Vibronic Effects in EPR Spectra of (NH₄)₂Cu(BeF₄)₂·6H₂0 Single Crystal

Abstract

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. = 2.428, gy = 2.138, and gx = 2.058. The gx value very slowly increases linearly on heating, whereas gz and gy 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 (NH4)2Cu(BeF4)2 •6H20 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.

Vibronic Behavior and Electron Spin Relaxation of Jahn−Teller Complex Cu(H2O)62+ in (NH4)2Mg(SO4)2·6H2O Single Crystal

“…Moreover, still open questions exist in paramagnetic salts with Jahn−Teller active ions, particularly for Cu 2+ in Tutton salt type crystals of general formula M‘ 2 M‘‘(SO 4 ) 2 ·6H 2 O where M‘ = NH 4 , K, Rb, Cs, Tl and M‘‘ = Cu, Zn, Cd, Mg, Co, Ni, Mn. The fluxional behavior of the Cu(H 2 O) 6 2+ octahedra was observed by EPR both in copper(II)-doped diamagnetic Tutton salts − and in pure Cu(II) salts. − The molecular and crystal structure details and their temperature variations were studied in recent years with increasing accuracy by X-ray and neutron diffraction in hydrogeneous − and deuterated salts, − and a unified interpretation of the structural and fluxional behavior has recently been proposed …”

“…The models assume that the adiabatic potential surface is temperature-independent. This assumption was questioned, and it was suggested that the lattice strains can vary with temperature, producing variations in energy barrier between the wells. ,, …”

“…The Jahn−Teller dynamics can be influenced by cooperative elastic coupling between Cu 2+ ions. Effects such as an increase in the barrier height, a change in the shape of the g -factor temperature dependence, an increase in the Jahn−Teller radius ρ, and change in the linear coupling coefficient A 1 in copper(II) Tutton salts as compared to the Cu 2+ -doped diamagnetic salts were pointed out as a possible effects due to a cooperative coupling. ,, The cooperative elastic coupling can appear also in magnetically diluted systems, but the percolation threshold for an influence of the coupling on the Jahn−Teller dynamics in Tutton salts is not known, although our preliminary data suggest that it can be below of 1% of copper(II) concentration. This is in contradiction to the Jahn−Teller systems with the three-dimensional array of pulsating octahedra linked with each other by the common corners, where the threshold concentration of Cu 2+ producing cooperative Jahn−Teller phase transition was found as x = 0.23 in Ba 2 Zn 1 - x Cu x O 6 , as x = 0.40 in K 2 Zn 1 - x Ca x F 4 , and as x = 0.59 in Rb 2 Mn x Mg 1 - x F 4 for the Jahn−Teller deformation ordering.…”

Electron Paramagnetic Resonance Characteristics and Crystal Structure of a Tutton Salt Analogue: Copper-Doped Cadmium Creatininium Sulfate

“…Using this approach, Silver and Getz were able to successfully explain the variation of the g and A Cu tensors in ZKH, finding an energy difference δ 12 = 75 cm –1 between the lowest two valleys . This work was soon followed by similar SG analysis of this fluxation behavior of Cu 2+ (H 2 O) 6 in other Tutton salts ,,− and analogues , and found δ 12 values ranging from ∼75 to 300 cm –1 for the magnetically dilute crystals and ∼300 to 500 cm –1 for the pure copper Tutton crystals.…”

“…Several of these EPR studies revealed limitations of the SG model with the most apparent being the fact that δ 12 is not constant over the entire temperature range. ,,,− In other systems, the tensor variations were not fully predicted, ,, which was initially suggested as due to a non-Boltzmann thermal equilibrium at low temperatures but later also from temperature-dependent changes in the lattice strains. In a few other cases, g min was observed to vary with temperature, , and therefore a three-state SG model was applied. ,, …”