Phonons and Thermal Expansion Behavior of NiSi and NiGe

Abstract

We have carried out first principles calculations of the vibrational and thermodynamic behavior in NiSi and isostructural compound NiGe. Phonon density of states has also been measured in NiSi using inelastic neutron scattering techniques. We find that the vibrational spectra of the two compounds are very different, due to the difference in the size and mass of Si and Ge. Interesting anomalous thermal behavior of NiSi due to anharmonic phonons is brought out well in our calculations, particularly the negative thermal expansion (NTE) along the b-axis of the orthorhombic unit cell. Large difference in thermal expansion behavior of NiSi and NiGe is very well reproduced by the calculations. Additionally, calculations enable to identify the phonon modes which lend major contribution to the negative thermal expansion behavior in NiSi, and reasons for negligible NTE in NiGe. Such typical representative modes at the zone-boundary along b-axis involve transverse vibrations of Si/Ge along c-axis.PACS numbers: 78.70.Nx, 63.20.-e, 65.40.–b.

Kondo-Induced Giant Isotropic Negative Thermal Expansion

“…Adding further interest to this material are reports of an exceptional occurrence of strong, isotropic, thermally robust and tunable negative thermal expansion (NTE) [12][13][14]. Although discoveries of materials with strong NTE over wide temperature ranges have been greatly accelerated during the past two decades, thereby boosting the potential for integration in state-of-the-art technologies [15,16], detailed understanding of the underpinning mechanisms, and the extent to which they can be enumerated is still under debate.…”

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

“…FeSi, CoSi and similar compounds have been extensively studied with above applications in mind, and also for their various unusual physical and chemical properties. FeSi, a fascinating material, has been studied for its unusual magnetic and thermodynamic properties like thermal expansion, elastic behaviour, heat capacity, Seebeck coefficients, magnetic susceptibility and insulator-metal transition with increasing temperature 30 . Phonon density of states have been studied in FeSi, both 31 experimentally and using abinitio molecular dynamics, to investigate softening of phonons and to understand the coupling between phonons and electronic structure in order to explain anomalous thermal expansion.…”