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51V nuclear magnetic resonance in polycrystalline ferroelastic BiVO4
Solid State Communications, Vol~54,No.5, pp.403-404, Printed in Great Britain.
1985.
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
51V NUCLEAR MAGNETIC RESONANCE IN POLYCRYSTALLINE FERROELASTIC BiVO 4 Stanley Segel Department of Physics, Queen's University Kingston, Ontario, Canada K7L 3N6 (Received December 17, 1984 by R.H. Silsbee) Nuclear magnetic resonance studies on polycrystalline ferroelastic BiVO 4 indicate that the 51V electric field gradient asymmetry parameter is an order parameter in the ferroelastic transition. Using n = A(T-Tc)B, B is found to be 0.48(5), in good agreement with earlier studies of this material. Near the phase transition above and below, the vanadium nuclear quadrupole coupling is constant with a value of 4.8(1) MHz.
At room temperature BiVO~ is monoclinic, 12/a, and with increasing temperature transforms uniformly and continuously to a tetragonal scheelite structure, 14/a, at 528K. The continuous nature of the transition is evidenced by observing the lattice parameters as a function of temperature and pressurel,Z, ~, Brillouin scattering ~,5, refractive index as a function of temperature G, and the softening of the optic mode as function of temperature and pressure 7,s. As one might expect, there is no detectable latent heat associated with the transition. So, not only is this transformation one of strict "non-first order" (allowing for second and higher order), but it also satisfies the crystallographic conditions for a ferroelastic transition, which in the Aizu 9 formulation is written f/mF2/m. All experimental results indicate a mean field approximation to be valid. Our NMR results support this conclusion within experimental error. All the data were taken with a common crossed coil spectrometer of the Torgeson type operating in superfast passage mode. l° High temperatures were achieved using a gas flow dewar pipe arrangment with cooling gas around the outside to prevent warming of the rf probe. The 51V nuclear quadrupole coupling is sufficiently large to produce 2nd order broadening of the central transition. While first order satellites were observed, they were sufficiently broadened by the asymmetry parameter, ~, in the electric field gradient, efg, to be of little analytical use in the monoclinic phase. Analysis of the central transition is complicated by the presence of large anisotropic chemical shifts; the spread in shifts is of the order of 0.02%. At high magnetic fields the chemical shifts confuse and almost obliterate the quadrupole coupling. It was found expedient to work at ii MHz and 8 MHz. In Fig. i we show the "classic" decrease in n with temperature. Measurements of the quadrupole coupling gave a value of 4.8 (i) MHz, constant within experimental error above and below the transition temperature. A fit of n = A(T-Tc)B yielded values A = 0.029 (5) and B = 0.48 (5) and it is this equation which is sketched through the data points in Fig. i. That the square of B is proportional to the temperature is consistent with bi-refringence G and soft phonon measurements.7, 8
403
I
0.4' • % ~,,o..
eislv04 ' e~'J va T
°~ox~
0.3
o 11.5 NHz x 8.8 MHz = SkTELLITES
-0.1 -D.O I
0
Q
!
100
T(°C)
I
ZOO
Q
1300
Figure I. 51V electric field gradient asymmetry parameter in biVO 4 as a function of temperature, o-I].5 MHz data; x-8.8M}{z data; m - 1st order satellite data. insert; ~-~ 5lv transition at 8 Ml{z at 77 K, ~ =0.505. Width of spectrum is approximately 40 gauss. See text for further details. It seems reasonable to conclude as in the case of metavanadates 11 that a small distortion of the VO~ tetrahedra gives rise to the asymmetry parameter and that, as the unit cell smoothly move~ into the tetragonal phase, the distortion, and therefore n, is removed. The resonance details for measuring n are the shoulders on the central transition (see figure). As we attempt to measure n values less than 0.2, the dipolar line width interferes with the resolution. This is true despite the favourable situation of having the V atom surrounded by large and spinless oxygen atoms so the dipolar line width is less than 0.2 mT. In many respects, these results are similar to those of Jeffrey 12 who studied the Na NMR in NaN 3 and found a similar (T-To) 0-5 behavior. With increasing temperature NaN 3 is thought to transform continuously from a monoclinic C2/m to
404
5~V NUCLEAR ~iAGNETIC RESONANCE IN POLYCRYSTALLINE FERROELASTIC BiVO 4
a high temperature rhombohedral R3m. There is a very small thermal anomaly at the transition temperature 13 of 293K, and this is accompanied by a slight softening of an A_ librational phonon. However, Banda et al, I~ in'their study of the alpha to beta transformation in quartz have noted possible ambiguities in interpretation of the data and have warned that "the observation of a critical like exponent for behavior of the long range order parameter does not, in itself, imply nonclassical critical behavior." These NMR techniques may prove useful in the study of ferroelastic materials for which it is difficult to grow single crystals.
