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Reverse Monte Carlo neutron scattering study of the "ordered-ice" oxide pyrochlore Pb2Ru2O6.5

We employ high-resolution total neutron scattering in conjunction with reverse Monte Carlo simulations to examine, in a detailed and unbiased manner, the crystal structure of the vacancy-ordered oxide pyrochlore Pb2Ru2O6.5 in light of its structural analogy with proton-ordering in the structures of ice. We find that the vacancy and the O' ion are completely ordered, and that the average structure in the F43m space group describes the vacancy ordering precisely. We complement these results with an examination of the Pb2+ lone pair network using density functional electronic structure calculations, and a comparison of the low-temperature lattice-only heat capacity of Pb2Ru2O6.5 with that of other related pyrochlores.