An Investigation for the Spin Hamiltonian Parameters and the Local Structure for Co 2+ in ZnWO 4

Transcription

1 MATEC Web of Conferences 7, DOI: / matecconf/01700 SMAE 01 An Investigation for the Spin Hamiltonian Parameters and the Local Structure for Co + in ZnWO 4 Chang-Chun DING 1,a, Shao-Yi WU 1,b, Yong-Qiang XU 1,c and Li-Juan ZHANG 1,d 1 Department of Applied Physics, School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 10054, P. R. China a ccding@13.com *, b wushaoyi@uestc.edu.cn, c qiangyongxu@13.com, d ljzhang@13.com Abstract. Based on the perturbation formulae of the spin Hamiltonian parameters SHPs, i,e,. g factors g x, g y, g z, and hyperfine structure constants A x, A y, A z, for a 3d 7 ion in rhombically distorted octahedra, the defect structure and the experimental EPR spectra are theoretically investigated for the rhombic Co + centre in ZnWO 4. Due to the Jahn-Teller effect, the impurity ion Co + on host Zn + site is found to undergo the local rhombic elongation distortion, characterized by the axial elongation Z Å of the impurity-ligand bonds and the perpendicular distortion angle 3.5 related to an ideal octahedron. Compared with host Zn + site in ZnWO 4, the Co + centre exhibits a more regular oxygen octahedron. All the calculated SHPs show good agreement with the observed values. The present work may be helpful to understand the local structures and properties of metal tungstate with transition-metal dopants. 1. Introduction As a stable and hard metal tungstate, ZnWO 4 is attractive in microwave, acoustic, catalytic, magnetic and phase-change optical recording properties and widely used in optical fibers, humidity sensors, photoluminescence and scintillation materials [1 4]. More important, this material containing transition-metal dopants exhibits novel performances such as catalyst [5-7], electrochemistry [8,9], lasing efficiency enhancement [10] and bright green upconversion luminescence emission [11]. For example, Co + doped ZnWO 4 is also an extension of the use of ruby as a maser material [1]. Usually, the properties of function materials have close relationships with the features of local environments e.g., coordination number, occupation, local symmetry, strength of crystal-fields in the vicinity of the doped impurities. The involved local features of impurities can be effectively revealed by use of electron paramagnetic resonance EPR technique. The EPR experiments have been performed for Co + doped ZnWO 4, and the spin Hamiltonian parameters SHPs * Corresponding author:ccding@13.com The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0

2 MATEC Web of Conferences 7, DOI: / matecconf/01700 SMAE 01 g factors g x, g y, g z and hyperfine structure constants A x, A y, A z were measured for the rhombic Co + centre [1], with the properties of g x, g y < g z and A x, A y <A z. But, up to now, there are no theoretical explanations to the above EPR results. In this work, the SHPs for the rhombic Co + centre in ZnWO 4 are studied from the perturbation formulae of these parameters for a 3d 7 ion under rhombically distorted octahedra. The calculated results and the local structure of the impurity centre are discussed.. Calculations ZnWO 4 has wolframite structure with C h point group symmetry and P/c space group [13]. When impurity Co + is doped into ZnWO 4, it may substitute for host Zn + and undergo local lattice distortion due to size mismatch. The formulae of g factors and hyperfine structure constants for a 3d 7 ion in rhombic distorted octahedra can be written as follows [14]: g x = {4[ ' k ] X 4 } / X 1/ ' x '' x x ' x x x ' '' x H g y = {4[ '' k ] Y 4 }/ H Y 1/ '' x ' x x '' x x x ' '' x g z = X 3Y { 4 k [ ] [ ] 1 1 }/H X Y 1/ x x x x ' '' x x 1 A x = { / [ ] k WXY WX WXZ P WZ } 1/ ' '' x x x ' '' x x x x ' '' / H A y '{ 5 7 X P 4 X } /H 1/ ' x x x x ' '' = { / [ ] k WXY WY WYZ P W } / Z 1/ ' '' x x x ' '' x x x x ' '' H A z '{ 5 7Y P 4 }/H Y 1/ ' x x x x ' '' = 9 4 WZ P{ / [ 8 ] 4k W } X W / Y x x x x x x ' '' H W XZ WYZ 3X 3Y P x x P'{ x x }/H 1/ X Y 1/ Z ' '' x x ' ''

