Phase transitions between NiAs- and MnP-type phases

Abstract

The Landau theory of phase transitions is applied to the case of a transition from ${D}_{6h}^{4}$ symmetry to ${D}_{2h}^{16}$ symmetry. The restraints placed upon various thermodynamic quantities by the symmetry change are reported. It is shown that the MnP-type structure (space group ${D}_{2h}^{16}$) and the low-temperature form of NbS (space group ${C}_{6\ensuremath{\nu}}^{4}$) are related distortions, as both correspond to the same irreducible representation ${M}_{4}^{\ensuremath{-}}$ of the NiAs-type structure (space group ${D}_{6h}^{4}$).