The attenuation and velocity of ultrasonic waves of frequencies in the range of 10 to 70 MHz have been measured in Li2OxB2O31−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 hightemperature 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 BO” 3 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.

ULTRASONIC RELAXATIONS, ANHARMONICITY AND FRAGILITY OF LITHIUM BORATE GLASSES

CARINI, GIOVANNI;CARINI, Giuseppe;D'ANGELO, Giovanna;TRIPODO, Gaspare;
2005

Abstract

The attenuation and velocity of ultrasonic waves of frequencies in the range of 10 to 70 MHz have been measured in Li2OxB2O31−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 hightemperature 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 BO” 3 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.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11570/1433944
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