EPR study of the Jahn-Teller effect onCu+2in CsCdCl3

A strong Jahn-Teller effect is shown to be present for ${\mathrm{Cu}}^{+2\mathrm{}}$ in CsCd${\mathrm{Cl}}_3$ by studying the electron-paramagnetic-resonance spectra at temperatures between 1.6°K and room temperature. The ${\mathrm{Cu}}^{+2\mathrm{}}$ ions occupy two magnetically inequivalent crystal sites of the same type (either $3m$ or $3\overline{m}$ in CsCd${\mathrm{Cl}}_3$ which has a $D_{6h}^4$ structure. The 1.6°K spectrum is a superposition of six single-line axial spectra caused by the three possible "tetragonal" distortions of each magnetically inequivalent site. The room-temperature spectrum is a single line independent of magnetic field orientation which is thermally averaged from the six low-temperature lines. The original trigonal distortions of the sites in the host crystal are completely suppressed by the Jahn-Teller interaction. These determinations are supported by the excellent match between theoretically predicted $g$ values and measured $g$ values which have unusual angular variations and by the type of temperature variations exhibited by the spectra. The observed hyperfine splittings are also consistent with a strong Jahn-Teller interaction. The ${\mathrm{Cu}}^{+2\mathrm{}}$ ground states are determined and the Jahn-Teller distortions are shown to be elongations of the ${\mathrm{Cu}}^{+2\mathrm{}}$ crystal site.