Materials Today Communications
Insight into structural, electronic, elastic and thermal properties of A15-type Nb3X (X = Si, Ge, Sn and Pb) compounds
Graphical abstract
Introduction
Recently, A3B compounds with A15-type structure, in which A is transition metal and B is an element on the right side of the periodic table [1], had been widely used as controllers of the plasma in fusion engineering test reactors due to good phase stabilities and excellent superconducting properties [[2], [3], [4], [5], [6], [7], [8], [9]]. The superconductivity of silicides and germanides of transition metals were measured, and the results showed that the transition temperature TC of V3Si is the highest (17.1 K) among all known binary compounds [10]. Moreover, it had been proved that the high TC and unusual electronically derived properties of V3Ga and V3Si are related to the sharp peak in electronic density of states near to the Fermi level [11]. Therefore, electronic properties are very important for superconducting materials.
Meanwhile, niobium-based A3B compounds with A15-type structure are also important superconducting materials [[12], [13], [14], [15], [16], [17], [18], [19]]. Niobium can form the wide Tc range superconducting compounds with other elements, such as Nb3Ge (23.6 K), Nb3Sn (18.9 K), Nb3Al (18.8 K) [20]. As early as the 1950s and 1960s, Nb3Ge was discovered, and the improvement of the structural order made it possible to increase Tc; meanwhile, it was also known as the compound with high upper critical field value in many A15 superconductors [21,22]. Due to the high Tc values, high critical magnetic fields and high critical current densities, the related properties of niobium-based A3B compounds have been extensively studied theoretically and experimentally [7,17,20,[23], [24], [25], [26]]. For example, the FP-LAPW method in WIEN2K code was used to investigate the electronic properties of Nb3X (X = Al, Ga, In, Ge, and Sn) compounds, the results indicated that Nb-d states can obviously hybridize with X-p states near the Fermi level, and Nb-X chemical bonds show the covalent characteristic [27]. In the investigations using first-principles calculations, there have been many reports about mechanical and electronic properties of Nb3Sn [[28], [29], [30], [31], [32]], mechanical properties of Nb3Ge [33] and Nb3Si [34,35]. However, at present, first-principles calculations of mechanical properties for Nb3Sn, Nb3Ge and Nb3Si are mainly concerned with elastic constants and moduli. Meanwhile, Nb3Pb, in which Pb is in the same group as Si, Ge and Sn, was discovered as a superconductor in 2004 [36], however, no first-principles calculations about Nb3Pb can be found so far. Besides, the discussions of elastic anisotropies and thermal properties of these Nb3X compounds are not deep and systematic.
Generally, the elastic anisotropy of compounds can introduce microcracks under stress due to the relatively large brittleness of compounds. Moreover, thermal properties, such as the sound velocity and Debye temperature, are undoubtedly important for the applications of solids. Furthermore, mechanical properties are related to the bonding characteristics in the compound, and the charge transfer and bonding behavior between atoms can be explored deeply by Mulliken population analysis. Therefore, in this work, the structural, electronic, elastic and thermal properties of Nb3X (X = Si, Ge, Sn and Pb) compounds were investigated using the first-principles calculations. Thess will provide a theoretical basis for future related work on the related compounds.
Section snippets
Computational methods
The CASTEP code [37] based on the density functional theory (DFT) [38] was employed to perform the first-principles calculations to investigate electronic structures, elastic properties, and Debye temperatures of Nb3X (X = Si, Ge, Sn and Pb) compounds. The OTFG ultrasoft pseudo-potentials were used to describe the interactions between ionic-core and valence-electrons. The generalized gradient approximation (GGA) within the Perdew-Burke-Ernzerhof (PBE) program was employed to treat the
Structural properties and phase stabilities
Fig. 1 shows the three-dimensional and top views of A15-type Nb3X (X = Si, Ge, Sn and Pb) compounds with the space group of Pm n (No. 223). Each Nb3X unit cell has six Nb atoms and two X atoms. The structural parameters were optimized firstly before the calculations of elastic and thermal properties, and the optimized parameters, as well as the experimental parameters [40], are listed in Table 1. As can be seen from Table 1, the symmetry of all Nb3X compounds is unchanged after the geometry
Conclusions
In summary, the calculated formation enthalpies, phonon dispersion curves and elastic constants indicate that these Nb3X compounds are thermodynamically, dynamically and mechanically stable. According to electronic properties, Nb-Nb and Nb-X bonds in Nb3X are ionic-covalent mixed bonds. Nb3Si has the largest B, G and E due to the strongest bond strength of chemical bonds in Nb3Si. Poisson's ratio ν, GH/BH, and Cauchy pressure (C12-C44) values indicate that all of these Nb3X are ductile. The
Declaration of Competing Interest
We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the submitted manuscript entitled “Insight into structural, electronic, elastic and thermal properties of A15-type Nb3X (X = Si, Ge, Sn and Pb) compounds”.
Acknowledgments
This work was supported by the Yunnan Ten Thousand Talents Plan Young & Elite Talents Project under Grant no. YNWR-QNBJ-2018-044, the Candidate Talents Training Fund of Yunnan Province under Grant no. 2015HB019, and the National Natural Science Foundation of China under Grant no. 51761023.
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