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 Pb(2)Ru(2)O(6)O(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 F 43m 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 Pb(2)Ru(2)O(6)O(0.5)(') with that of other related pyrochlores.