Relaxation in a Jahn-Teller System. I. Copper in Octahedral Water Coordination

Abstract

A study has been made, using EPR and electron-spin-echo methods, on the Jahn-Teller systems ${\mathrm{La}}_2$ ${\mathrm{Mg}}_3$ ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_{12}$·24${\mathrm{H}}_2$O and its deuterated counterpart and Zn ${\left(\mathrm{B}\mathrm{r}{\mathrm{O}}_3\right)}_2$·6${\mathrm{H}}_2$O, all containing substitutional divalent copper. At low temperatures, it is found that the ${\mathrm{Cu}}^{2+}$·6${\mathrm{H}}_2$O complex exists as a tetragonally distorted octahedron. Anisotropies in the $g$ tensor or random strains may stabilize a distortion. Phonon-induced reorientation of the distortions is observed, using electron-spin-echo methods, and is found to yield a rate ${\pi }^{-1\mathrm{}}$ linearly dependent on temperature with a proportionality factor of 5×${10}^4$ ${\mathrm{deg}}^{-1\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ for ${\mathrm{La}}_2$ ${\mathrm{Mg}}_3$ ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_{12}$·24${\mathrm{D}}_2$O at temperatures up to 12°K. The spin-lattice relaxation rate $T_1^{}{}_{}{}^{-1\mathrm{}}$ has been measured using the pulse-saturation method and is given by $T_1^{}{}_{}{}^{-1\mathrm{}}\approx 100T+3.3\times {10}^{-2\mathrm{}}{T}^5$ for $1.3°\mathrm{K}<T<\mathrm{}20\mathrm{}°\mathrm{K}$. Such a low-temperature rate is some four orders of magnitude faster than that observed for ${\mathrm{Cu}}^{2+}$ in the static octahedral water coordination of the KZn Tutton salt. Over the range that both have been measured, $T_1^{}{}_{}{}^{-1\mathrm{}}\propto {\tau }^{-1\mathrm{}}$.