Covalent Si–H Bonds in the Zintl Phase Hydride CaSiH1+x (x ≤ 1/3)

The crystal structure of the Zintl phase hydride CaSiH≈4/3 was discussed controversially, especially with respect to the nature of the silicon-hydrogen interaction. We have applied X-ray and neutron powder diffraction as well as total neutron scattering on a deuterated sample, CaSiD1.1. Rietveld refinement (CaSiD1.1, Pnma, a = 14.579(4) Å, b = 3.8119(4) Å, c = 11.209(2) Å) and an analysis of the neutron pair distribution function show a silicon-deuterium bond length of 1.53 Å. The Si–H bond may thus be categorized as covalent and the main structural features described by a limiting ionic formula Ca2+H−(Si−)2/3(SiH−)1/3. Hydrogen atoms decorating the ribbon-like silicon polyanion made of three connected zigzag chains are under-occupied, resulting in a composition CaSiH1.1. Hydrogen-poor Zintl phase hydrides CaSiH<1 with hydride ions in Ca4 tetrahedra only were found in an in situ neutron diffraction experiment at elevated temperature. Hydrogen (deuterium) uptake and release in CaSiDx (0.05 ≤ x ≤ 0.17) is a very fast process and takes less than 1 min to complete, which is of importance for possible hydrogen storage applications.

Infrared and Raman spectra of cubic form of magnesium caesium arsenate hexahydrate

Fourier transform infrared (FT-IR) spectra recorded at room temperature (RT) and at the boiling temperature of liquid nitrogen (LNT), as well as Raman spectra recorded at room temperature for the cubic polymorph of magnesium cesium arsenate hexahydrate (MgCsAsO4·6H2O) and its partially deuterated analogues, were interpreted with respect to the normal modes of the water molecules and the arsenate ions and water librations. The spectral characteristics of MgCsAsO4·6H2O were compared to the cubic form of the phosphate analogue. A spectral similarity between the two isostructural salts was established, except for the obvious differences due to the nature of the anions (AsO43– vs PO43–). The spectroscopic data for the uncoupled OD stretching mode of the matrix-isolated HDO molecules revealed that the hydrogen bonds formed in the arsenate salt were stronger than those in the phosphate. In the Raman spectrum of the protiated compound, only one very intensive band at 811 cm–1 was observed in the region of the stretching vibrations of the AsO43– ion, which was insensitive to deuteration. In accordance with the expectation, one band appeared in the same spectral range in the infrared spectra of the protiated and highly deuterated sample at 792 cm–1 and 810 cm–1, respectively, which can be attributed with certainty to the asymmetric stretching ν3(AsO4) modes.

Production, crystallization and neutron diffraction of fully deuterated human myelin peripheral membrane protein P2

The molecular details of the formation of the myelin sheath, a multilayered membrane in the nervous system, are to a large extent unknown. P2 is a peripheral membrane protein from peripheral nervous system myelin, which is believed to play a role in this process. X-ray crystallographic studies and complementary experiments have provided information on the structure–function relationships in P2. In this study, a fully deuterated sample of human P2 was produced. Crystals that were large enough for neutron diffraction were grown by a ten-month procedure of feeding, and neutron diffraction data were collected to a resolution of 2.4 Å from a crystal of 0.09 mm3in volume. The neutron crystal structure will allow the positions of H atoms in P2 and its fatty-acid ligand to be visualized, as well as shedding light on the fine details of the hydrogen-bonding networks within the P2 ligand-binding cavity.

Jarosite–butlerite intergrowths in non-stoichiometric jarosites: crystal chemistry of monoclinic natrojarosite–hydroniumjarosite phases

Monoclinic, non-stoichiometric natrojarosite—hydroniumjarosite solid solution phases have been synthesized hydrothermally over a range of temperatures, starting compositions and reaction times, and have been characterized using Rietveld refinement of synchrotron X-ray diffraction data, and chemical and thermal analyses. The H atom locations have been obtained from refinement of neutron diffraction data on a deuterated sample. The results confirm a direct relationship between the monoclinic distortion and the ordering of iron site vacancies in one of two independent iron sites. Ordering of iron vacancies gives rise to domains containing butlerite-like 7 Å chains of corner-connected octahedra and tetrahedra. The formation of these chains within (100) planes results in an expansion of the monoclinic a lattice parameter and a contraction of the c parameter relative to stoichiometric jarosites. The results support a recent model for iron deficiency, whereby an iron vacancy is compensated by the replacement of four coordinated OH– ions by H2O molecules, with one of the H2O molecules coming from deprotonation of H3O+. The general formula, based on intergrowth of stoichiometric jarosite and non-stoichiometric, butlerite-like regions, is [(Na,H3O)Fe3(SO4)2(OH)6]1–y[(H2O)Fe2(SO4)2(OH)2(H2O)4]y.

