Structural discrimination via density functional theory and lattice dynamics: Monoclinic ${\text{Mg}}_{2}{\text{NiH}}_{4}$

Structural discrimination via density functional theory and lattice dynamics: Monoclinic ${Mg}_{2} {NiH}_{4}$

J. F. Herbst and L. G. Hector, Jr.

Phys. Rev. B 79, 155113 – Published 16 April 2009

Abstract

Two distinct crystal structures for the monoclinic, low-temperature phase of ${Mg}_{2} {NiH}_{4}$ , which we designate as LTI and LTII, are available in the published literature. We demonstrate that density functional theory and lattice dynamics can easily identify LTII as the preferable structure at two levels of inquiry. First, enthalpies of formation $Δ H$ calculated using three different forms for the exchange-correlation energy functional are in better agreement with experiment for LTII. Second, the phonon spectrum calculated for LTII contains no anomalies while that for LTI exhibits a variety of soft modes. By analyzing the soft modes in LTI as well as those we find for the known ${CaMgNiH}_{4}$ structure with Ca replaced by Mg we derive a crystal structure that closely approximates LTII.