Predicting the vibrational contribution to thermal and EOS properties of Ni₃X (X = In, Sn) intermetallic compounds

Two computational approaches to the vibrational part of thermal properties and equation-of-state parameters of the Ni3Sn(hP8), Ni3Sn(cP4), Ni3In(hP8) and Ni3In(cP4) intermetallic phases, of interest in the study of lead-free soldering materials, have been critically compared. The first approach, referred to as the "phonon method" (PHM) combines the zero-kelvin energy versus volume values obtained ab initio using the VASP, with an account of the vibrational frequencies provided by the PHONOPY code. The second method, the "quasi-harmonic Debye approach" (QDA), implemented in the GIBBS Program, introduces a volume-dependent Debye temperature theta(D)(V). Extensive comparisons between the predictions of the PHM and the QDA are reported, which show significant discrepancies. In particular, it is found that the QDA systematically underestimates the entropy of the current phases. The discrepancy is explained in terms of the difference between theta(D)(V), estimated by the GIBBS Program from the elastic properties, and the Debye temperature theta(D)(0), which accounts for the high-temperature entropy, to be determined by the logarithmic average of all phonon frequencies. A discussion is also reported of the possible effect upon the discrepancies of varying the only parameter kept constant in the QDA calculations, viz., the Poisson ratio.