Phonon and elastic instabilities in rocksalt calcium oxide under pressure: a first-principles study

Abstract

The lattice and elastic instabilities of rocksalt (RS) calcium oxide CaO under pressure are extensively studied to reveal the physically driven mechanism of the phase transition from RS to CsCl structure by using the pseudopotential plane-wave method within density-functional theory. The predicted phase transition pressure is 66.38 GPa, employing the total energy method, which falls in the experimental transition pressure range of 60-70 GPa. A pressure-induced soft transverse acoustic (TA) phonon mode is identified at the zone boundary X point in the Brillouin zone, signifying a structural instability. A predicted charge transfer from Ca to O with pressure might be attributable to the phonon softening. Moreover, a softening behavior in the C(44) shear modulus with pressure is predicted. Analysis of the calculated results suggested that, with increasing pressure, the predicted TA phonon softening behavior, instead of C(44) shear modulus instability, is mainly responsible for the pressure-induced structural phase transition. We also find that the bond between Ca and O becomes more ionic under compression from Mulliken population analysis.

Calcium peroxide from ambient to high pressures

“…CaO undergoes a transition from the rocksalt (F m 3m) to the CsCl (P m 3m) structure (the B1-B2 transition) around 60-65 GPa. Diamond-anvil-cell experiments indicate a transition pressure of 60±2 GPa at room temperature [25], while DFT calculations give a transition pressure of 65-66 GPa [26,27]. We find a pressure of 65 GPa in the present study, and we therefore use the F m 3m rocksalt structure below 65 GPa and the CsCl structure at higher pressures.…”

“…Diamond-anvil-cell experiments indicate a transition pressure of 60 AE 2 GPa at room temperature, 25 while DFT calculations give a transition pressure of 65-66 GPa. 26,27 We find a pressure of 65 GPa in the present study, and we therefore use the Fm% 3m rocksalt structure below 65 GPa and the CsCl structure at higher pressures. The bulk modulus of Fm% 3m-CaO has been measured to be 104.9 GPa, 28 while our static-lattice DFT calculations give a value of 108.5 GPa.…”

Polymorphs of CaSeO4 under Pressure: A First-Principles Study of Structural, Electronic, and Vibrational Properties

“…57, Z = 4) structures have also been considered as postscheelite phases . Finally, we included in our study the decomposition of CaSeO 4 under pressure to form CaO + SeO 3 . However, we found that the subproducts are not energetically competitive against the polymorphs considered in this work.…”

Thermal properties of materials from ab-initio quasi-harmonic phonons

“…Besides, pressure dependence of ab initio compressional and shear sound velocities is in consistent with seismic observations for the Earth's lower mantle (Karki et al 1999). In contrast with these successes, the calculated thermal properties of the B1 and B2 phases of CaO (Karki & Wentzcovitch 2003) are inconsistent with experimental data, and this is most likely due to the too small lattice parameter predicted by the LDA, as later investigations based on a GGA XC functional seem to indicate (Zhang & lai Kuo 2009).…”