Real-space rigid-unit-mode analysis of dynamic disorder in quartz, cristobaliteand amorphous silica

Abstract

We use a recently developed tool based on geometric algebra toanalyse the phase transition in quartz, the nature of thedisordered high-temperature phase of cristobalite and thedynamics of silica glass. The approach is to analyseconfigurations generated by the reverse Monte Carlo or moleculardynamics simulations in terms of rigid-unit-mode (RUM) motions,but concentrating on quantifying the real-space distortionsrather than performing a reciprocal-space analysis in terms ofRUM phonons. One of the important results is a measure of theextent to which the amplitudes of motion are directlyattributable to RUMs, and how the RUM fraction changes as aresult of a phase transition.