First-principles investigations of intermetallics in the Ca–Ge system

Abstract

Ca–Ge intermetallics were studied by first-principles calculations within density functional theory (DFT) with the projector augmented-wave technique (PAW); both LDA and GGA were considered. Structural parameters and enthalpies of formation of observed intermetallics in the phase diagram were calculated. Other structures observed in other isoelectronic systems A–B (A=Ca, Sr, Ba; B=Si, Sn, Pb, Ge) were also investigated. GGA results are very close to available experimental data and proved to be more accurate than the LDA to describe this system. Results show that (i) Ca7Ge6 is not a ground state compound; (ii) a new polymorph of CaGe2, not yet reported experimentally, is found to be more stable than all other forms at 0°K at this composition with the GGA; this is qualitatively in disagreement with the LDA that gives it less stable than the accepted one in the phase diagram; (iii) the new structure of Ca3Si4 reported in the Ca–Si system is very competitive energetically in the Ca–Ge system and indications suggest that it may represent a new compound in Ca–Ge; (iv) the experimentally reported cell parameters for Ca7Ge, and even Ca33Ge, are probably of some other compounds and not of the supposed ones; (v) all of the energetically competitive compounds are mechanically stable.