We report a detailed first-principles study of the structural, electronic and vibrational properties of the superconducting C32 phase of the ternary alloy CaAl2−xSix, both in the experimental range 0.6≤x≤1.2, for which the alloy has been synthesised, and in the theoretical limits of high aluminium and high silicon concentration. Our results indicate that, in the experimental range, the dependence of the electronic bands on composition is well described by a rigid-band model, which breaks down outside this range. Such a breakdown, in the (theoretical) limit of high aluminium concentration, is connected to the appearance of vibrational instabilities, and results in important differences between CaAl2 and MgB2. Unlike MgB2, the interlayer band and the out-of-plane phonons play a major role on the stability and superconductivity of CaAlSi and related C32 intermetallic compounds.
@article{arxiv.cond-mat/0509242,
title = {Electrons and phonons in the ternary alloy CaAl$_{2-x}$Si$_x$} as a function of composition},
author = {Matteo Giantomassi and Lilia Boeri and Giovanni B. Bachelet},
journal= {arXiv preprint arXiv:cond-mat/0509242},
year = {2009}
}