Structure of the Ne-Xe mixture near the 26 MPa demixing curve at T=275 K

We report neutron diffraction measurements for two Ne-Xe mixtures at $T=275$ K and $P=26$ MPa, having a Xe molar fraction of 0.81 and 0.45, respectively. These thermodynamic points are chosen to be in the single phase region, close to the gas-gas demixing surface. Total structure factors and neutron weighted pair distribution functions have been extracted. Partial pair distribution functions ${g}_{\mathrm{ij}}(r)$ have been calculated by means of molecular dynamics simulation using Lennard-Jones interaction potentials and the Lorentz-Berthelot mixing rules, obtaining a good agreement with the experimental data. These partial pair distribution functions have been analyzed in terms of density expansion and also compared with those derived in a previous experiment on a He-Xe mixture. The main difference found between the structural properties of the two mixtures is that, in the Ne containing mixture, a definitely larger fraction of atoms of the lighter species occupies the Xe-Xe first neighbor shell. The model already used in interpreting the thermodynamic behavior of the He-Xe mixture in terms of microscopic properties suggests that this structural difference is related to the different behavior of the pressure-temperature projection of the critical line in the two mixtures.