NMR Determination of Phase Diagrams in Crystals with Orientational Disorder

Pulsed nmr techniques have been used to determine the melting curves of cyclohexane and hexamethyldisilane (HMDS) and the brittle-to-plastic solid state transition in HMDS over a pressure range to 2.8 k bar. Inherent pressure hysteresis at both melting and solid-solid transitions is attributed to a “superpressing” phenomenon analogous to supercooling. The solid-solid transition in HMDS exhibits a “region of indifference” attributed to a distribution of local pressures within the polycrystalline sample. The nmr technique has the advantage that the proportion of different phases present in a sample may be determined.