VIBRONIC EFFECTS IN EPR-SPECTRA OF (NH4)2CU(BEF4)2.6H2O 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 dependenc e of the g-factors was measured along the principal g-tensor axes in t he temperature range from 4.2 K to 300 K. The rigid lattice g-factors at 4.2 K are: g(z) = 2.428, g(y) = 2.138, and g(x) = 2.058. The g(x) v alue very slowly increases linearly on heating, whereas g(x) and g(y) nonlinearly tend towards their average value. This is a typical behavi our of Cu(II) EPR in Tutton salt crystals, however the g(T) dependence is much less pronounced in (NH4)2Cu(BeF4)2.6H2O as compared to the Cu (II) doped salts. The results are described in terms of the two vibron ic coupling models of the strong Jahn-Teller effect. It is shown that the models are not adequate to describe the vibronic g-factor behaviou r in paramagnetic Cu(II) crystals which is attributed to the cooperati ve Jahn-Teller effect existing in these crystals.