Ultrasonic Measurement of LaCoO₃ –Evidence for the Orbital-Order Fluctuation–

Abstract

The sound velocity propagating along [111] of LaCoO 3 has been measured from 4 to 580 K for longitudinal ( L ) and transverse ( T ) sounds with frequencies, f , of 10 to 100 MHz. A comparison of the data for the single-domain rhombohedral crystal with those for a twinned pseudocubic specimen reveals the anisotropy of the elastic constants with the rhombohedral symmetry. The temperature dependence of the velocity of the L mode with f =10 MHz is analyzed consistently with the anomalous lattice expansion associated with the spin state transition. A frequency dispersion of the sound velocity appears in the L mode above 100 K. The amplitude of the dispersion increases pronouncedly with increasing temperature up to the measured highest temperature. The relaxation rate of the lattice evaluated based on the Debye model shows an activation type of temperature dependence with an attempt frequency of (1/τ) 0 =187×10 6 s -1 and the energy E a =11 meV. The slow lattice relaxation is attributed to a fluctuation of the orbital-order of Co atoms in the intermediate spin state and also to the relaxation between the intermediate- and the high-spin states, the rate of which is slowed down to that of the orbital fluctuation.