The combined inelastic neutron scattering (INS) and solid-state dft study of hydrogen-atoms dynamics in kaolinite-dimethylsulfoxide intercalate

Vibrational spectra of two kaolinite-dimethylsulfoxide intercalates, obtained using inelastic neutron scattering (INS), were analyzed with a view to understanding the dynamics of the hydrogen atoms in the structure. The main focus was on the spectral region 0-1700 cm, which is difficult to analyze using optical spectroscopy. The experimental vibrational spectra of kaolinite: dimethylsulfoxide and kaolinite:d6-dimethylsulfoxide collected using two different spectrometers were interpreted by means of the solid-state DFT calculations. Calculated spectra were obtained by both normal-mode analysis and molecular dynamics going beyond the harmonic approximation. The Al-O-H bending modes were found to be spread over the large interval 100 - 1200 cm, with the dominant contributions located between 800 and 1200 cm. The shape of the individual hydrogen spectrum depends on whether or not the respective hydrogen atom is involved in an O-H- • O hydrogen bond and on its strength. The modes corresponding to the in-plane movements of the inner-surface hydrogen atoms are well defined and always appear at the top of the intervals of energy transfer. In contrast, the modes generated by the out-of-plane movements of the hydrogen atoms are spread over large energy intervals extending down to the region of external (lattice) modes. The C-H modes are concentrated mainly in the three regions 1200 - 1450 cm, 800-1100 cm, and 0-400 cm. While the first two regions are typical of the various deformational modes of methyl groups, the low-energy region is populated by the modes corresponding to the movements of the whole dimethylsulfoxide molecule

Trimetallic borohydride Li₃MZn₅(BH₄)₁₅ (M=Mg, Mn) containing two weakly interconnected frameworks

The compounds, Li3MZn5(BH4)15, M = Mg and Mn, represent the first trimetallic borohydrides and are also new cationic solid solutions. These materials were prepared by mechanochemical synthesis from LiBH4, MCl2 or M(BH4)2, and ZnCl2. The compounds are isostructural, and their crystal structure was characterized by in situ synchrotron radiation powder X-ray and neutron diffraction and DFT calculations. While diffraction provides an average view of the structure as hexagonal (a = 15.371(3), c = 8.586(2) Å, space group P63/mcm for Mg-compound at room temperature), the DFT optimization of locally ordered models suggests a related ortho-hexagonal cell. Ordered models that maximize Mg-Mg separation have the lowest DFT energy, suggesting that the hexagonal structure seen by diffraction is a superposition of three such orthorhombic structures in three orientations along the hexagonal c-axis. No conclusion about the coherent length of the orthorhombic structure can be however done. The framework in Li3MZn5(BH4)15 is of a new type. It contains channels built from face-sharing (BH4)6 octahedra. While X-ray and neutron powder diffraction preferentially localize lithium in the center of the octahedra, thus resulting in two weakly interconnected frameworks of a new type, the DFT calculations clearly favor lithium inside the shared triangular faces, leading to two interpenetrated mco-nets (mco-c type) with the basic tile being built from three tfa tiles, which is the framework type of the related bimetallic LiZn2(BH4)5. The new borohydrides Li3MZn5(BH4)15 are potentially interesting as solid-state electrolytes, if the lithium mobility within the octahedral channels is improved by disordering the site via heterovalent substitution. From a hydrogen storage point of view, their application seems to be limited as the compounds decompose to three known metal borohydrides

Potassium zinc borohydrides containing triangular [Zn(BH4)3]-- and tetrahedral [Zn(BH4)xCl4-x]2-- anions

Three novel potassium-zinc borohydrides/chlorides are described. KZn(BH4)3 and K2Zn(BH4)xCl4-x form in ball-milled KBH4:ZnCl2 mixtures with molar ratios ranging from 1.5:1 up to 3:1. On the other hand, K3Zn(BH4)xCl5-x forms only in the 2:1 mixture after longer milling times. The new compounds have been studied by a combination of in situ synchrotron powder diffraction, thermal analysis, Raman spectroscopy, and the solid state DFT calculations. Rhombohedral KZn(BH4)3 contains an anionic complex [Zn(BH4)3]− with D3 (32) symmetry, located inside a rhombohedron K8. KZn(BH4)3 contains 8.1 wt % of hydrogen and decomposes at 385 K with a release of hydrogen and diborane similar to other Zn-based bimetallic borohydrides like MZn2(BH4)5 (M = Li, Na) and NaZn(BH4)3. The decomposition temperature is much lower than for KBH4. Monoclinic K2Zn(BH4)xCl4-x contains a tetrahedral complex anion [Zn(BH4)xCl4-x]2- located inside an Edshammar polyhedron (pentacapped trigonal prism) K11. The compound is a monoclinically distorted variant of the paraelectric orthorhombic ht-phase of K2ZnCl4 (structure type K2SO4). K2Zn(BH4)xCl4-x releases BH4 starting from 395 K, forming Zn and KBH4. As the reaction proceeds and x decreases, the monoclinic distortion of K2Zn(BH4)xCl4-x diminishes and the structure transforms at 445 K into the orthorhombic ht-phase of K2ZnCl4. Tetragonal K3Zn(BH4)xCl5-x is a substitutional and deformation variant of the tetragonal (I4/mcm) Cs3CoCl5 structure type possibly with the space group P42/ncm. K3Zn(BH4)xCl5-x decomposes nearly at the same temperature as KZn(BH4)3, i.e., at 400 K, with the formation of K2Zn(BH4)xCl4-x and KBH4, indicating that the compound is an adduct of the two latter compounds