Borate anomaly and anharmonicity in sodium borate glasses

Vie thermal expansion coefficient in each binary system of alkali borate glasses shows a minimum termed the borate anomaly. The borate anomaly relates essentially to anharmonic vibrational properties. In order to study the vibrational anharmonicity in sodium borate glasses xNa(2)O-(1-x)B2O3 (0 <= x <= 0.35), the following physical quantities have been measured: the density, the longitudinal and transverse ultrasonic velocities, the linear expansion coefficient, and the temperature coefficients of travel times for longitudinal and transverse ultrasonic waves. From these physical quantities, the four different Anderson-Gruneisen constants representing vibrational anharmonicity are calculated. Each of these constants first decreases to a distinct minimum at x approximate to 0.09 with increasing x, and then increases gradually in the range 0.09 <= x <= 0.17 and steeply in the range 0.17 <= x <= 0.35 with further increasing x. The presence of the minimum at x approximate to 0.09 is a new type of borate anomaly relating to vibrational anharmonicity.