Structural and electronic properties of alkali metal peroxides at high pressures

Abstract

Different phase transition mechanisms and structures have been observed for alkali metal peroxides (Li2O2, Na2O2, K2O2, Rb2O2) under pressure. Phase transition pressures of the studied compounds closely related to the alkali metal electronegativity.

Theoretical predictions for low-temperature phases, softening of phonons and elastic stiffnesses, and electronic properties of sodium peroxide under high pressure

“…5 and Föppl. 6 The diamond represents the Pbam structure predicted by Deng et al 10 Föppl's structures, there was a small signicant atomic displacement of the Na1 and Na2 atoms between the Tallman's and Föppl's structures at room temperature (Fig. S13, ESI †) as discussed before.…”

“…S2, ESI †). Here the Pbam structure is same as the structure proposed by Deng et al 10 when the structure is shied to (0, 0, 1/ 2).…”

“…Thus, the rst structural phase transition is predicted at 22 GPa, the Amm2 structure transforming to the P2 1 /c structure, and the second structural phase transition is predicted at 28 GPa, the P2 1 /c structure transforming to the Pbam structure. The transition pressure at 28 GPa is the same value as predicted by Deng et al 10 To investigate the driving force for the structural phase transitions, the changes of lattice constants and volume for the Amm2, P2 1 /c, and Pbam structures at high pressures were considered (Fig. S9, ESI †).…”

“…They predicted that the hexagonal P 62m structure transforms to the orthorhombic Pbam structure at around 28 GPa. 10 However, since the high-pressure phases of Na 2 O 2 are waiting for experimental conrmation, the structural phase transition is an open question for further theoretical prediction, which has been conducted in the present work.…”

“…As for previous high-pressure study, Deng et al 10 investigated the structural phase transition of Na 2 O 2 in the range of 0-100 GPa using rst-principles calculations. They employed the particle swarm optimization algorithm (CALYPSO) to search the lowest enthalpy structures at given pressures and optimized them using the projected augmented-wave potentials with 3s 1 p 0 and 2s 2 2p 4 as valence electrons for Na and O, respectively, and the Perdew-Burke-Ernzerhof (PBE) functional as implemented in the Vienna Ab initio Simulation Package (VASP).…”