First-principles solution to the problem of Mo lattice stability

Abstract

The energy differences between the ground state body-centred structure (bcc) and closed-packed face-centred structure (fcc) structures for transition metals in the middle of the series show unusually large disagreements when they are obtained by the thermochemical approach based on the analysis of experimental data or by first-principles electronic structure calculations. Considering a typical example, the lattice stability of Mo, we present a solution to this long-standing problem. In contrast to conventional total energy calculations within Density Functional Theory framework, we carry out ab initio molecular dynamics simulations for the two phases at high temperature. We show that at these conditions both bcc and fcc structures of Mo are dynamically stable, and the difference in their configurational energies decreases dramatically as compared to the zero temperature result, approaching the value derived by means of the thermochmical approach. We show that the main contribution to the effect comes from the modification of the canonical band structure for bcc and fcc phases due to lattice vibrations at high temperature, and discuss consequences of our finding for future first-principles simulations of phase stability.

Theoretical and experimental study of metastable solid solutions and phase stability within the immiscible Ag-Mo binary system

“…This is in agreement with theoretical and experimental studies, where the fcc Mo-phase has been found to be dynamically unstable at low temperatures. 46,47,48 This means that even though the calculated mixing enthalpies of the Mo-rich bcc and Ag-rich fcc ideal random alloys in Fig. 1 appear symmetric with respect to composition, short-range Ag and Mo isostructural clustering in the bcc structure could lower the driving force for global phase separation in Mo-rich alloys more than in Ag-rich alloys with fcc structure.…”

Quantifying the anomalous self-diffusion in molybdenum with first-principles simulations

“…Thermophysical properties such as the specific heat show unusually large anharmonic contributions, and until recently, electronic-structure calculations were in stark qualitative and quantitative disagreement with experiments, e.g., concerning the relative stability between competing phases. 1,2 Turning to lattice defects in bcc metals, a similarly involved situation exists. For instance, the core structure of screw dislocations, which governs the resistance to plastic deformation, is notoriously difficult to model.…”

Temperature dependence of TiN elastic constants fromab initiomolecular dynamics simulations

“…21 It has been demonstrated that the errors for lattice stabilities due to the neglect of temperature effects in simulations can be as large as a factor of 2. 22 One class of properties that has attracted particular interest is the elastic behavior of transition-metal nitrides. [16][17][18][19][20] Indeed, it influences many important phenomena that determine the hard-coating performance.…”