Specific heat analyses on optical-phonon-derived uniaxial negative thermal expansion system $Tr$Zr$_{2}$ ($Tr$ = Fe and Co$_{1-x}$Ni$_{x}$)

Yuto Watanabe; Ceren Tayran; Md. Riad Kasem; Aichi Yamashita; Mehmet Ãakmak; Takayoshi Katase; Yoshikazu Mizuguchi

doi:10.1038/s41598-024-79353-8

paper

Specific heat analyses on optical-phonon-derived uniaxial negative thermal expansion system $T r$ Zr $_{2}$ ( $T r$ = Fe and Co $_{1 - x}$ Ni $_{x}$ )

arXiv:2406.18291 · doi:10.1038/s41598-024-79353-8

Abstract

Recently, huge uniaxial negative thermal expansion (NTE) along a $c$ -axis has been observed in transition-metal ( $T r$ ) zirconides $T r$ Zr $_{2}$ with a tetragonal CuAl $_{2}$ -type structure. In a recent study on FeZr $_{2}$ [M. Xu et al., Nat. Commun. 14, 4439 (2023)], the importance of optical phonons to the emergence of the $c$ -axis NTE in FeZr $_{2}$ has been proposed. In this study, the physical properties of $T r$ Zr $_{2}$ ( $T r$ = Fe and Co $_{1 - x}$ Ni $_{x}$ ) have been studied by specific heat, sound velocity measurements, and theoretical phonon calculations to discuss the importance of optical phonons to the emergence of the $c$ -axis NTE in CoZr $_{2}$ and FeZr $_{2}$ . From analyses of lattice specific heat, we found that Ni substitution results in a systematic decrease in oscillator strength for the Einstein modes with 8.74 meV (CoZr $_{2}$ ). From phonon calculations, the low-energy optical phonon branches at the $I^$ point were observed for CoZr $_{2}$ and FeZr $_{2}$ with $c$ -axis NTE, but not in NiZr $_{2}$ with positive thermal expansion. The enhancement of phonon density of states near the above-mentioned optical phonon energy in CoZr $_{2}$ and FeZr $_{2}$ is consistent with the specific heat analyses. We propose the importance of the low-energy optical phonons to the emergence of the $c$ -axis NTE in TrZr $_{2}$ .

14 pages, 9 figures, and 3 tables

Specific heat analyses on optical-phonon-derived uniaxial negative thermal expansion system TrZr2​ (Tr = Fe and Co1−x​Nix​)

Abstract

Specific heat analyses on optical-phonon-derived uniaxial negative thermal expansion system $T r$ Zr $_{2}$ ( $T r$ = Fe and Co $_{1 - x}$ Ni $_{x}$ )