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

The relaxation losses and the corresponding velocity variations, observed at ultrasonic frequencies in alkali borate glasses between 1.5 and 300 K, have been modelled by an asymmetric double well potential model having a distribution of both the barrier potential and the asymmetry. It is shown that the relaxation strength I and the spectral density of asymmetries w0 decreases markedly with decreasing cation size. Below 10 K the sound attenuation is regulated by the phonon-assisted relaxation of tunnelling systems and exhibits a tunnelling strength C, ranging between 10-4 and 10-3. At variance with the behaviour observed for I, C slightly increases with decreasing cation size and is more than one order of magnitude smaller than I. It is concluded that, differently from classical relaxing states, tunnelling systems are independent of bond strengths and of structural changes characterizing a glassy network, confirming their inherent universality. Above about 120 K the ultrasonic velocity is mainly regulated by vibrational anharmonicity and shows a nearly linear decrease as the temperature is increased, the slope scaling with the cation size. Taken together, the observations point to the existence of a distinct correlation between anharmonicity and local mobility in the glassy network.