Vol. 54, No. 5
An extensive search was made for the 2~9Bi resonance in this material, using both NMR and pure quadrupole resonance techniques, the latter extending to 150 MHz. No resonance were found and we must conclude the the Z°gBi resonances reported by Volkov et al Is were in a different phase of Bivo~ Acknowledgments - This work was supported by the Natural Sciences and Engineering Research Council of Canada. We would like to thank R.D. Heyding for confirming the monoclinic structure of the Bivo~ used, which was purchased from CERAC, Inc.
REFERENCES i. 2. 3. 4. 5. 6.
7.
W. David and I.G. Wood, J. Phys. C 16 5127 (1983). W.I.F. David, A.M. Glazer and A.W. Hewat, Phase Transitions I, 155 (1979). R.M. Hazen and J.W. Mariathasan, Science 216, 991 (1982). A. Tokumoto and H. Unoki, Phys. Rev. 327, 3748 (1983). G. Benyuan, M. Copic and H.Z. Cummins, Phys. Rev. B24, 4098 (1981). L.P. Avkayants, D.F. Kiselev and A.V. Chervyakov, Soy. Phys. Solid State 25, 1603 (1984). A. Pinczuk et al., Sol. State Com. 24, 163 (1977).
8. 9. I0. II. 12. 13. 14.
15.
A. Pinczuk et al., Sol. State Com. 29, 515 (1979). K. Aizu, J. Phys. Soc. Japan 27, 387 (1969). S. Segel, R.B. Creel and D.R. Torgeson, J. Mol. Struct. I!i , (1983). S.L. Segel and R.B. Creel, Can. J. Phys. 48, 2673 (1970). K. Jeffrey, J. Chem. Phys. 66, 4677 (1977). H.P. Fritzer and K. Torkar, Monatsh. Chem. 97, 703 (1966). E.J.K. Banda, R.A. Craven, R.D. Parks, P.M. Horn, and M. Blume Solid State Commun. 17, ii (1974). A.F. Volkov, L.A. Ivanova and Y. Nvenetsev, Izv. Akad. Nauk SSR, Neorg. Mater. 14, 782 (1978).
1985.
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
51V NUCLEAR MAGNETIC RESONANCE IN POLYCRYSTALLINE FERROELASTIC BiVO 4 Stanley Segel Department of Physics, Queen's University Kingston, Ontario, Canada K7L 3N6 (Received December 17, 1984 by R.H. Silsbee) Nuclear magnetic resonance studies on polycrystalline ferroelastic BiVO 4 indicate that the 51V electric field gradient asymmetry parameter is an order parameter in the ferroelastic transition. Using n = A(T-Tc)B, B is found to be 0.48(5), in good agreement with earlier studies of this material. Near the phase transition above and below, the vanadium nuclear quadrupole coupling is constant with a value of 4.8(1) MHz.
At room temperature BiVO~ is monoclinic, 12/a, and with increasing temperature transforms uniformly and continuously to a tetragonal scheelite structure, 14/a, at 528K. The continuous nature of the transition is evidenced by observing the lattice parameters as a function of temperature and pressurel,Z, ~, Brillouin scattering ~,5, refractive index as a function of temperature G, and the softening of the optic mode as function of temperature and pressure 7,s. As one might expect, there is no detectable latent heat associated with the transition. So, not only is this transformation one of strict "non-first order" (allowing for second and higher order), but it also satisfies the crystallographic conditions for a ferroelastic transition, which in the Aizu 9 formulation is written f/mF2/m. All experimental results indicate a mean field approximation to be valid. Our NMR results support this conclusion within experimental error. All the data were taken with a common crossed coil spectrometer of the Torgeson type operating in superfast passage mode. l° High temperatures were achieved using a gas flow dewar pipe arrangment with cooling gas around the outside to prevent warming of the rf probe. The 51V nuclear quadrupole coupling is sufficiently large to produce 2nd order broadening of the central transition. While first order satellites were observed, they were sufficiently broadened by the asymmetry parameter, ~, in the electric field gradient, efg, to be of little analytical use in the monoclinic phase. Analysis of the central transition is complicated by the presence of large anisotropic chemical shifts; the spread in shifts is of the order of 0.02%. At high magnetic fields the chemical shifts confuse and almost obliterate the quadrupole coupling. It was found expedient to work at ii MHz and 8 MHz. In Fig. i we show the "classic" decrease in n with temperature. Measurements of the quadrupole coupling gave a value of 4.8 (i) MHz, constant within experimental error above and below the transition temperature. A fit of n = A(T-Tc)B yielded values A = 0.029 (5) and B = 0.48 (5) and it is this equation which is sketched through the data points in Fig. i. That the square of B is proportional to the temperature is consistent with bi-refringence G and soft phonon measurements.7, 8
403
I
0.4' • % ~,,o..
eislv04 ' e~'J va T
°~ox~
0.3
o 11.5 NHz x 8.8 MHz = SkTELLITES
-0.1 -D.O I
0
Q
!
