First-principles study of structural phase transitions in hexagonal KNiCl3

The whole phonon-band dispersion of KNiCl3 with the CsNiCl3 structure was calculated by using density functional perturbation formalism. A transverse acoustic (TA) band, in which modes propagate normal to the c axis having displacements of NiCl3 chains along the c axis, was found to be totally unstable. The most unstable mode locates at q = (1/3 1/3 0) in the reciprocal space. The instability of this mode derives from the structural transition from the prototype CsNiCl3 structure (P63/mmc) to the room-temperature KNiCl3 structure (P63 cm). In comparison with RbNiCl3 and CsNiCl3, KNiCl3 has a phonon-dispersion curve indicating the presence of two kinds of instabilities at least in the prototype CsNiCl3 phase. The successive structural transitions of KNiCl3 are discussed from the viewpoint of these instabilities.