COMPRESSION MECHANISMS AND EQUATIONS OF STATE

The derivations of equations of state to describe the volume-pressure variation of a solid are based upon certain assumptions about the prop erties of the solid. For finite strain equations of state, these assum ptions include homogeneity and isotropy of the strain distribution in the sample. the continuous differentiability of the equation of state parameters with respect to extensive variables, and the assumption tha t terms involving higher-order powers of the finite strain do not cont ribute significantly to the free energy of the material. We examine th ese assumptions and demonstrate that, within the experimental uncertai nties, crystalline solids with no or limited degrees of internal struc tural freedom compress in the manner predicted by finite strain equati ons of state, even though in some cases the assumptions involved in th e derivation of the equation of state are demonstrably violated. In mo re complex structures with a larger number of degrees of structural fr eedom, a variety of behaviour is observed; most undergo continuous str uctural change with increasing pressure and the evolution of the volum e with pressure again follows that predicted by the finite strain equa tions of state. However, a significant number of complex structures un dergo changes in compression mechanism which, in some cases, result in significant deviations from the behaviour predicted by the equations of state.