RESUMEN
In this work, trimethoxyboroxine (TMB) is identified as a small-molecule glass former. In its viscous liquid as well as glassy states, static and dynamic properties of TMB are explored using various techniques. It is found that, on average, the structure of the condensed TMB molecules deviates from threefold symmetry so that TMB's electric dipole moment is nonzero, thus rendering broadband dielectric spectroscopy applicable. This method reveals the super-Arrhenius dynamics that characterizes TMB above its glass transition, which occurs at about 204 K. To extend the temperature range in which the molecular dynamics can be studied, 11B nuclear magnetic resonance experiments are additionally carried out on rotating and stationary samples: Exploiting dynamic second-order shifts, spin-relaxation times, line shape effects, as well as stimulated-echo and two-dimensional exchange spectroscopy, a coherent picture regarding the dynamics of this glass former is gained.
RESUMEN
Two new chalcogenogallates Cs4 Ga6 Q11 (Q=S, Se) were obtained by a controlled thermal treatment of CsN3 in the presence of stoichiometric amounts of Ga2 Q3 and the elemental chalcogens at elevates temperatures. Both isotypic compounds crystallize in the space group P 1 â¾ (no. 2). The most prominent structural feature in these chalcogenogallates are the complex anionic Dreier double chains 1 ∞ [Ga6 Q11 4- ] formed by condensed GaQ4 tetrahedra. The semiconductors Cs4 Ga6 S11 (Eg =3.14â eV) and Cs4 Ga6 Se11 (Eg =2.41â eV) were further studied by using UV/Vis, 133 Cs and 71 Ga solid-state NMR spectroscopy, and complementary DFT calculations. The 133 Cs MAS NMR spectra are characteristic for cationic cesium and vibrational spectra show two distinct regions, attributed to the Ga-Q valence and deformation vibrations, respectively. High-temperature studies revealed incongruent melting of both solids, which is also depicted in updated binary phase-diagrams Cs2 Q-Ga2 Q3 (Q=S, Se).
RESUMEN
High-pressure silicates making up the main proportion of the earth's interior can incorporate a significant amount of water in the form of OH defects. Generally, they are charge balanced by removing low-valent cations such as Mg2+. By combining high-resolution multidimensional single- and double-quantum 1H solid-state NMR spectroscopy with density functional theory calculations, we show that, for ringwoodite (γ-Mg2SiO4), additionally, Si4+ vacancies are formed, even at a water content as low as 0.1 wt %. They are charge balanced by either four protons or one Mg2+ and two protons. Surprisingly, also a significant proportion of coupled Mg and Si vacancies are present. Furthermore, all defect types feature a pronounced orientational disorder of the OH groups, which results in a significant range of OH···O bond distributions. As such, we are able to present unique insight into the defect chemistry of ringwoodite's spinel structure, which not only accounts for a potentially large fraction of the earth's entire water budget, but will also control transport properties in the mantle. We expect that our results will even impact other hydrous spinel-type materials, helping to understand properties such as ion conduction and heterogeneous catalysis.
