ANOMALOUS HYPERSONIC BEHAVIOR OF CUGEO3 PRIOR TO THE SPIN-PEIERLS TRANSITION

The temperature dependence of the longitudinal elastic constants c(22) , c(33), and of the transverse elastic constant c(44), as obtained by Brillouin spectroscopy, reveals an unexpected elastic behavior in CuGe O3 prior to the spin-Peierls magnetic transition (T(SP)approximate to 14 1 K) At room temperature c(33)much greater than c(44)>c(22). Below about 50 K a steep increase in the values of the longitudinal elastic constants leads to a crossover of the values of c(22) and c(44) with c (22)>c(44). The existence of translation-rotation coupling in CuGeO3 c ould account for the elastic behavior above T-SP. The anisotropy of th e magnetoelastic coupling at the spin-Peierls transition temperature i s reflected in the different temperature behavior of the elastic const ants (c(33), c(44), and c(22)) at T(SP)approximate to 14.1 K.