Nature of lithium trapping sites in the quantum solids para-hydrogen and ortho-deuterium

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Abstract
Quantum mechanical studies of a lithium impurity in solid para‐hydrogen and ortho‐deuterium have been performed using the path integral formulation of statistical mechanics. Since an isolated lithium atom is much larger than the host molecules, trapping sites consisting of from one to six vacancies have been investigated. Interestingly, all of the sites are comparable in energy. This is due to the large compressibility of para‐hydrogen and ortho‐deuterium solids, which permits the lattice to relax to comfortably accommodate the impurity. The inhomogeneously broadened dipole spectrum of the lithium impurity in the various sites was calculated using the radial fast Fourier transform Lanczos method and compared to experiments by Fajardo [J. Chem. Phys. 98, 110 (1993)]. Based on the present calculations, lithium atoms appear to occupy preferentially a three‐vacancy trapping site in para‐hydrogen while in ortho‐deuterium a four‐vacancy trapping site seems to be favored. Complementary variational Einstein model calculations predict that the four‐vacancy trapping site is favorable in both para‐hydrogen and in ortho‐deuterium.