Ultrasonic investigations of the Jahn--Teller effect

Several transition metal (i.e., Ni/sub x/Zn/sub 1 - x/Cr2O4 and CsCuCl3) and rare earth (lanthanum antimonides, TmCd, Pr metal) compounds were investigated ultrasonically, the changes in sound velocity being used as a probe. The coupling of the ultrasonically produced strain field to the energy levels of the transition metal and rare earth ions (a manifestation of the Jahn--Teller effect) causes readily discernible changes in the sound velocity. Three general types of behavior are noted as the temperature is changed. If the coupling between the strain and the energy levels is vanishingly small, or if the ground state multiplet is a singlet or Kramer's doublet (which cannot be split by a strain), the usual behavior of a solid is seen, that is, the velocity increases as the temperature is lowered, and becomes constant at temperatures near absolute zero. If the lowest level of the ground state multiplet is a singlet or Kramer's doublet, and the coupling constant is less than some critical value, there is a minimum in the velocity vs. temperature curve, at a temperature which is related to the energy of the lowest excited state. If the ground state is a singlet or Kramer's doublet, and the coupling is above the critical value, or if the lowest level is a multiplet other than a Kramer's doublet, there exists a temperature below which the crystal distorts. Studies of the velocity anisotropy in nematic and cholesteric liquid crystals, nematic-isotropic and cholesteric-nematic phase transitions were performed in conjunction with the investigations and are also reported