ANTIFERRODISTORTIVE STRUCTURAL PHASE-TRANSITION IN THE JAHN-TELLER SYSTEM DYKMO2O8 .1. RAMAN-SCATTERING AND INFRARED-ABSORPTION STUDIES
A cooperative Jahn-Teller effect is observed in the orthorhombic layered rare-earth compound DyKMoO at T=14.3 K. Raman and infrared spectra show a mode near 30 cm at 2 K which decreases in energy in a mean-field fashion with increasing temperature up to T. The Raman spectra show also a well defined...
| Main Authors: | , , |
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| Format: | Journal article |
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1987
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| Summary: | A cooperative Jahn-Teller effect is observed in the orthorhombic layered rare-earth compound DyKMoO at T=14.3 K. Raman and infrared spectra show a mode near 30 cm at 2 K which decreases in energy in a mean-field fashion with increasing temperature up to T. The Raman spectra show also a well defined mode at ∼ 22 cm which becomes overdamped at 10 K and whose intensity decreases continuously to zero at T. This soft mode is neither Raman or infrared active above T. Estimates of the ion-lattice coupling show that a B-symmetry local distortion is favoured, and a comparison of the spectroscopic data with group theoretical predictions suggest that the transition is antiferrodistortive. A two-sublattice Ising model which includes intra- and inter-sublattice effective exchange interactions, J(k) and H(k), is developed and is found to display the two-mode behaviour observed: the lower-energy soft mode follows the order parameter (S) whereas the upper mode has the energy of the mean field. A fit to the Raman data yields the result J(0) ≈ = H(0)=7.5 cm. © 1987 IOP Publishing Ltd. |
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