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The structural, elastic and electronic properties of XCoSb along with the impact of pressure on the structure of the compounds are discussed in this paper. Calculations have been carried out using the pseudopotential plane wave method within generalized gradient approximation according to the Perdew-Burke-Ernzerhof (GGA-PBE) method. At ambient conditions, XCoSb crystallizes in a C1b-type cubic structure and we observed that at high-pressure VCoSb, CrCoSb and MnCoSb undergo a phase transition from cubic to the LiGaGe-type hexagonal structure. Elastic constants are reported for the first time. The electronic band structure and density of states (DOS) show that ScCoSb and TiCoSb are semiconductors and non-magnetic while VCoSb, CrCoSb and MnCoSb are half-metallic ferromagnetic. In addition, we have also calculated elastic moduli, Zener anisotropic factor, Poisson's ratio and transverse and longitudinal sound velocities from the second-order elastic constant. [ABSTRACT FROM AUTHOR]

The classical Oseen-Frank theory of liquid crystal elasticity is based on the experimentally verified fact that there are three independent modes of distortion, each with its associated elastic constant. On the other hand, the arguably more first-principles order parameter-based Landau-de Gennes theory only involves two independent elastic modes. The resulting ‘elastic constants problem’ has led to a considerable amount of vexation among theorists. In a series of papers at the turn of the century, Fukuda en Yokoyama suggested that the resolution of this problem could be found in the proper treatment of non-local effects in the ideal part of the free energy. They used an ingenious, but technically complex, technique based on a field-theoretic approach to semi-flexible polymers. Here we revisit their idea but now in the more accessible framework of density functional theory of rigid particles. Our work recovers their main results for rod-like particles, in that generically an ordered assembly of non-interacting rods has three independent elastic constants associated to it that all scale as the square of the length of the particles and obey the inequalities . We also consider the case of disc-like particles, and then find in line with expectations that . [ABSTRACT FROM AUTHOR]

Zn-doped PbTiO3has been developed to derive mechanical properties and phase transition. In this paper, a Zn atom 0–16% wt which occupied PbTiO3structure was investigated by using first principles density functional theory. The generalized gradient approximation was employed to determine the exchange correlation potential. The findings revealed that Zn-doped PbTiO3, calculated through Mulliken population analysis, had atomic (both ionic and covalent) bonding. In addition, the elastic constant satisfied the traditional mechanical stability conditions. Bulk modulus, Young's Modulus and Poisson's Ratio were found to increase with increasing Zn contents. Moreover, the result from the calculated longitudinal sound velocity and transverse sound velocity indicated that Debye temperature for Zn-doped PbTiO3had more than PbTiO3. Although the Zn-doped PbTiO3has no reported data for comparisons, it is predicted that Zn-doped PbTiO3can improve mechanical and dynamics behavior. [ABSTRACT FROM AUTHOR]