Anomalous hypersonic behavior of CuGeO₃ prior to the spin-Peierls transition

The temperature dependence of the longitudinal elastic constants c₂₂, c₃₃, and of the transverse elastic constant c₄₄, as obtained by Brillouin spectroscopy, reveals an unexpected elastic behavior in CuGeO₃ prior to the spin-Peierls magnetic transition (T_SP≈14.1K). At room temperature c₃₃≫c₄₄>c₂₂. Below about 50 K a steep increase in the values of the longitudinal elastic constants leads to a crossover of the values of c₂₂ and c₄₄ with c₂₂>c₄₄. The existence of translation-rotation coupling in CuGeO₃ could account for the elastic behavior above T_SP. The anisotropy of the magnetoelastic coupling at the spin-Peierls transition temperature is reflected in the different temperature behavior of the elastic constants (c₃₃, c₄₄, and c₂₂) at T_SP≈14.1K.