100
T(°C)
I
ZOO
Q
1300
Figure I. 51V electric field gradient asymmetry parameter in biVO 4 as a function of temperature, o-I].5 MHz data; x-8.8M}{z data; m - 1st order satellite data. insert; ~-~ 5lv transition at 8 Ml{z at 77 K, ~ =0.505. Width of spectrum is approximately 40 gauss. See text for further details. It seems reasonable to conclude as in the case of metavanadates 11 that a small distortion of the VO~ tetrahedra gives rise to the asymmetry parameter and that, as the unit cell smoothly move~ into the tetragonal phase, the distortion, and therefore n, is removed. The resonance details for measuring n are the shoulders on the central transition (see figure). As we attempt to measure n values less than 0.2, the dipolar line width interferes with the resolution. This is true despite the favourable situation of having the V atom surrounded by large and spinless oxygen atoms so the dipolar line width is less than 0.2 mT. In many respects, these results are similar to those of Jeffrey 12 who studied the Na NMR in NaN 3 and found a similar (T-To) 0-5 behavior. With increasing temperature NaN 3 is thought to transform continuously from a monoclinic C2/m to
404
5~V NUCLEAR ~iAGNETIC RESONANCE IN POLYCRYSTALLINE FERROELASTIC BiVO 4
a high temperature rhombohedral R3m. There is a very small thermal anomaly at the transition temperature 13 of 293K, and this is accompanied by a slight softening of an A_ librational phonon. However, Banda et al, I~ in'their study of the alpha to beta transformation in quartz have noted possible ambiguities in interpretation of the data and have warned that "the observation of a critical like exponent for behavior of the long range order parameter does not, in itself, imply nonclassical critical behavior." These NMR techniques may prove useful in the study of ferroelastic materials for which it is difficult to grow single crystals.
Vol. 54, No. 5
An extensive search was made for the 2~9Bi resonance in this material, using both NMR and pure quadrupole resonance techniques, the latter extending to 150 MHz. No resonance were found and we must conclude the the Z°gBi resonances reported by Volkov et al Is were in a different phase of Bivo~ Acknowledgments - This work was supported by the Natural Sciences and Engineering Research Council of Canada. We would like to thank R.D. Heyding for confirming the monoclinic structure of the Bivo~ used, which was purchased from CERAC, Inc.
REFERENCES i. 2. 3. 4. 5. 6.
7.
W. David and I.G. Wood, J. Phys. C 16 5127 (1983). W.I.F. David, A.M. Glazer and A.W. Hewat, Phase Transitions I, 155 (1979). R.M. Hazen and J.W. Mariathasan, Science 216, 991 (1982). A. Tokumoto and H. Unoki, Phys. Rev. 327, 3748 (1983). G. Benyuan, M. Copic and H.Z. Cummins, Phys. Rev. B24, 4098 (1981). L.P. Avkayants, D.F. Kiselev and A.V. Chervyakov, Soy. Phys. Solid State 25, 1603 (1984). A. Pinczuk et al., Sol. State Com. 24, 163 (1977).
8. 9. I0. II. 12. 13. 14.
15.
A. Pinczuk et al., Sol. State Com. 29, 515 (1979). K. Aizu, J. Phys. Soc. Japan 27, 387 (1969). S. Segel, R.B. Creel and D.R. Torgeson, J. Mol. Struct. I!i , (1983). S.L. Segel and R.B. Creel, Can. J. Phys. 48, 2673 (1970). K. Jeffrey, J. Chem. Phys. 66, 4677 (1977). H.P. Fritzer and K. Torkar, Monatsh. Chem. 97, 703 (1966). E.J.K. Banda, R.A. Craven, R.D. Parks, P.M. Horn, and M. Blume Solid State Commun. 17, ii (1974). A.F. Volkov, L.A. Ivanova and Y. Nvenetsev, Izv. Akad. Nauk SSR, Neorg. Mater. 14, 782 (1978).