Dutt Rajeev, Pandey Dhanshree, Chakrabarti Aparna
Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai-400094, India.
Theory and Simulations Laboratory, Human Resources Development Section, Raja Ramanna Centre for Advanced Technology, Indore-452013, India.
J Phys Condens Matter. 2020 Oct 28;33(4). doi: 10.1088/1361-648X/abbb40.
In this work, using density functional theory based electronic structure calculations, we carry out a comparative study of geometric, mechanical, electronic, magnetic, and thermoelectric properties of CoTaalloys, where= Si, Ge and Sn and= 1 and 2. In the present study, a systematic approach has been taken to perform calculations to probe the possibility of existence of a tetragonal (martensite) phase in these alloys and also to perform a comparative study of various physical properties of the six systems, mentioned above, in the cubic and possible tetragonal phases. From our calculations, a tetragonal phase has been found to be stable up to about 400 K in case of CoTaSi and CoTaGe alloys, and up to about 115 K for CoTaSn, indicating the presence of room temperature cubic phase in the latter alloy unlike the former two. Further, the results based on the energetics and electronic structure have been found to corroborate well with the elastic properties. All the above-mentioned full Heusler alloys (FHAs) show magnetic behavior with metallicity in both the phases. However, their half Heusler counterparts exhibit non-magnetic semi-conducting behavior in the cubic phase. We calculate and compare the thermoelectric properties, in detail, of all the materials in the cubic and possible tetragonal phases. In the cubic phase, the half Heusler alloys exhibit improved thermoelectric properties compared to the respective FHAs. Furthermore, it is observed that the FHAs exhibit higher (by about an order of magnitude) values of Seebeck coefficients in their cubic phases, compared to those in the tetragonal phases (which are of the order of only a few micro-volts/Kelvin). The observed behaviors of the transport properties of the probed materials have been analyzed using the topology of the Fermi surface.
在本工作中，我们基于密度泛函理论进行电子结构计算，对CoTa合金（其中 = Si、Ge和Sn且 = 1和2）的几何、力学、电子、磁性和热电性质开展了一项对比研究。在本研究中，我们采用了一种系统的方法来进行计算，以探究这些合金中四方（马氏体）相存在的可能性，并对上述六个体系在立方相和可能的四方相中的各种物理性质进行对比研究。通过我们的计算发现，对于CoTaSi和CoTaGe合金，四方相在约400 K以下是稳定的，而对于CoTaSn合金，四方相在约115 K以下是稳定的，这表明与前两种合金不同，后一种合金在室温下为立方相。此外，基于能量学和电子结构的结果与弹性性质得到了很好的印证。所有上述全Heusler合金（FHAs）在两个相中均表现出具有金属性的磁行为。然而，它们对应的半Heusler合金在立方相中表现出非磁性的半导体行为。我们详细计算并比较了所有材料在立方相和可能的四方相中的热电性质。在立方相中，半Heusler合金相较于各自对应的全Heusler合金表现出改善的热电性质。此外，观察到全Heusler合金在立方相中的塞贝克系数值比在四方相中的更高（大约高一个数量级），四方相中的塞贝克系数值仅为几微伏/开尔文量级。我们利用费米面拓扑结构分析了所探究材料的输运性质的观测行为。
