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A three-body-interaction potential (TBP) model has been formulated by incorporating the effects of long-range Coulomb and three-body interactions and short-range van der Waals and overlap repulsion, effective up to second neighbors. The three-body interactions arise from the electron-shell deformation when the nearest-neighbor ions overlap. This TBP has been employed for detailed studies of pressure-induced phase-transition and high-pressure behavior of divalent metal oxides. The model has yielded somewhat more realistic predictions of the phase-transition and highpressure behavior as compared to those derived from the usual two-body potentials based on phenomenological and ab initio approaches.

A theoretical study of the phase transition and high‐pressure behaviour of MgO and CaO crystals using a three‐body potential model is carried out. The quantities calculated are relative stability, phase‐transition pressure, relative volume changes in the rock‐salt (B1)‐cesium chloride (B2) phase trunsition, and shear instability. The high‐pressure elastic behaviour is compared with other theoretical model predictions. The effect of three‐body interactions is found to shift the predictions of phase‐transition pressure and shear instability to higher values and relative volume changes to lower values compared to the two‐body potential model of Mackrodt and Stewart. The present results of phase transition for MgO (302 GPa) and CaO (115 GPa) are also close to those of the modified electron gas (MEG) theory. The three‐body effects are found more pronounced in MgO than in CaO, consistent with the higher values of Cauchy deviations and lower valucs of the cation to anion ratio.

The phase transition pressure and relative stability of MgO and CaO crystals are successfully predicted by means of two well‐known potentials developed by Sangster and Stoneham and Mackrodt and Stewart. The transition pressures and volume decrease in these oxides are found to be 202 and 108 GPa and 4 and 6%, respectively, and compare reasonably with the measured data on CaO (70 ± 10 GPa). The calculated phase diagrams for these oxides are well in agreement with the experimental compression curves and electron gas model calculations.

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