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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]

By performing calculations based on density functional theory (DFT), while adopting different exchange and correlations functionals, we have investigated pressure-induced phase transitions in calcium carbonate (CaCO3) up to 30 GPa. We have focused on the effect of zero-point vibrations on the stability sequence of three CaCO3 polymorphs, i.e. calcite-III, calcite-IIIb and calcite-VI, as well as on the evolution of their elastic properties. Our results show that the zero-point vibrations have a minor effect on the bulk modulus of the three CaCO3 polymorphs, in the inspected pressure range. However, we demonstrate that the zero-point vibrations have a major impact on phase transitions leading to the following polymorphic sequence: calcite-IIIb→calcite-III→calcite-VI, where transitions of the first order occur at 4 and 18 GPa. The obtained sequence shows that the boundary phase of calcite-II is calcite-IIIb; and the latter doesn't reappear at higher pressure, as previously predicted by theory. [ABSTRACT FROM AUTHOR]

We discuss theoretical description of pressure-induced phase transitions by means of first-principles calculations in the framework of density functional theory. We illustrate applications of theoretical tools that allow one to take into account configurational and vibrational disorders, considering Ti–V alloys as a model system. The universality of the first-principles theory allows us to apply it in studies of different phenomena that occur in the Ti–V system upon compression. Besides the transitions between different crystal structures, we discuss isostructural transitions in bcc Ti–V alloys. Moreover, we present arguments for possible electronic transitions in this system, which may explain peculiar behaviour of elastic properties of V upon compression. [ABSTRACT FROM PUBLISHER]