A comment on the paper "The structural, elastic, electronic and optical properties of MgCu under pressure: A first-principles study"

Rahman et al. [International Journal of Mod. Phys. B 30, 1650199 (2016)] found that MgCu intermetallic compound with CsCl-type (B2) structure becomes unstable under pressure at about 15GPa. This prediction has been made using an elastic instability condition for second-order elastic constants given by Born. They determined the elastic constants using Hooke's law for the linear stress-strain relationship which does not hold good at high pressures. It has been shown here that the values of elastic constants for MgCu at high pressures determined by Rahman et al. are not consistent with the values of bulk modulus computed from the Birch-Murnaghan equation of state (BM-EOS). We have used the Tallon formulation based on the thermodynamics of elastic deformation to estimate elastic constants which yields results for the bulk modulus of MgCu at different pressures in good agreement with those determined from the BM-EOS. It has been found that MgCu remains stable up to about 20 GPa which is significantly higher than the limit of elastic stability predicted by Rahman et al.