Ultrasonic relaxations, anharmonicity, and fragility in lithium borate glasses

The attenuation and velocity of ultrasonic waves of frequencies in the range of 10 to 70 MHz have been measured in (Li2O)x(B2O3)1-x borate glasses as a function of temperature between 1.5 and 300 K. Two distinct features characterize the attenuation behavior: (i) a plateau at temperatures below 10 K, (ii) a broad high temperature peak. The former feature is interpreted in terms of the phonon-assisted relaxation of two level systems and the latter by assuming the existence of a distribution of thermally activated relaxing centers. The spectral density of two-level systems results to be independent on the alkali oxide content, while the density of relaxing particles decreases with increasing lithium ion concentration supporting their association to the triangular BO3 units building up the borate skeleton (O=oxygen atom bridging between two network-forming ions, i.e., boron ions). The comparison between the number densities of two-level systems and of relaxing particles indicates that only a small fraction of the locally mobile defects are subjected to tunneling motions. At temperatures below 100 K the sound velocity is mainly governed by the relaxation contribution, while above 100 K it is regulated by the vibrational anharmonicity and shows a nearly linear temperature dependence, whose slope strongly depends on the concentration of network modifier ions (Li+ ions). The determination of the average thermal Grüneisen parameters permits us also to disclose the existence of a distinct correlation between anharmonicity and fragility of lithium borate glasses: a growing fragility is predictive of an increasing anharmonicity.