Metal−Metal Bonding in ScTaN₂. A New Compound in the System ScN−TaN

Abstract

Gray microcrystalline powders of ScTaN(2) were prepared from solid-state reactions of delta-ScN with Ta(3)N(5) powders at T = 1770 K. According to thermal analyses the compound is stable against oxidation by O(2) up to temperatures of T = 800 K. In an Ar atmosphere ScTaN(2) decomposes above T = 1250 K and in a N(2) atmosphere above T = 2000 K under release of N(2) to form delta-ScN and beta-Ta(2)N. The crystal structure (space group P6(3)/mmc, No. 194, a = 305.34(3) pm, c = 1056.85(9) pm, Z = 2) was refined on the basis of X-ray and neutron powder diffraction data. It comprises alternating layers of ScN(6/3) octahedra and trigonal TaN(6/3) prisms, which are also observed in the binary nitrides delta-ScN and theta-TaN, respectively. A small degree of anti-site defects (about 5%) was detected. Only a small solubility of ScN in epsilon-TaN was observed, while the solubility of TaN in delta-ScN is >/=10 mol % at T = 1820 K. ScTaN(2) is a diamagnetic small gap semiconductor or a semimetal, as inferred from magnetization and electrical resistivity measurements, consistent with band structure calculations. Chemical bonding analyses with the COHP method yield significant covalent Ta-Ta interactions. Topological analyses of the electron localization function reveal unexpected Ta-Ta three-center bonding basins within seemingly empty trigonal prisms of the TaN(6/3) layers.

Phase stability of ScN-based solid solutions for thermoelectric applications from first-principles calculations

“…These results of our calculation agree with the fact that this phase has been experimentally synthesized in bulk as ScTaN 2 and ScNbN 2Àd . 45,46 Our results indicate that the corresponding phase, ScVN 2 , should exist in the Sc-V-N system. However, the latter system might deserve a more detailed future study as calculations have obtained energies for WC-type VN to be lower than the experimentally observed B1 VN phase making predictions in the VN-related system more difficult.…”

“…It consists of a layered structure with two subsystems which are rocksalt ScN-like and a hexagonal related structure of TaN (WC structure type). 45 When we consider the ground state structure of the binary phase of TaN and NbN, there are competitive phases with hexagonal base structure (TaN has e-phase structure and NbN has WC structure). [54][55][56] These two binary systems have lower formation enthalpy than their rocksalt phase as plotted in Figure 4.…”

“…Finally, the ScTaN 2 -type inherently nanolaminated structures could potentially be promising structure for thermoelectric application in themselves, since they are thermodynamically stable and have been reported to have semimetallic character. 45 Thus, Nb or Ta is a good choice to use for ScTaN 2 -type structures since both of them have heavy mass difference which give more effect on lattice thermal conductivity reduction as compared to V.…”

Metal to semiconductor transition and phase stability of Ti1−xMgxNyalloys investigated by first-princi

“…The SrSiN 2 -type structure is 0.044 eV/atom higher in energy than the B1-L1 1 structure. The structure of LiUN 2 , the inverse MAX-phase structure of, e.g., ScTaN 2 [65], and the structure of LiPN 2 , also referred to as KCoO 2 -type structure, as well as all other considered cases, are even higher in energy. As a last verification step of the stability of TiMgN 2 , I search for possibly relevant competing ternary Ti-Mg-N phases based on other stoichiometries with structures that are observed in related ternary systems.…”

Octahedral and trigonal-prismatic coordination preferences in Nb-, Mo-, Ta-, and W-based ABX2 layered oxides, oxynitrides, and nitrides

“…The structural difference between octahedral and trigonal-prismatic coordination has been theoretically studied in layered sulfides and selenides by Kertesz and Hoffmann: They attributed the differences to variations in the electron count and overlap of the metal-metal orbitals [15]. Although some studies have computationally predicted this metal-metal overlap [16,17], it has rarely been experimentally visualized. Compounds beyond layered ABX2 oxides and nitrides have not been systematically studied, which is necessary for further investigation of related materials.…”