Pressure-induced changes in mode Gruneisen parameters and general equations of state for solids

The simple models used in a recent paper by the author (see ibid., vol.13, p.4601, 1980) are extended to include higher-order terms. These can be used to predict the pressure-induced changes in mode Gruneisen parameters and the curvatures of frequency versus pressure curves. This then provides a framework for a definition of the 'normal behaviour' of simple solids up to much high pressures than those considered in the earlier paper. The implications for more complex structures are considered, and generalisations for approximate equations of state for solids are attempted.