Crystal structure of nitromethane up to the reaction threshold pressure.

Angle dispersion X-ray diffraction (AXDX) experiments on nitromethane single crystals and powder were performed at room temperature as a function of pressure up to 19.0 and 27.3 GPa, respectively, in a membrane diamond anvil cell (MDAC). The atomic positions were refined at 1.1, 3.2, 7.6, 11.0, and 15.0 GPa using the single-crystal data, while the equation of state (EOS) was extended up to 27.3 GPa, which is close to the nitromethane decomposition threshold pressure at room temperature in static conditions. The crystal structure was found to be orthorhombic, space group P2(1)2(1)2(1), with four molecules per unit cell, up to the highest pressure. In contrast, the molecular geometry undergoes an important change consisting of a gradual blocking of the methyl group libration about the C-N bond axis, starting just above the melting pressure and completed only between 7.6 and 11.0 GPa. Above this pressure, the orientation of the methyl group is quasi-eclipsed with respect to the NO bonds. This conformation allows the buildup of networks of strong intermolecular O...H-C interactions mainly in the bc and ac planes, stabilizing the crystal structure. This structural evolution determines important modifications in the IR and Raman spectra, occurring around 10 GPa. Present measurements of the Raman and IR vibrational spectra as a function of pressure at different temperatures evidence the existence of a kinetic barrier for this internal rearrangement.

Resolving the controversies about the 'nearly cubic' and other phases of Sr(1-x)Ca(x)TiO(3) (0≤x≤1): II. Comparison of phase transition behaviours for x = 0.40 and 0.43.

The phase transition behaviour of two 'nearly cubic' compositions, x = 0.40 and 0.43, of Sr(1-x)Ca(x)TiO(3), representative of Pbnm and Pbcm space group structures, has been investigated using temperature dependent dielectric, x-ray diffraction and Raman scattering techniques. For x = 0.40, a first order antiferroelectric phase transition occurs around 364 K followed by an antiferrodistortive phase transition involving the R point ([Formula: see text]) instability at ∼800 K. For x = 0.43, on the other hand, there is no antiferroelectric phase transition. Instead, two antiferrodistortive transitions around 463 and 850 K linked with the M ([Formula: see text]) and R ([Formula: see text]) point instabilities, respectively, are observed. Both the compositions also exhibit qualitatively different types of phase transitions below room temperature. Evidence for a new phase transition at low temperatures is presented for x = 0.43.