Revealing the relaxation mechanisms of hidden states in transition metal dichalcogenides leads to control of metastability.

An overview of recent structural work focusing on the gas hydrates of methane and carbon dioxide is given. Both the crystal structure and the microstructure are considered. We report on the pressure-dependent molecular structure of methane clathrate hydrate using laboratory-made hydrogenous and deuterated samples investigated by neutron and hard-X-ray synchrotron diffraction experiments. The isothermal compressibilities are determined for hydrogenated and deuterated CH4 hydrate, and isotopic differences between both compounds are established for the first time. The cage filling of carbon dioxide and methane hydrate is determined and compared with predictions from statistical thermodynamic theory. In the case of small cages in methane hydrate, experimental results and predictions do not agree. Field-emission scanning electron microscopy reveals the meso- to macro-porous nature of gas hydrates formed with an excess of free gas. Furthermore, in situ measurements of the formation kinetics of porous hydrates are reported in which differences between methane and carbon dioxide are established quantitatively and the transient existence of a type II carbon dioxide structure is found. PACS Nos.: 82.75-z, 61.10Nz, 61.12Ld, 68.37Hk

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Reactions between AF (A = Li, Na, K, Rb, Cs) and TiF (with starting n(AF):n(TiF) molar ratios in the range from 3:1 to 1:3) in anhydrous hydrogen fluoride yield [TiF], [TiF], [TiF], [TiF], and [TiF] salts. With the exception of the ATiF compounds, which consist of A cations and octahedral [TiF] anions, all of these materials arise from the condensation of TiF units. The anionic part in the crystal structures of A[TiF] (A = K, Cs) and A[TiF]·HF (A = Na, K, Rb) is composed of infinite ([TiF]) chains built of TiF octahedra sharing joint vertices. Each structure shows a slightly different geometry of the ([TiF]) chains. The crystal structure of Na[TiF]·HF is constructed from polymeric ([TiF]) anions that appear as two parallel infinite zigzag chains comprising TiF units, where each TiF unit of one chain is connected to a TiF unit of the other chain through a shared fluorine vertex. Slow decomposition of single crystals of K[TiF]·8HF and Rb[TiF]·6HF ( Shlyapnikov , I. M. ; et al. Chem. Commun. 2013 , 49 , 2703 ) leads to the formation of [TiF] (Rb) and [TiF] (K, Rb) salts. The former displays the same ([TiF]) double chain as in Na[TiF]·HF, while the anionic part in the latter, ([TiF]), represents the first example of a three-dimensional network built of TiF octahedra. The ([TiF]) anion was also found in [HO][TiF]. The crystal structure determination of Cs[TiF] revealed a new type of polymeric fluoridotitanate(IV) anion, ([TiF]). Similar to the ([TiF]) anion, it is also built of zigzag double chains comprising TiF units. However, in the former there are fewer connections between TiF units of two neighboring chains than in the latter.