Origin of the c-axis ultraincompressibility of Mo2GaC above about 15GPa from first principles

The mechanical properties and structural evolution of Mo2GaC are calculated by first-principles under pressure. Our results unexpectedly found that the c axis is always stiffer than a axis within 0-100 GPa. An ultraincompressibility of c axis within 15-60 GPa is observed, with a contraction of about 0.2 angstrom, slightly larger than that of a axis (0.14 angstrom). The abnormal expansion of c axis and the fast decrease in a axis above about 15 GPa and 70 GPa failed to induce the structural instability, whereas such behavior caused the elastic softening in many mechanical quantities. The shrinkage anomaly of c axis is closely reflected by the internal coordinate (u) shift of Mo atom as it shows three different slopes within 0-15 GPa, 20-60 GPa, and 70-100 GPa, respectively. The longest Mo-Mo bond is responsible for the unusual shrinkage of c-axis under pressure as they experience nearly identical pressure dependences, whereas the a axis presents certain response with the variation of C-Mo bond particularly at 70 GPa. The electronic properties are investigated, including the energy band and density of states, and so on. At G point of K-M line, the energy decreases at 10 GPa first and increases at 30 GPa subsequently, the critical point is at about 15 GPa, with respective values of -0.17 of 0 GPa, -0.18 of 10 GPa, -0.16 of 15 GPa, and -0.13 of 30 GPa, respectively. This alternative energy change of G point, which is the symmetry center of the rhombic parallelogram of Ga atoms and the midpoint of the two bonded Mo atoms, convincingly reveal the origin of the anomalous ultraincompressibility of c axis as the Mo-Mo bond length shrinkage has to overcome the increasing energy barrier height. The Mo- Mo bond population and the electronegativity investigations of the Mo atom further reveal the most likely origin of the ultraincompressibility of c axis. This interesting result expects further experimental confirmation as this is the first nanolaminate ceramics compound presenting quite low-pressure axial ultraincompressibility. (c) 2016 AIP Publishing LLC.