# Phonons and Anisotropic Thermal Expansion Behaviour of NiX (X = S, Se, Te)

## TL;DR

This study combines first-principles calculations and experimental measurements to analyze phonons and anisotropic thermal expansion in NiX (X = S, Se, Te), revealing good agreement between theory and experiment.

## Contribution

It provides the first combined theoretical and experimental analysis of phonons and anisotropic thermal expansion in NiX compounds, including inelastic neutron scattering and X-ray diffraction data.

## Key findings

- Phonon spectrum matches well between calculations and experiments.
- NiSe exhibits significant anisotropic thermal expansion.
- Displacement patterns of phonons relate to anisotropic expansion behavior.

## Abstract

Metal Chalcogenides have been known for important technological applications and have attracted continuous interest in their structure, 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 a parameter. The ab-initio calculated anisotropic Gr\"uneisen parameters of the different phonon modes in all the chalcogenides along with the elastic constants are used to compute anisotropic thermal expansion behviour, which is found in good agreement with experiments. The displacement pattern of phonons indicate that difference in amplitudes of Ni and X atoms follow the anisotropy of thermal expansion behavior along c- and a-axis.

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Source: https://tomesphere.com/paper/1902.08005