Abstract
The lattice and elastic instabilities of rocksalt (RS) calcium oxide CaO under pressure areextensively studied to reveal the physically driven mechanism of the phase transition fromRS to CsCl structure by using the pseudopotential plane-wave method withindensity-functional theory. The predicted phase transition pressure is 66.38 GPa, employingthe total energy method, which falls in the experimental transition pressure rangeof 60–70 GPa. A pressure-induced soft transverse acoustic (TA) phonon modeis identified at the zone boundary X point in the Brillouin zone, signifying astructural instability. A predicted charge transfer from Ca to O with pressure mightbe attributable to the phonon softening. Moreover, a softening behavior in theC44 shear modulus with pressure is predicted. Analysis of the calculated results suggested that,with increasing pressure, the predicted TA phonon softening behavior, instead ofC44 shear modulus instability, is mainly responsible for the pressure-induced structural phasetransition. We also find that the bond between Ca and O becomes more ionic undercompression from Mulliken population analysis.
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