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≤2000K . 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(H₂)₂ crystals; we find that the MgCu₂ Laves structure, which has been ignored in all RG-H₂ 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.