Structure and stability of high pressure synthesized MgTM2H6 (TM = Zr, Nb) hydrides
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The structure of two isomorphous high pressure synthesized metal hydrides MgZr2H6 and MgNb2H6 is described in the R-3m space group with the hexagonal unit cells a = 3.3592(2) angstrom, c = 25.131(3) angstrom and a = 3.2901(9) angstrom, c = 23.09(1) angstrom, respectively (Z = 3). The metal atom structures were refined from powder XRD data, with Mg in the 3a site (0, 0, 0,) and Zr and Nb in the 6c site (0, 0, z) with the only refineable atomic parameter z = 0.225 and z = 0.220, respectively. As only a minute sample could be produced in the anvil cell, a conventional structural determination with neutron diffraction on a deuterated sample could not be made. Instead formation energies of the all proposed structural models were calculated using density functional theory (DFT) for accessing the most stable structure of the lowest energy level. The measured lattice parameters and the refined atomic positional parameters from the powder X-ray diffraction pattern were reproduced well by the OFT calculation. The hydrogen atoms were found to fully occupy an octahedral site coordinated by 3Mg and 3Zr atoms, a tetrahedral site coordinated by 4Zr atoms and another tetrahedral site coordinated by 3Mg and 1Zr atoms. Chemical, mechanical and dynamical stabilities of the MgZr2H6 structure were further discussed based on a chemical potential equilibrium diagram constructed from the calculated formation energies, calculated elastic constants and phonon spectra respectively. It is also shown that a high pressure increases the stability of the new hydrides. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.