3 MATEC Web of Conferences 7, DOI: / matecconf/01700 SMAE 01 Here H = 8. is the core polarization constant. x can be ' '' x x determined from the energy splittings =E{ 4 B 1 [ 4 T 1 F]} E{ 4 B 3 [ 4 T 1 F]} and =E{ 4 B [ 4 T 1 F]} E{ 4 B 3 [ 4 T 1 F]} of 4 T 1 ground state in rhombic crystal-fields by the ' ' ' expression = [ ] x+3. The splittings and can be 3 x x calculated from the d-d transition energy matrices for 3d 7 ion in rhombic symmetry. The related parameters f i, q i, i, ij and W ij in the above formulae are related to the admixtures of the ground and excited states in rhombic symmetry and can be found in Ref. [14]. Based on the cluster approach, the spin-orbit coupling coefficients, ', the orbital reduction factors k, k' and the dipole hyperfine structure parameters P and P for 3d 7 ion can be written as [14,15] = N t 0 d + t p 0 /, ' = N t N e 1/ 0 d t e p 0 /, k = N t 1 + t /, k = N t N e 1/ 1 t e / P = N t P 0, P' = N t N e 1/ P 0, where 0 d and 0 p are the spin-orbit coupling coefficients of a free 3d 7 and ligand ions, respectively. P 0 is the dipolar hyperfine structure parameter of the free 3d 7 ion. N and are, respectively, the normalization factors and the orbital admixture coefficients. They can be determined from the normalization conditions and the approximate relationships [14,15] N 1S 1, N 1 S S 1. t dpt t e e dpe s ds e s t t dpt t dpt e e dpe s dps e dpe s ds t N N 1 S S, N N 1 S S S S. 3 Here N is the covalency factor, and S dp and S ds are the group overlap integrals. The orbital admixture coefficients and the group overlap integrals have the proportionality relationship e / S dpe s /S ds. The host Zn + site in ZnWO 4 is surrounded by a considerably by about 0.31 Å elongated oxygen octahedron with obvious perpendicular planar rhombic angular distortion [13]. Nevertheless, substitution of Zn + by Co + may lead to quite different local environment around the impurity. Importantly, the Jahn-Teller ion Co + at octahedral crystal-fields can suffer the Jahn-Teller effect i.e., the phenomenon of removal of the degeneracy of energy levels and results in lower symmetry and energy via the vibrational interactions [1,17]. For example, the [CoO ] 10 cluster may experience a local axial elongation Z along Z-axis and the planar angular variation. According to the superposition model [18] and the geometrical relationship of the studied Co + centre, the rhombic field parameters can be expressed as follows: t t t t t t D s /7 A R/ R1 R/ R R/ R3 R/ R4 R/ R5 R/ R, D t t4 t4 t4 t4 1/ 1 A4 R/ R cos4 R/ R cos4 R/ R cos4 R/ R cos4 D t4 R R R R t4 / 5 /, 4 3

4 MATEC Web of Conferences 7, DOI: / matecconf/01700 SMAE 01 A R R R R R R R R t t t t / 7 / cos / cos / cos / cos, D t4 t4 t4 t4 5 / 1 A4 R/ R1 cos 1 R/ R cos R/ R3 cos 3 R/ R4 cos 4 Here A and A 4 are the intrinsic parameters related to the reference average cation-anion distance R.114 Å [13] of Zn + site in ZnWO 4. For 3d n ions in octahedral environments [191], the expressions A 4 7/1 Dq and A 1 A 4 have been proved valid and are reasonably utilized in this work. From the Slater-type self-consistent field functions [,3] and R, the integrals S dpt 0.09, S dpe 0.038, S ds 0. and A are calculated. The spectral parameters Dq, B and C of the studied system can be obtained from those of similar octahedral Co + cluster in MgO [4], i.e., Dq 80 cm 1, B 985 cm 1, C 3858 cm 1. 5 From the free ion values B cm 1 and C 0 43 cm 1 [] of Co +, one can obtain N Then the molecular orbital coefficients N t 0.943, N e 0.951, t 0.57 and e 0.31 from Eq. 3. Substituting the free-ion parameters 0 d 533 cm 1 [5] and P cm 1 [] for Co + and 0 p 151 cm 1 for O [7] into Eq., we have 507 cm 1, 500 cm 1, k 0.973, k 0.89, P cm 1, P cm 1. For Co + in Tutton salts, the core polarization constant 0.13 [8] is obtained and also adopted here. There are only two unknown quantities local axial elongation Z and planar angular variation in the formulae of SHPs. Substituting the above parameters into Eq. 1 and fitting the calculated SHPs to the experimental data, one can obtain the optimal values Z Å, The corresponding SHPs Cal. b are shown in Table 1. TABLE 1 THE G FACTORS G I AND HYPERFINE STRUCTURE CONSTANTS A I IN UNITS OF 10 4 CM 1 FOR THE RHOMBIC CO + CENTRE IN ZNWO 4. g x g y g z A x A y A z Cal. a Cal. b Expt. [1] For comparison, the results Cal. a based on the host structural data i.e., R 1H R 3H.0 Å, R H R 4H.090 Å, R 5H R H.7 Å; 1H 53.17º, H º, 3H 30.83º, 4H 8.17º [13] of Zn + in ZnWO 4 i.e., omission of the local lattice distortion are also collected in Table 1. 4

