High-pressure phases, vibrational properties, and electronic structure of Ne(He)2 and Ar(He)2: A first-principles study

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 MgZn2 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 MgZn2 -> MgCu2, at a pressure of P-t=120(1) GPa. In Ar(He)(2), we find that the system stabilizes in the MgCu2 Laves phase at low pressures but it transitates toward the AlB2-type structure by effect of compression at P-t=13.8(4) GPa. The phonon spectra of the Ne(He)(2) crystal in the MgZn2 and MgCu2 Laves structures, and that of Ar(He)(2) in the AlB2-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 MgCu2 Laves structure, which has been ignored in all RG-H-2 works so far, might turn out to be competitive with respect to the MgZn2 and AlB2-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.