A phase transition starting at 84 K and indicated by a marked color change of the sample has been found in the organic insulator tetrathiafulvalene-chloranil. Optical, infrared, and Raman measurements indicate that this is a reversible transition from a nominally neutral (N) solid to a nominally ionic (D salt. Surprisingly, the N-I transition is not first order, but occurs over a broad temperature region (~ 30 K), in which there is a coexistence of N and I molecules.
D2O ice II has a fully deuteron-ordered structure in which two crystallographically independent types of water molecule form a tetrahedrally linked network of H bonds, with the deuterons lying near but not on the O···O centerlines. Orientation of the molecules is nearly, but not exactly, symmetrical in the donor O···O···O angles. The D–O–D angles (average, 105.4°) do not differ significantly from the angle in D2O vapor (104.5°) although the corresponding O···O···O angles are smaller (average, 93.8°). Agreement between D–O–D and O···O···O angles is improved about 6° (on average) by a distortion of the bond network, from symmetry R3̄c to R3̄. The approach to a match between D–O–D and O···O···O angles is only partly responsible for the existence of proton ordering in ice II. Twinning of R3̄ individuals is established by variations in certain diffraction intensities from crystal to crystal. Mean O–D bond length (0.98 Å) is slightly longer than in the vapor (0.957 Å). Variations among the individual O–D distances (range 0.95–0.01 Å) do not correlate with the H-bonded O···O distances (range 2.77–2.84 Å) and hence are probably not significant.
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