Relativistic effects and pressure-induced phase transition in CsAu.

Cesium auride (CsAu) is an intriguing compound formed by two metals that, upon reacting, exhibits properties of an ionic salt. In this study, we employ computer simulations to explore the influence of relativistic effects on the structure and some physical properties of CsAu, as well as on a potential pressure-induced structural phase transition, the effect of high pressures on its electronic gap, and the possible transition to a conducting state. We have found that including relativistic effects reduces the lattice parameter of CsAu and brings its volumetric properties closer to the trend observed in alkali halides. It also enhances the charge transfer from cesium to gold, resulting in a difference of up to 0.15e, at ambient pressure, between non-relativistic and fully relativistic calculations. Additionally, upon increasing pressure, in the absence of intervening structural phase transitions, the closing of CsAu's band gap is expected at approximately 31.5 GPa. The inclusion of relativistic effects stabilizes the CsAu Pm3̄m structure and shifts the transition pressure to a possible high-pressure P4/mmm phase from 2 GPa (non-relativistic calculation) to 14 GPa (fully-relativistic calculation). Both the Pm3̄m and P4/mmm structures become dynamically unstable around 15 GPa, thus suggesting that the tetragonal structure may be an intermediate state towards a truly stable high-pressure CsAu phase.

Pressure-induced second-order phase transition in fluorine.

The relative stability between the crystal structure of α-F2, space group C2/c, and a hypothesized high-pressure phase, space group Cmce, was explored using Density Functional Theory at the PBE0+D3(ABC)/TVZP level of theory and further assessed by Quantum Monte Carlo calculations. The analysis of the phonon dispersion spectra reveals that, at ambient pressure, besides the energy difference favoring the C2/c structure, the Cmce phase also presents a dynamical instability near the Γ-point, which disappears with increasing pressure. The unstable vibrational mode can be related to the absence of σ-holes in the fluorine molecule, which renders a repulsive head-to-head interaction between molecules, as opposed to heavier halogens, in which the presence of σ-holes stabilizes the orthogonal Cmce structure. The results show that the pressure-induced phase transition C2/c → Cmce is of second-order.