Effect of cation sizes on tunnelling states, relaxations and anharmonicity of alkali borateglasses

Abstract

The relaxation losses and the corresponding velocity variations, observed at ultrasonic frequencies in(M2O)0.14(B2O3)0.86 alkali borateglasses (M = Li, K, Cs) between 1.5 and 300 K, have been modelled by an asymmetricdouble-well potential model having a distribution of both the barrierpotential and the asymmetry. It is shown that the relaxation strengthC* and the spectraldensity of asymmetries f0 decreases markedly with decreasing cation size. Below 10 K the sound attenuation is regulatedby the phonon-assisted relaxation of tunnelling systems and exhibits a tunnelling strengthC, rangingbetween 10−4 and 10−3. At variance with the behaviour observed forC*,C slightly increases with decreasing cation size and is more than one order of magnitude smaller thanC*. It is concluded that, differently from classical relaxing states, tunnelling systems areindependent of bond strengths and of structural changes characterizing a glassy network,confirming their inherent universality. Above about 120 K the ultrasonic velocity is mainlyregulated by vibrational anharmonicity and shows a nearly linear decrease as thetemperature is increased, the slope scaling with the cation size. Taken together, theobservations point to the existence of a distinct correlation between anharmonicity andlocal mobility in the glassy network.