Magnetic destabilization of Ni₇Al

Previously unknown cubic ordered Ni₇Al and Cu₇Pt compounds have recently been theoretically predicted to be stable phases in the Ni-Al and Cu-Pt systems. While Cu₇Pt was subsequently synthesized and identified, Ni₇Al remains experimentally unobserved. Using first-principles total energy calculations, we reinvestigate the stability of this Ni₇Al compound. We find the stability of this compound to be qualitatively effected by spin polarization, ignored in previous calculations. The effect of ferromagnetism is to stabilize the two-phase mixture of Ni+Ni3Al relative to the Ni₇Al compound such that the latter is stable in nonmagnetic calculations, but unstable when spin polarization is taken into account. This reversal of relative stabilities of Ni₇Al and Ni+Ni₃Al with magnetism also has a dramatic effect on the calculated Ni₃Al/Ni interfacial energy s and spin-polarized calculations lead to a positive value of s, which is in qualitative agreement with values obtained from precipitation experiments.