rise to the ground state 1A1 and four C
DOI: 10.1021/acs.inorgchem.6b01368 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
configuration coordinate model, we obtained a higher excitation with broader energy bandwidth and a lower emission with narrower energy bandwidth compared with componentaveraged case.
emission spectra. In solid-solution, the irregular site occupations of Ba2+ and Sr2+ ions adjust the structural environment around all VO4 dimers, and the interaction between different VO4 dimers transforms the discrete charge-transfer excited energy levels (localized electron states) into an energy band. As mentioned above, the emission is related to spin-triplet excited states. Considering the nonradiative relaxation among all spin triplets, the effective radiative transition occurs mainly from the lowest spin triplet. The relationship between fwhm and Huang−Rhys parameter S is given by20 fwhm = hυ ×
8 ln 2 × coth
hυ × 2kT
S
4. CONCLUSION In conclusion, we observed extraordinary spectroscopic properties that the spectral shift and the variation of spectral bandwidth were both nonlinearly dependent on the composition in solid-solution vanadate Ba2xSr2−2xV2O7. We measured the XRD patterns of all samples, which confirmed the nonlinear composition dependence of spectroscopic properties cannot be ascribed to structural anomaly. The novel behaviors were related to the disordered distributions of Ba2+ and Sr2+ ions around VO4 dimers in solid-solution and the deviation of configuration coordinates between the ground and excited states. A tentative model is proposed to explain the results. The phenomena can be further exploited in the study of new colortunable luminescent materials.
(1)
where hυ represents the vibrational phonon energy, k is Boltzmann constant, and T is the samples’ temperature. Equation 1 shows that the smaller S, the smaller is the fwhm at a given temperature. Hence the experiment data of fwhm as a convex function of x indicates that in solid-solution, the lowest spin triplets tend to have smaller coordinate difference from the spin-singlet ground state. The larger fwhm in excitation spectra is attributed to the combination of larger configurational deviation between the spin-singlet excited states and the ground state and inhomogeneous broadening. In the excitation process, when Ba2+ and Sr2+ ions coexist, different structural environment around all VO4 dimers will lead to inhomogeneous broadening, and discrete spin singlet levels turn into a broad spin singlet bands. Different from the case in emission spectrum, which is due to the transition from the lowest spin triplet to ground spin singlet, the excitation spectrum is due to the transition from the ground spin singlet to all the excited spin singlet states in an inhomogeneously broadened energy range. As higher spin singlets tend to have larger coordinate difference from the spin singlet ground state, larger fwhm than component-averaged fwhm can be expected. Moreover, both inhomogeneous broadening and configuration differences contribute smaller deviations of configuration from the ground state than that of the spin-triplet excited states. From the analysis above, we constructed a schematic configuration coordinate diagram. As plotted in Figure 7, in
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
H.W.F. and S.S.Z. designed the experiment; H.W.F. synthesized the materials, performed the XRD measurements, and performed the spectral measurements; C.K.D. and H.W.F. analyzed the data; H.W.F. and C.K.D. wrote the manuscript; M.Y., Y.H.C., X.T.W., and C.K.D. participated in discussions of the data. All authors commented on the revision of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Key Basic Research Program of China (Grant No. 2013CB921800), the National Natural Science Foundation of China (Grant Nos. 11274299, 11374291, 11574298, and 11404321), and Anhui Provincial Natural Science Foundation (Grant No. 1308085QE75).
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Figure 7. Schematic configuration coordinate diagram for the excitation and emission in Ba2xSr2−2xV2O7 at a certain x. Discrete localized state levels (dotted parabolas) transform into a distribution of excited energy bands including some energy levels with higher and lower energy (solid parabolas). The configurational deviation rather than inhomogeneous broadening governs larger fwhm in excitation, which is not depicted to scale. D
DOI: 10.1021/acs.inorgchem.6b01368 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b01368 Inorg. Chem. XXXX, XXX, XXX−XXX
