| Abstrakt: |
First-principles calculations were used to study intermetallic compounds in the Nb-Ge system, to gain a better understanding of the phase diagram and resolve conflicts reported in the literature. The enthalpy of formation with regard to temperature was calculated for all the intermetallic compounds, to investigate phase stabilities and phase equilibria at low and elevated temperatures. These results, combined with the electronic DOS, suggest that the tI32 (WSi-type) NbGe and NbGe compounds are stable over the whole temperature range. The stoichiometric cP8 NbGe becomes stable close to its melting temperature. Regarding different compositions of the cP8 NbGe, the calculations suggest the (Nb)(Nb,Ge) model for the NbGe phase instead of the proposed model, (Nb)(Nb,Ge,Va), where Va represents vacancy. The calculations show that the tI32 (CrB-type) NbGe, hP16 (MnSi-type) NbGe and NbGe compounds should be considered metastable. The elastic constants, bulk, shear, and Young's modulus, Poisson's ratio, and Debye temperature of the Nb, Ge, cP8 NbGe, tP32 NbGe, tI32 (CrB-type) NbGe, tI32 (WSi-type) NbGe, hP16 (MnSi-type) NbGe, NbGe and NbGe were calculated. These phases were found to be mechanically stable. Using the Cauchy pressure, Pugh's index of ductility, and the Poisson's ratio as criteria, the calculations suggest that the tI32 (CrB-type) NbGe and NbGe intermetallics should be brittle (with the latter being the most brittle) and the cP8 NbGe, tP32 NbGe, hP16 NbSi and NbGe ductile (with cP8 NbGe being the most ductile). [ABSTRACT FROM AUTHOR] |
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