Di-hydrogen contact induced lattice instabilities and structural dynamics in complex hydride perovskites.

The structural phase transitions occurring in a series of perovskite-type complex hydrides based on the tetrahydroborate anion BH4(-) are investigated by means of in situ synchrotron x-ray powder diffraction, vibrational spectroscopy, thermal methods and ab initio calculations in the solid state. Structural dynamics of the BH4 anion are followed with quasi-elastic neutron scattering. We show that unexpected temperature-induced lattice instabilities in perovskite-type ACa(BH4)3 (A = K, Rb, Cs) have their origin in close hydridic di-hydrogen contacts. The rich lattice dynamics lead to coupling between internal B-H vibrations and phonons, resulting in distortions in the high-temperature polymorph that are identical in symmetry to well-known instabilities in oxide perovskites, generally condensing at lower temperatures. It is found that anion-substitution BH4(-) <-> (X = Halide) can relax distortions in ACa(BH4)3 by eliminating coulomb repulsive H(-)···H(-) effects. The interesting nature of phase transition in ACa(BH4)3 enters an unexplored field of weak interactions in ceramic-like host lattices and is the principal motivation for this study. Close di-hydrogen contacts suggest new concepts to tailor crystal symmetries in complex hydride perovskites in the future.

Hydrogen-fluorine exchange in NaBH4-NaBF4.

Hydrogen-fluorine exchange in the NaBH4-NaBF4 system is investigated using a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed using in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as a new Rock salt type compound with idealized composition NaBF2H2 in the temperature range T = 200 to 215 °C. Combined use of solid-state (19)F MAS NMR, FT-IR and DFT calculations supports the formation of a BF2H2(-) complex ion, reproducing the observation of a (19)F chemical shift at -144.2 ppm, which is different from that of NaBF4 at -159.2 ppm, along with the new absorption bands observed in the IR spectra. After further heating, the fluorine substituted compound becomes X-ray amorphous and decomposes to NaF at ~310 °C. This work shows that fluorine-substituted borohydrides tend to decompose to more stable compounds, e.g. NaF and BF3 or amorphous products such as closo-boranes, e.g. Na2B12H12. The NaBH4-NaBF4 composite decomposes at lower temperatures (300 °C) compared to NaBH4 (476 °C), as observed by thermogravimetric analysis. NaBH4-NaBF4 (1:0.5) preserves 30% of the hydrogen storage capacity after three hydrogen release and uptake cycles compared to 8% for NaBH4 as measured using Sievert's method under identical conditions, but more than 50% using prolonged hydrogen absorption time. The reversible hydrogen storage capacity tends to decrease possibly due to the formation of NaF and Na2B12H12. On the other hand, the additive sodium fluoride appears to facilitate hydrogen uptake, prevent foaming, phase segregation and loss of material from the sample container for samples of NaBH4-NaF.

Synthesis and characterization of NaBD3H, a potential structural probe for hydrogen storage materials.

Specifically labeled NaBD(3)H has been synthesized and characterized using X-ray diffraction, NMR, and vibrational spectroscopy. The isotopic purity of the compound, as estimated from NMR spectra, was found to be about 85% with the compound NaBD(2)H(2) as the second product. IR spectra confirm the relatively strong intensity of the single B-H stretching mode predicted from DFT calculations. Anharmonic DFT calculations show that for the BD(3)H(-) ion Fermi resonances with the single B-H stretching mode are very limited, making this mode a promising structural probe for complex borohydrides which can be prepared by metathetical reactions.

[Clinical suggestions on the use of calcium hydroxide in therapy of pulp necrosis]. / Suggerimenti clinici sull'impiego dell'ossido di calcio nella terapia della necrosi pulpare.

The Aa. discuss the Bernard's method for therapy of pulpar necrosis, based on using of calcium oxide, know as "ocalexic method". They report clinical procedure and results of their experience.

[Neurotrophic lesions of cutaneous and mucosal areas with trigeminal innervation]. / Lesioni neurotrofiche nelle aree cutanee e mucose innervate dal trigemino.

The authors consider of trigeminal neurotrophic ulcerations features hypotising relationships with trophic diseases of ora mucosa, creating conditions for development of other oral mucosa diseases (lichen and mycosis).

[Esthetic restorations in conservative dentistry]. / I restauri estetici in odontoiatria conservativa.

The AA. discuss how to use some esthetic materials, as the Masking Agents and Tints, considering their technique characteristics and tackling the problem of right dental color remark.