KNH2 - KH: a metal amide - hydride solid solution

We report for the first time the formation of a metal amide-hydride solid solution. The dissolution of KH into KNH2 leads to an anionic substitution, which decreases the interaction among NH2 - ions. The rotational properties of the high temperature polymorphs of KNH2 are thereby retained down to room temperature.Fil: Santoru, Antonio. Helmholtz-zentrum Geesthacht; Alemania. Università di Torino; ItaliaFil: Pistidda, Claudio. Helmholtz-zentrum Geesthacht; AlemaniaFil: Sørby, Magnus H.. Institute for Energy Technology. Physics Department; NoruegaFil: Chierotti, Michele R.. Università di Torino; ItaliaFil: Garroni, Sebastian. University of Sassari; ItaliaFil: Pinatel, Eugenio. Università di Torino; ItaliaFil: Karimi, Fahim. Helmholtz-zentrum Geesthacht; AlemaniaFil: Cao, Hujun. Helmholtz-zentrum Geesthacht; AlemaniaFil: Bergemann, Nils. Helmholtz-zentrum Geesthacht; AlemaniaFil: Le, Thi T.. Helmholtz-zentrum Geesthacht; AlemaniaFil: Puszkiel, Julián Atilio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Gobetto, Roberto. Università di Torino; ItaliaFil: Baricco, Marcello. Università di Torino; ItaliaFil: Hauback, Bjorn C.. Institute for Energy Technology. Physics Department; NoruegaFil: Klassen, Thomas. Helmholtz-zentrum Geesthacht; AlemaniaFil: Dornheim, Martín. Helmholtz-zentrum Geesthacht; Alemani

The Role of Ca(BH4)(2) Polymorphs

This study compares the structure and decomposition behaviors of theα,β, and γ polymorphs of Ca(BH 4) 2 for hydrogen storage. Samples with different polymorphic contents are characterized using powder X-ray diffraction and vibrational spectroscopy. Decomposition paths and formation of decomposition products are monitored by differential scanning calorimetry and temperature programed desorption as well as in situ synchrotron radiation powder diffraction. Vibrational spectroscopy in the <1000 cm -1 range shows different sharp librational bands for α- andγ-, which are not seen inβ-Ca(BH 4) 2. In the 1000-2700 cm -1 range, all three polymorphs show the vibrational features of the C 2 local structure corresponding to the internal vibrations of BH4 -. Shifts of these vibrational bands toward larger wavenumbers are observed forγ andβ-Ca(BH 4) 2. The increase in wavenumber coincides with an increase of the decomposition temperatures that can be up to 15°C between α- andγ-Ca(BH 4) 2 depending on the polymorphic content. The decomposition temperature of pureβ-Ca(BH 4) 2 is found to be about 6°C lower than the decomposition of the high-temperature modification obtained via the polymorphic transformation of α-Ca(BH 4) 2. This confirms that the pure Ca(BH 4) 2 polymorphs have slightly different kinetic barriers and that the polymorphic content determines the decomposition kinetics of the samples. In addition, simultaneous thermogravimetric and differential scanning calorimetry analyses show increasing mass losses from approximately 7 to 10 mass% depending on the polymorph and the heating rate. The largest hydrogen release occurs for the purest α-Ca(BH 4) 2 at a heating rate of 10°C/min. Calculated activation energies lead to 184 (14), 192 (3) and 230 (1) kJ/mol forγ-, α- andβ-Ca(BH 4) 2 samples, respectively. This is in agreement with the observed decomposition behavior. The results illustrate the complexity of the decomposition of Ca(BH 4) 2 and how the polymorphic content and the formation of intermediates can affect or not affect the decomposition reaction pathways. In particular, the origins of CaB 2Hx and the borohydride borate Ca 3(BH 4) 3(BO 3) seem to be unrelated to the nature of the polymorph. © 2012 American Chemical Society

