Vibronic Effects in EPR Spectra of (NH$\text{}_{4}$)$\text{}_{2}$Cu(BeF$\text{}_{4}$)$\text{}_{2}$·6H$\text{}_{2}$0 Single Crystal

Angular variations of the g-factors and linewidth value were recorded at X-band, at room temperature, and at 77 K. The temperature dependence of the g-factors was measured along the principal g-tensor axes in the temperature range from 4.2 K to 300 K. The rigid lattice g-factors at 4.2 K are: g$\text{}_{z}$ = 2.428, g$\text{}_{y}$ = 2.138, and g$\text{}_{x}$ = 2.058. The g$\text{}_{x}$ value very slowly increases linearly on heating, whereas g$\text{}_{z}$ and g$\text{}_{y}$ nonlinearly tend towards their average value. This is a typical behaviour of Cu(II) EPR in Tutton salt crystals, however the g(T) dependence is much less pronounced in (NH$\text{}_{4}$)$\text{}_{2}$Cu(BeF$\text{}_{4}$)$\text{}_{2}$·6H$\text{}_{2}$0 as compared to the Cu(II) doped salts. The results are described in terms of the two vibronic coupling models of the strong Jahn-Teller effect. It is shown that the models are not adequate to describe the vibronic g-factor behaviour in paramagnetic Cu(II) crystals which is attributed to the cooperative Jahn-Teller effect existing in these crystals.