New high-pressure phase of MgH₂: An ab initio constant-pressure study

Department of Physics, Texas Tech University - Lubbock, TX, 79409, USA and Department of Advanced Materials and Nanotechnology, Abdullah Gül University - Kayseri, 38039, Turkey

Received: 29 November 2013
Accepted: 5 February 2014

Abstract

The stability of magnesium hydride (MgH₂) at high pressure is studied using a constant-pressure ab initio technique. Two phase transformations are successfully observed through the simulations. The rutile structure undergoes a phase transformation into a CaCl₂-type phase. Further increase in pressure results into a first-order phase transition into an orthorhombic state within Pbcm symmetry. This phase can be considered as a distorted CaF₂-type crystal and does not correspond to the previously proposed MgH₂ phases. The transformation mechanism of the CaCl₂-Pbcm phase change at the atomistic level is successfully characterized and it is found that the CaCl₂-to-Pbcm phase change proceeds via an ideal CaF₂-type intermediate phase. These phase transformations are also analyzed using total energy-volume calculations.