Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups (1) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/trans isomer undergoes photoinduced phase transitions (crystal–isotropic phase–crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be −0.84. Heating enhances the thermal cis‐to‐trans isomerization and subsequent cooling returned crystals of the trans/trans isomer.

A metal to insulator transition in integer or half integer charge systems can be regarded as crystallization of charges. The insulating state tends to have a glassy nature when randomness or geometrical frustration exists. We report that the charge glass state is realized in a perovskite compound PbCrO3, which has been known for almost 50 years, without any obvious inhomogeneity or triangular arrangement in the charge system. PbCrO3 has a valence state of Pb(2+)(0.5)Pb(4+)(0.5)Cr(3+)O3 with Pb(2+)-Pb(4+) correlation length of three lattice-spacings at ambient condition. A pressure induced melting of charge glass and simultaneous Pb-Cr charge transfer causes an insulator to metal transition and ∼10% volume collapse.

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The mechanism of the dehydrogenation-rehydrogenation of Ti-doped NaAlH 4 was investigated through X-ray diffraction (XRD) studies and Doppler broadening measurements of positron annihilation. Phase transformations during dehydrogenation and hydrogenation half-cycles were studied by the Rietveld analysis of XRD profiles. Changes in lattice defects were monitored by determination of the S parameter of the Doppler broadening spectra, which shows the ratio of positron annihilation to valence electrons. No significant changes in lattice parameters and the S parameter were observed directly following Ti doping. However, the S parameter increased upon dehydrogenation and decreased following rehydrogenation. These results indicate that vacancies are not introduced into NaAlH 4 upon Ti doping, but rather, they arise during dehydrogenation and disappear during rehydrogenation.

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The effect of the rare earth elements and alloy composition on the hydrogenation properties and crystal structures of hydrides in Mg2–x RE x Ni4 (RE = La, Pr, Nd, Sm, and Gd; x = 0.6 and 1.0) was investigated. All Mg2–x RE x Ni4 alloys had a C15b Laves phase before hydrogenation. Mg1.4 RE 0.6Ni4 (RE = Pr, Sm, and Gd) alloys were hydrogenated through one plateau to form Mg1.4 RE 0.6Ni4H∼3.6 while maintaining the C15b structure. Mg1.0 RE 1.0Ni4 (RE = La, Pr, and Nd) alloys were hydrogenated to ∼1.0 H/M proceeding through two plateaus, and Mg1.0 RE 1.0Ni4 (RE = Sm and Gd) alloys were hydrogenated to 0.6–0.7 H/M through one plateau. Mg1.0 RE 1.0Ni4 alloys initially transformed into Mg1.0 RE 1.0Ni4H∼4 with an orthorhombic structure. In addition it was experimentally confirmed that Mg1.0 RE 1.0Ni4H∼4 with La, Pr, and Nd transformed into Mg1.0 RE 1.0Ni4H∼6 with a C15b structure, while no formation of Mg1.0 RE 1.0Ni4H∼6 (RE = Sm and Gd) was observed at 40 MPa at 250 K. Theoretical calculations suggest that Mg1.0 RE 1.0Ni4H∼4 with Sm and Gd also transform to Mg1.0 RE 1.0Ni4H∼6 at higher pressures than those used in our experiments (264 MPa for Mg1.0Sm1.0Ni4 and 8.5 GPa for Mg1.0Gd1.0Ni4 at 253 K). It was found that the hydrogenation properties and crystal structure of the hydrides in Mg2–x RE x Ni4 are dependent on the alloy composition, i.e., the ratio of Mg to RE in the alloy phase, but independent of the choice of rare earth element.