Neutral and charged thorium impurity in solid argon

Abstract

We study the neutral and positively charged thorium impurity (${\mathrm{Th}}^{n+}, n=0$, 1, 2, 3, 4) in solid argon by exploring the nature of chemical bonding in the ThAr diatomic molecule and in clusters ${\mathrm{ThAr}}_{4}, {\mathrm{ThAr}}_{12}, {\mathrm{Th}}^{n+}{\mathrm{Ar}}_{18}$ at the Hartree-Fock level with the second-order perturbation (MP2) correction accounting for the van der Waals forces. The chemical bonding is formed from the valence states of thorium and polarized states of argon in the clusters with ${\mathrm{Th}}^{n+}$ ($n=0$, 1, 2, 3) and solely from polarized states in the clusters with ${\mathrm{Th}}^{4+}$. In all cases with two or more valence electrons of Th, the ground state, influenced by the first Hund rule for the thorium impurity, is the high spin state. Allowing for the cubic to orthorhombic (${D}_{2h}$) symmetry lowering in ${\mathrm{Th}}^{n+}{\mathrm{Ar}}_{18}$, we find that the averaged Th-Ar bond length decreases whereas the binding energy increases with $n$, accounted for by the weakening of the Th-Ar repulsion and the strengthening of polarization. For ${\mathrm{Th}}^{4+}{\mathrm{Ar}}_{18}$, two conformations (cubic and orthorhombic) are found. We conclude that with the Th-Ar bond lengths lying very close to the Ar-Ar bond lengths in a fcc lattice, the solid argon is a material that is well suited for the accommodation of thorium impurities.