Thermally induced phase transitions of barium oxalates

The thermal decomposition of BaC2O4 center dot 3.5H(2)O and BaC2O4 center dot 0.5H(2)O was investigated using in situ synchrotron X-ray and neutron powder diffraction. The decomposition routes for the barium oxalate hydrates were observed to depend on the applied heating rate. Thermal decomposition of BaC2O4 center dot 0.5H(2)O showed transformation to alpha-BaC2O4 and to beta-BaC2O4 prior to the formation of BaCO3. The decomposition of BaC2O4 center dot 3.5H(2)O showed formation of BaC2O4 center dot 0.5H(2)O at 58 degrees C and the hemi hydrate transforms to alpha-BaC2O4 at 187 degrees C using a relatively fast heating rate of 6.25 degrees C/min. The phase transitions were more complicated using lower heating rate, which also reveal formation of beta-BaC2O4 coexisting with alpha-BaC2O4 along with an unidentified compound. Heating alpha- and beta-BaC2O4 to higher temperatures (T > 400 degrees C) produced BaCO3. A sample of alpha-BaC2O4 was prepared in situ by thermal decomposition of BaC2O4 center dot 3.5H(2)O on a powder neutron diffractometer. The neutron diffraction data has broad diffraction peaks due to small crystallite sizes and overlapping Bragg reflections. [A structural model for alpha-BaC2O4 was derived from the neutron pattern, triclinic, space group P-1, a = 5.127(7), b = 8.905(12), c = 9.068(12) angstrom, alpha = 82.74(1), beta = 99.46(2), gamma = 100.10(1)degrees measured at T= 300 degrees C. The average Ba-O distances are 2.84(3) angstrom and 2.66(3) angstrom for Ba 1 and Ba2 respectively, C-O atom distances in the oxalate ions were found in the range 1.25(3)-1.26(4) angstrom, and C-C distances were 1.60(1)-1.61(1) angstrom]. (C) 2011 Elsevier Masson SAS. All rights reserved

Anion Substitution in Ca(BH4)(2)-CaI2: Synthesis, Structure and Stability of Three New Compounds

The substitution of the complex borohydride anion BH4- in calcium borohydride by the larger iodide anion I- is explored in order to tailor the hydrogen storage properties. Three new compounds are identified in the Ca(BH4)(2)-CaI2 system and are structurally characterized using the Rietveld method and synchrotron radiation powder X-ray diffraction (SR-PXD) data. Calcium borohydride readily dissolves in the trigonal calcium iodide structure during ball milling, forming a solid solution Ca((BH4)(1-x)I-x)(2) with a CaI2-type structure and an anisotropically contracted trigonal unit cell, a = 4.311(1) and c = 6.867(2) angstrom for x similar to 0.3 (T = 28 degrees C), space group P (3) over bar m1. The trigonal tri-Ca((BH4)(0.70)I-0.30)(2) transforms at similar to 180 degrees C to an orthorhombic phase of similar composition, ort-Ca((BH4)(0.64)I-0.36)(2), with a CaCl2-type structure (a distorted beta-Ca(BH4)(2) type structure) and cell parameters a = 7.271(2), b = 7.042(1), and c = 4.4601(7) angstrom (T = 322 degrees C), space group Pnnm. Further heating of the CaCl2-type compound to similar to 330 degrees C leads to a transition to a tetragonal phase with cell parameters, a = 4.1062(2) and c = 24.822(2) angstrom (T = 340 degrees C, x similar to 0.62), space group I4mm. This iodide-rich compound tet-Ca((BH4)(0.38)I-0.62)(2), has a new structure type. The tetragonal phase finally decomposes to CaHI and CaB6 at T > 345 degrees C. All three novel compounds found in the Ca(BH4)(2)-CaI2 system are stable at room temperature. The anion substitution ultimately changes the decomposition reaction pathway in which hydrogen is released from the tetragonal Ca((BH4)(1-x)I-x)(2) via CaHI, but unfortunately the temperature of hydrogen release is still fairly high and similar to that for Ca(BH4)(2)

Interstitial carbon in bcc HfNbTiVZr high-entropy alloy from first principles

The remarkable mechanical properties of high-entropy alloys can be further improved by interstitial alloying. In this work we employ density functional theory calculations to study the solution energies of dilute carbon interstitial atoms in tetrahedral and octahedral sites in bcc HfNbTiVZr. Our results indicate that carbon interstitials in tetrahedral sites are unstable, and the preferred octahedral sites present a large spread in the energy of solution. The inclusion of carbon interstitials induces large structural relaxations with long-range effects. The effect of local chemical environment on the energy of solution is investigated by performing a local cluster expansion including studies of its correlation with the carbon atomic Voronoi volume. However, the spread in solution energetics cannot be explained with a local environment analysis only pointing towards a complex, long-range influence of interstitial carbon in this alloy