5 MATEC Web of Conferences 7, DOI: / matecconf/01700 SMAE Discussions From Table 1, one can find that the calculated g factors and hyperfine structure constants show reasonable agreement with the experimental data, except that A z is smaller than the observed value. For 3d 7 ions in rhombic distorted octahedra, g factors and hyperfine structure constants normally exhibit similarly consistently anisotropies related to the local structures of the impurity centres. In fact, the ratios are A x /g x and cm 1, A y /g y and cm 1 and A z /g z and cm 1 for the similar rhombic Co + centres in NaF [9] and AgCl [30], respectively. Thus, the ratio A x /g x cm 1, A y /g y cm 1 and A z /g z cm 1 based on the present calculations for Co + in ZnWO 4 show the comparable ratios for y and z components with the above two systems and can be regarded as suitable. However, the ratio A z /g z cm 1 [1] based on the experimental data for ZnWO 4 :Co + are much about 50% larger than A y /g y cm 1 [1], reflecting unsuitably higher magnitude of A z. So, further experimental verification for the hyperfine structure constants would be helpful. The moderate axial anisotropies g [= g z g x +g y /] and A [= A z A x +A y /] of g and A factors may be attributed to the moderate axial elongation of the system, characterized by the intermediate axial elongation Z. The relatively smaller perpendicular anisotropies g = g x g y and A = A x A y originate mainly from the planar angular distortion, characterized by the deviation of the angle from that = 45 in regular octahedra. So, the oxygen octahedron transforms from considerable elongation i.e., large Z 0.31 Å in host Zn + site to slight elongation i.e., small Z Å in the impurity Co + center. And the four nearly planar Co + O bonds experience the angular variations via bending the bonds, yielding a more regular [CoO ] 10 octahedron. 4. Acknowledgement This work was financially supported by the Fundamental Research Funds for the Central Universities [ZYGX014J13]. References 1. G. Huang, Y. Zhu, Synthesis and photocatalytic performance of ZnWO 4 catalyst, Mater. Sci. Eng. B G. Huang, C. Zhang, Y. Zhu, ZnWO 4 photocatalyst with high activity for degradation of organic contaminants, J. Alloys Compd F. Yang, C. Tu, H. Wang, Y. Wei, Z. You, G. Jia, J. Li, Z. Zhu, X. Lu, Y. Wang, Growth and spectroscopy of Dy 3+ doped in ZnWO 4 crystal, Opt. Mater J. Lin, J. Lin, Y. Zhu, Controlled synthesis of the ZnWO 4 nanostructure and effects on the photocatalytic performance, Inorg. Chem L. Zhang, Z. Wang, L. Wang, Y. Xing, Y. Zhang, Preparation of ZnWO 4 /graphene composites and its electrochemical properties for lithium-ion batteries, Mater. Lett M. Qamar, A. Khan, Mesoporous hierarchical bismuth tungstate as a highly efficient visible-light-driven photocatalyst. RSC Adv F. Chambon, F. Ratabou, C. Pinel, A. Cabiac, E. Guillon and N. Essayem, Cellulose Conversion with Tungstated-Alumina-Based Catalysts: Influence of the Presence of Platinum and Mechanistic Studies, ChemSusChem

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