Reverse Monte Carlo neutron scattering study of the 'ordered-ice' oxide pyrochlore Pb₂Ru₂O_6.5

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 Pb ₂Ru₂O₆O_0.5^′ 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 space group describes the vacancy ordering precisely. We complement these results with an examination of the Pb^{2 +} lone pair network using density functional electronic structure calculations, and a comparison of the low-temperature lattice-only heat capacity of Pb₂Ru₂O₆O_0.5 ^′ with that of other related pyrochlores.