Phase transitions between NiAs- and MnP-type phases

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 $C_{6\nu }^4$) are related distortions, as both correspond to the same irreducible representation $M_4^{-}$ of the NiAs-type structure (space group $D_{6h}^4$).