ON THE THERMAL-EXPANSION OF BETA-CRISTOBALITE

We have measured the temperature dependence of the cell parameter of c ubic beta-cristobalite up to 1300 degrees C by high-precision X-ray po wder diffraction. The thermal expansion coefficient decreases on heati ng, until above 1000 degrees C the cell parameter is virtually constan t in value. We discuss this change in the thermal expansion with refer ence to the behaviour of low-frequency rigid unit modes and fluctuatio ns associated with the alpha-beta phase transition.