EPR Spectrum of a Jahn-Teller System, NaCl:Cu+2

The EPR spectrum of isolated ${\mathrm{Cu}}^{+2\mathrm{}}$ ions in NaCl obeys an axial spin Hamiltonian at low temperatures with $g_{\parallel }=2.\mathrm{}373$, $g_{\perp }=2.\mathrm{}07$, $A_{\parallel }=95\mathrm{}$ G, $A_{\perp }=40\mathrm{}$ G, $Q^{\prime }=20\mathrm{}$ G with the $z$ axis lying parallel to one of the cube edges. Upon warming toward room temperature, the ligand huperfine spectrum disappears as the linewidth increases from 16 G at 4.2 K to 25 G at 77 K. At 95-100 K, as this spectrum disappears, a single isotropic line appears with $g=2.16\mathrm{}$ and a linewidth of 60 G. These low- and high-temperature spectra are interpreted as the static and dynamic Jahn-Teller distortions of the CuCl complex. A static strain experiment performed at 77 K results in a repopulation of the three distortion sites and enables a measurement of the strain coefficient $V_2$ to be made.