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The structural B1–B2 (B1 is the NaCl phase and B2 is the CsCl phase) phase transition of MgS and the elastic properties of the B1 and B2 phases of MgS are investigated by ab initio plane-wave pseudopotential density functional theory method. The dependences of the elastic constants c ij , the aggregate elastic modulus B S , G , and the Debye temperature Θ D on pressure are successfully discussed. From the usual condition of equal enthalpies, we find that the structure B1–B2 phase transition of MgS occurs at 255.5 GPa. Moreover, the dependences of the aggregate elastic modulus and the Debye temperature on the pressure P have been investigated for the first time.

The structural properties of Mo2Ga2C are simulated by first-principles under pressure. At 40 GPa, the axial compressibilities of [Formula: see text]- and [Formula: see text]-axes are 0.9763 and 0.9264, respectively, which are the known largest and smallest values in the [Formula: see text] compounds. A phase transition at 48 GPa is observed with an abrupt increase of [Formula: see text]-axis and a rapid decrease of [Formula: see text]-axis. The elastic properties and phonon imaginary frequencies confirmed the structural transition at 48 GPa, which probably should be the lowest-pressure transition in the [Formula: see text] ([Formula: see text], etc.) phases. The anti-expansion of Mo–Mo bond length is responsible for the [Formula: see text]-axis ultra-incompressibility as well as the structural transition at 48 GPa.

First-principles calculations of the crystal structures, phase transition, and elastic properties of B1-B2 phase calcium sulfide (CaS) have been carried out with the plane-wave pseudopotential density functional theory method. The calculated values (for crystal structures and the phase transition) are in very good agreement with experimental data as well as with some of the existing model calculations. The dependence of the elastic constants c(ij), the aggregate elastic modulus, the deviation from the Cauchy relation, and the elastic anisotropy on pressure have been investigated. The normalized elastic constants c(ij)' have been introduced to investigate the elasticity of CaS in detail. Moreover, the variation of the Poisson ratio, Debye temperature, and longitudinal and transverse elastic wave velocity with pressure P up to 70 GPa at 0 K have been investigated for the first time.