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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 F43m 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.

Filmy-QCC, an organic material synthesized in the laboratory, exhibits red photoluminescence (PL). The peak wavelength of the PL ranges from 650 to 690 nm, depending on the mass distribution of polycyclic aromatic hydrocarbon (PAH) molecules, and the emission profile is a good match for that of the extended red emission (ERE) in the Red Rectangle (RR) nebula. The quantum yield of the PL ranges from 0.009 to 0.04. When filmy-QCC is dissolved in cyclohexane, it exhibits blue PL in the wavelength range of 400-500 nm with a quantum yield of 0.12-0.16. The large width of the red PL and the large wavelength difference between the PL of the filmy-QCC as a solid film and in a solution indicate that there is a strong interaction between the components of filmy-QCC. The major components of filmy-QCC are PAHs up to 500 amu. Our laboratory data suggest that the blue luminescence observed in the RR nebula is probably caused by small PAHs in a gaseous state, and the ERE is caused by larger PAHs in dust grains.

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