The Hunt for Hydrogen in the High-Pressure Phase of Ca(OH)2 and Ca(OD)2: Neutrons Versus X-Rays

A recent measurement on the high-pressure phase of Ca(OH)2using high-resolution synchrotron X-ray powder diffraction has been carried out on a hydrogenated sample at high-temperature. It complements an earlier time-of-flight neutron measurement carried out in the US on a deuterated sample at low temperature. The two protons are found by Rietveld refinement using a new suitable non-Fourier based strategy, for which GSAS and FullProf yield very similar results. Our X-ray result agrees with the previous neutron study for one proton site only, prompting us to reinvestigate the neutron data. A second new and distinct non Fourier-based strategy coupled with GSAS is used to tackle the latter data and indeed confirms the US results. Whilst the reasons for this X-Ray vs Neutron discrepancy can only be guessed at this stage, the use of the two related proton-finding strategies could be extended beyond our specific study.

Hydrogen Sites In The α(TiCrSiO) 1/1 Approximant Phase

Crystal approximant phases are important because they are believed to have a similar local atomic structure to corresponding quasicrystals. Interstitial hydrogen is used as a probe of the local structures of the Ti-based quasicrystals and crystal approximants. Phase pure samples of ct(TiCrSiO), a Mackay type 1/1 crystal approximant, which were plasma etched and coated with a thin layer of Pd, were loaded with deuterium from the gas phase to a maximum deuterium atom to metal atom ratio (D/M) of 0.26. After deuteration, the sample becomes a fine powder and remains single-phase. A Rietveld structural refinement of powder x-ray and neutron diffraction data was made to determine the location of the interstitial sites. For the fully deuterated sample the deuterium atoms sit at both octahedral and tetrahedral sites. The octahedral sites are formed by six Ti atoms in the first and second shells of the icosahedra. Deuterium atoms are also located at the tetrahedral interstitial sites formed between the clusters along <100> directions. In the partially loaded samples with DiM = 0.11, the deuterium atoms occupy only octahedral sites, demonstrating a preference for these sites.

Zeolite Rho Loaded with Methylamines. III. Trimethylamine Loadings

Samples of two differently prepared zeolites rho loaded with different amounts of trimethylamine (TMA) were studied in their hydrated and dehydrated forms by X-ray and neutron diffraction experiments. Both zeolites are partially dealuminated, as indicated by nonframework Al, which is assumed to be Al2O3 or AlOOH. Series I was prepared from dry-calcined NHn-rho at 873 K, series II from steam-calcined NHn-rho at 773 K. The samples were loaded with different amounts of deuterated TMA. Rietveld refinements yielded the following results for series I: (1) H3.8(H-TMA)5Cs0.2Al9 Si39O96.Al2O3.22H2O, X-ray data collection at room temperature, Im\overline 3m, a = 15.0542 (2) Å, R wp = 0.094; (2) composition as in (1) (anhydrous), neutron data collection at 5 K of dehydrated and deuterated sample, Im3m, a = 15.0467 (4) Å, R wp = 0.034. Series II: (3) H0.3(H-TMA)5Cs0.7Al6Si42O96.2.5Al2O3.22H2O, X-ray data collection at room temperature, Im\overline 3m, a = 15.0574 (2) Å, R wp = 0.118; (4) composition as in (3) (anhydrous), neutron data collection at 5 K of dehydrated and deuterated sample, lm\overline 3m, a = 15.0761 (5) Å, R wp = 0.037. In all determinations the TMA molecules reside with the N and H (D in neutron diffraction analysis) atoms on the central axes (x, 0, 0) pointing towards the center of the single eight-ring, whilst the three methyl groups point away to the center of the \alpha-cage.