On the Nb-Ge Binary System

Experimental and thermodynamic assessment of the Ge-Nb-Si ternary phase diagram

“…It is suggested that the difference is related to the pseudo potentials used. In this study we are basing the phase diagram on our calculations [8], which appear to be confirmed by the experimental results in the current work (e.g. XRD analysis reported in section 4.0).…”

“…When plotting the diagram using the published dataset a small inadvertent miscibility gap was seen at the Nb 3 Ge-liquid phase boundary (Figure 2a In the previously reported literature [11][12][13][14] there is a conflict over the prototype structure of the Nb 5 Ge 3 phase, whether it has the Cr 5 B 3 or W 5 Si 3 prototype. First principles calculations reported in our previous study using CASTEP [8] indicate that the W 5 Si 3 prototype is more stable than the Cr 5 B 3 structure, over the whole temperature range (at 0 K by -0.1 kJ/mol atom). However, Colinet et al [15] recently performed similar calculations using VASP but instead found that the Cr 5 B 3 prototype was more stable than the W 5 Si 3 prototype (by -0.3 kJ/mol at 0 K).…”

“…2 is based on the latter). Our first principles calculations indicate that this phase should not be stable over the whole temperature range, and is likely metastable or stabilised by impurities [8]. Colinet et al reports a more negative value for Nb 10 Ge 7 , calculated using VASP, however at 0 K this phase is still not stable [15].…”

“…As such in the following assessment, Nb 10 Ge 7 is assumed to be metastable and hence not included in the model. Papadimitriou et al [8] and Colinet et al [15] report the enthalpy of formation for the intermetallics at 0 K calculated using first principles. These values are more negative than those used in the CALPHAD assessment by Geng et al [10], which was fit to the available experimental values.…”

“…A thermodynamic description for the ternary system was developed by extrapolation of thermodynamic descriptions of binary systems (Ge-Si, Ge-Nb and Nb-Si) and optimisation using newly obtained data and previously published work. The thermodynamic description of the Ge-Nb binary phase diagram was modified to take into account recent ab initio data [8]. This paper presents the experimental data obtained along the pseudo binary and the outcomes of thermodynamic modelling.…”