Watanabe Yuto, Tayran Ceren, Kasem Md Riad, Yamashita Aichi, Çakmak Mehmet, Katase Takayoshi, Mizuguchi Yoshikazu
Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo, 192-0397, Japan.
Department of Physics, Faculty of Science, Gazi University, Ankara, 06500, Turkey.
Sci Rep. 2024 Nov 14;14(1):28018. doi: 10.1038/s41598-024-79353-8.
A large uniaxial negative thermal expansion (NTE) along the c-axis has recently been observed in the transition metal (Tr) zirconides TrZr with a tetragonal CuAl-type structure. A recent study on FeZr₂ [M. Xu et al., Nat. Commun. 14, 4439 (2023)] suggests that optical phonons play a critical role in inducing the NTE along the c-axis. In this study, we investigate the thermophysical properties of TrZr₂ compounds (Tr = Fe and CoNi(x = 0, 0.2, 0.4, 0.6, 0.8, 1)) using specific heat measurements, sound velocity data, and theoretical phonon calculations to achieve our aim of clarifying the contribution of optical phonons to the uniaxial NTE along the c-axis observed in both FeZr₂ and CoZr₂. We found that FeZr shows a lattice-specific heat peak structure at 8.90 meV, which corresponds to optical phonon energy with a high population of negative Grüneisen parameter along the c-axis in the phonon dispersion curves in FeZr. In an examination of a chemical substitution effect on the CoNiZr, we found that the lattice-specific heat peak structure disappeared for x ≥ 0.4 and the oscillator intensity decreased. Phonon calculations revealed the existence of low-energy optical phonon branches at the Γ point for CoZr and FeZr with uniaxial NTE along the c-axis. However, the low-energy phonon branches were not found in NiZr with uniaxial positive thermal expansion along the c-axis. The increase in phonon density of states near the above optical phonon energy in CoZr and FeZr is consistent with the lattice-specific heat analyses, and we propose that low-energy optical phonons are essential for the exhibiting of uniaxial NTE along the c-axis in TrZr.
最近在具有四方CuAl型结构的过渡金属（Tr）锆化物TrZr中观察到沿c轴的大的单轴负热膨胀（NTE）。最近对FeZr₂的一项研究[M. Xu等人，《自然·通讯》14, 4439 (2023)]表明，光学声子在诱导沿c轴的NTE中起关键作用。在本研究中，我们使用比热测量、声速数据和理论声子计算来研究TrZr₂化合物（Tr = Fe和CoNi(x = 0, 0.2, 0.4, 0.6, 0.8, 1)）的热物理性质，以实现我们的目标，即阐明光学声子对在FeZr₂和CoZr₂中观察到的沿c轴的单轴NTE的贡献。我们发现FeZr在8.90毫电子伏特处显示出晶格比热峰结构，这对应于FeZr中声子色散曲线沿c轴具有高负格鲁尼森参数的光学声子能量。在研究CoNiZr的化学取代效应时，我们发现当x≥0.4时晶格比热峰结构消失且振子强度降低。声子计算揭示了对于沿c轴具有单轴NTE的CoZr和FeZr，在Γ点存在低能光学声子分支。然而，在沿c轴具有单轴正热膨胀的NiZr中未发现低能声子分支。CoZr和FeZr中上述光学声子能量附近的声子态密度增加与晶格比热分析一致，并且我们提出低能光学声子对于TrZr中沿c轴表现出单轴NTE至关重要。
