A Dense Modification of BaO and Its Crystal Structure

Abstract

BaO displays a reversible phase transformation in the pressure range between approximately 110 and 180 kbar at room temperature. The high‐pressure modification is the first high‐pressure polymorph of an alkaline‐earth oxide to be observed directly. The x‐ray diffraction pattern of the high‐pressure from at about 180 kbar can be satisfactorily indexed as tetragonal with a = 4.397 ± 0.007 Å and c = 3.196 ± 0.005 Å. Although there are intensity discrepancies, the high‐pressure polymorph of BaO appears to have the structure of phosphonium iodide PH₄I (SG P4/nmm), which is a distorted CsCl‐type structure, and contains 2 molecule per unit cell. A transformation to this structure results in a volume decrease about 11–14% at the phase transformation. This study has also been carried out up to pressures of about 290 kbar, no further phase change was observed and the axial ratio c/a appears independent of pressure within experimental error.