Controlling electronic population through chemical doping is one way to tip the balance between competing phases in materials with strong electronic correlations. Vanadium dioxide exhibits a first-order phase transition at around 338 K between a high-temperature, tetragonal, metallic state (T) and a low-temperature, monoclinic, insulating state (M1), driven by electron-electron and electron-lattice interactions. Intercalation of VO2 with atomic hydrogen has been demonstrated, with evidence that this doping suppresses the transition. However, the detailed effects of intercalated H on the crystal and electronic structure of the resulting hydride have not been previously reported. Here we present synchrotron and neutron diffraction studies of this material system, mapping out the structural phase diagram as a function of temperature and hydrogen content. In addition to the original T and M1 phases, we find two orthorhombic phases, O1 and O2, which are stabilized at higher hydrogen content. We present density functional calculations that confirm the metallicity of these states and discuss the physical basis by which hydrogen stabilizes conducting phases, in the context of the metal-insulator transition.

Structural strain and a first-order phase transition in the crystalline DL-cysteine on cooling and on reverse heating were followed by Raman spectroscopy and X-ray diffraction. The transition is reversible and has a large hysteresis (over 100 K). The temperature at which the transition is observed depends strongly on the cooling/heating rate. The phase transition is accompanied by crystal fragmentation. The low-temperature phase could be obtained not only as a result of the solid-state transformation in situ as a polycrystalline sample (with strong preferred orientation, or without it, depending on the preparative technique), but also (using an original crystallization technique) as a single crystal of the quality suitable for structural analysis. For the first time, the crystal structure of the low-temperature phase was solved independently by powder and by single-crystal diffraction techniques. The spectral changes were correlated with the precise diffraction data on the intramolecular conformations and the intermolecular hydrogen bonding before and after the phase transition. The role of the distortion of the intermolecular hydrogen bonds and of the motions of the -CH(2)SH side chains in the phase transition is discussed in a comparison with the low-temperature phase transition in L-cysteine, which is of a different type and preserves the single crystals intact (Kolesov et al. J. Phys. Chem. B, 2008, 112 (40), 12827-12839).

, however, is regarded to be the key factor that prevents the rehydrogenation of dodecaborates.12 In order to elucidate the mechanism as well as to provide effective solutions to this problem, a novel solvent-free synthesis route of anhydrous M 2/n B 12 H 12 (here M means Li, Na, and K) has been developed. 13 Thermal stability and transformations of the anhydrous single phase Li 2 B 12 H 12 suggested the formation of the high temperature polymorph of Li 2 B 12 H 12 during the dehydrogenation of LiBH 4 , while concurrently emphasized the importance of further investigation on the decomposition mechanism of metal borohydrides and metal dodecaborates.14 The high stability of icosahedral [B 12 H 12 ] 2− , on the other hand, favors its potential application as solid electrolyte. Recently, Na + conductivity of Na 2 B 12 H 12 was reported to be 0.1 S/cm above its order−disorder phase transition at ∼529 K, 3 which is comparable to that of a polycrystalline β″-Al 2 O 3 (0.24 S/cm at 573 K) solid state Na-electrolyte. 15 Mechanistic understanding on the diffusion behavior of cation and further improvement of ionic conductivity at a lower temperature, however, are important in order to facilitate the practical application of metal dodecaborates as superionic conductors.Bimetallic compounds composed of two different metal elements often show different crystal structures and interesting chemical and physical properties, which are often distinguished from those of the monometallic counterparts. For example, bimetallic borohydrides have been proven as a way to tune the thermodynamics of metal borohydride decomposition.16−18 To our best knowledge, there has been no report of improving the ionic conductivity of M 2/n B 12 H 12 by introducing another metal to form a bimetallic dodecaborate, and we hypothesized that the coexistence of bimetallic elements could have a synergetic effect on the mobility change of each ion. LiNaB 12 H 12 was prepared through sintering of LiBH 4 , NaBH 4 , and B 10 H 14 with a stoichiometric molar ratio of 1:1:1. The successful synthesis was confirmed by X-ray diffraction, Raman spectra, and solid state nuclear magnetic resonance (NMR) measurements (see Supporting Information Figure S1).

The effect of pressure on L-alanine has been studied by X-ray powder diffraction (up to 12.3 GPa), single-crystal X-ray diffraction, Raman spectroscopy and optical microscopy (up to approximately 6 GPa). No structural phase transitions have been observed. At approximately 2 GPa the cell parameters a and b become accidentally equal to each other, but without a change in space-group symmetry. Neither of two transitions reported by others (to a tetragonal phase at approximately 2 GPa and to a monoclinic phase at approximately 9 GPa) was observed. The changes in cell parameters were continuous up to the highest measured pressures and the cells remained orthorhombic. Some important changes in the intermolecular interactions occur, which also manifest themselves in the Raman spectra. Two new orthorhombic phases could be crystallized from a MeOH/EtOH/H(2)O pressure-transmitting mixture in the pressure range 0.8-4.7 GPa, but only if the sample was kept at these pressures for at least 1-2 d. The new phases converted back to L-alanine on decompression. Judging from the Raman spectra and cell parameters, the new phases are most probably not L-alanine but its solvates.