High-pressure phase of GaP: Structure and chemical ordering

Abstract

The present paper concerns a combined x-ray diffraction and absorption study of gallium phosphide (GaP) at high pressure up to $39\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The aim of this study is twofold: To clarify the nature of the high pressure phase using x-ray diffraction and to determine the degree and the evolution of the short range chemical order using x-ray absorption. The analysis of x-ray diffraction shows that GaP transforms to a Cmcm structure and the absence of the ``difference reflections'' indicates that the Cmcm structure lacks long-range chemical order. In this system, the EXAFS is compatible with the hypothesis of a chemically ordered Cmcm local environment. The comparison between the XANES region of the spectra and multiple scattering calculations confirms this hypothesis, clearly showing that the Cmcm is short-range chemically ordered. The local environment of Ga is given by $6\phantom{\rule{0.3em}{0ex}}\mathrm{P}$ atoms and short-range Ga-Ga interactions are not likely to occur in this system, at least up to $39\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This result shows that even in a compound with a relatively low ionicity of the bonds, this parameter dictates the short-range interactions up to very high pressures.