Phonons and anisotropic thermal expansion behavior of NiX (X = S, Se, Te)

Abstract

Metal chalcogenides have been known for their important technological applications and have attracted continuous interest for their structural, electronic, thermal, and transport properties. Here, we present first-principles calculations of the vibrational and thermodynamic properties of NiX (X = S, Se, Te) compounds along with inelastic neutron scattering measurements of the phonon spectrum in NiSe. The measured phonon spectrum is in very good agreement with the computed result. We also report the measurement of thermal expansion behavior of NiSe using X-ray diffraction from 13 K to 300 K. The change in the hexagonal c lattice parameter in NiSe is considerably greater as compared to that in the a-parameter. The ab initio calculated anisotropic Grüneisen parameters of the different phonon modes in all the chalcogenides along with the elastic constants are used to compute anisotropic thermal expansion behavior, which is found in good agreement with experiments. The displacement pattern of phonons indicates that the difference in the amplitudes of Ni and X atoms follows the anisotropy of thermal expansion behavior along c- and a-axes.