High-pressure phases, vibrational properties, and electronic structure of Ne(He)_2 and Ar(He)_2: A first-principles study

摘要

We have carried out a comprehensive first-principles study of the energetic, structural, and electronic properties of solid rare-gas (RG)-helium binary compounds, in particular, Ne(He)_2 and Ar(He)_2, under pressure and at temperatures within the range of 0≤T≤2000 K. Our approach is based on density-functional theory and the generalized gradient approximation for the exchange-correlation energy; we rely on total Helmholtz free-energy calculations performed within the quasiharmonic approximation for most of our analysis. In Ne(He)_2, we find that at pressures of around 20 GPa the system stabilizes in the MgZn_2 Laves structure, in accordance to what was suggested in previous experimental investigations. In the same compound, we predict a solid-solid phase transition among structures of the Laves family of the type MgZn_2→MgCu_2, at a pressure of P_t = 120(1) GPa. In Ar(He)_2, we find that the system stabilizes in the MgCu_2 Laves phase at low pressures but it transitates toward the AlB_2-type structure by effect of compression at P_t= 13.8(4) GPa. The phonon spectra of the Ne(He)_2 crystal in the MgZn_2 and MgCu_2 Laves structures, and that of Ar(He)_2 in the AlB_2-type phase, are reported. We observe that the compressibility of RG-RG and He-He bond distances in RG(He)_2 crystals is practically identical to that found in respective RG and He pure solids. This behavior emulates that of a system of noninteracting hard spheres in closed-packed configuration and comes to show the relevance of short-range interactions on this type of mixtures. Based on size-ratio arguments and empirical observations, we construct a generalized phase diagram for all RG(He)_2. crystals up to a pressure of 200 GPa where we map out systematic structural trends. Excellent qualitative agreement between such generalized phase diagram and accurate ab initio calculations is proved. A similar construction is done for RG(H_2)_2 crystals; we find that the MgCu_2 Laves structure, which has been ignored in all RG-H_2 works so far, might turn out to be competitive with respect to the MgZn_2 and AlB_2-type structures. Furthermore, we explore the pressure evolution of the energy-band gap in RG(He)_2 solids and elaborate an argument based on electronic-band theory which explains the observed trends.