Bjørn C. Hauback

Magnetic properties of the one-dimensional Ca3Co2O6

Abstract The magnetic properties of Ca 3 Co 2 O 6 have been studied by magnetic susceptibility and neutron diffraction methods from 5 K to room temperature. Magnetic ordering occurs below 24±2K. Above 80 K, the Curie-Weiss law is followed and an effective magnetic moment of 5.7±0.2 μ B was found. The magnetic structure of Ca 3 Co 2 O 6 has been refined from Rietveld analysis of powder neutron diffraction data at 10 K. The one-dimensional chains in Ca 3 Co 2 O 6 were found to have alternating low magnetic moment (0.08±0.04 μ B ) for the octahedral cobalt and high magnetic moment (3.00±0.05 μ B ) for the trigonal prismatic cobalt ions. Magnetic moments close to zero were indicated in the ab -plane. Ca 3 Co 2 O 6 is ferrimagnetic at low temperatures, with ferromagnetic ordering dominating within the CoO chains. In high magnetic fields, above around 2.7 MA m −1 , a field-induced transition from ferrimagnetic to ferromagnetic structure occurs, implying turning of the spins in one chain.

LiSc(BH4)4: A Novel Salt of Li + and Discrete Sc(BH4)4 - Complex Anions

LiSc(BH4)4 has been prepared by ball milling of LiBH4 and ScCl3. Vibrational spectroscopy indicates the presence of discrete Sc(BH4)4(-) ions. DFT calculations of this isolated complex ion confirm that it is a stable complex, and the calculated vibrational spectra agree well with the experimental ones. The four BH4(-) groups are oriented with a tilted plane of three hydrogen atoms directed to the central Sc ion, resulting in a global 8 + 4 coordination. The crystal structure obtained by high-resolution synchrotron powder diffraction reveals a tetragonal unit cell with a = 6.076 A and c = 12.034 A (space group P-42c). The local structure of the Sc(BH4)4(-) complex is refined as a distorted form of the theoretical structure. The Li ions are found to be disordered along the z axis.

In situ synchrotron diffraction studies of phase transitions and thermal decomposition of Mg(BH4)(2) and Ca(BH4)(2)

Mg(BH4)2 and Ca(BH4)2 with 14.9 and 11.6 mass% hydrogen, respectively, are among the most promising materials for mobile hydrogen storage, but until now very little has been known about their hydrogen desorption properties. In this work the materials have been studied by time-resolved in situ synchrotron powder X-ray diffraction, thermal desorption spectroscopy and energy dispersive X-ray spectroscopy, and details of the phase transitions and decomposition routes are reported.

Neutron diffraction structure determination of NaAlD4

Abstract The structure of NaAlD4 has been determined from Rietveld-type refinements of powder neutron diffraction data at 8 and 295 K. The space group is I41/a with a=501.19(1) and c=1131.47(5) pm at 295 K. The Na atoms are surrounded by eight D atoms from eight different [AlD4]− tetrahedra in the geometry of a distorted square antiprism. The two different Na–D distances are nearly equal: 240.3(2) and 240.5(2) pm at 8 K and 243.1(2) and 243.9(2) pm at 295 K. The Al–D distance is 162.7(2) and 162.6(2) pm at 8 and 295 K, respectively.

Accurate structure of LiAlD4 studied by combined powder neutron and X-ray diffraction

The accurate structure of LiAlD4 was determined from combined Rietveld-type profile refinements of powder X-ray and neutron diffraction data at 295 K. In addition, neutron diffraction was used for structure analyses at 8 K. The space group is P21/c with a=482.54(1), b=780.40(1), c=789.68(1) pm and β=112.268(1)° at 295 K. The slight distortion of the AlD4 tetrahedra increases at low temperatures. The Al–D distances are in the range 160.3(7)–163.3(5) pm at 295 K and 159.6(6)–164.5(5) pm at 8 K. The lithium atoms are surrounded by a trigonal bipyramid of five deuterium atoms from five neighbouring [AlD4] tetrahedra.

Short hydrogen–hydrogen separations in novel intermetallic hydrides, RE3Ni3In3D4 (RE=La, Ce and Nd)

Abstract Crystal structure data for deuterides RENiInD x (RE=La, Ce and Nd) are provided on the basis of high-resolution powder X-ray and neutron diffraction data. The materials retain the hexagonal ZrNiAl type structure on deuteration. The formation of saturated deuterides is connected with anisotropic expansion along [001]. In the saturated hydrides, RE 3 Ni 3 In 3 D 4 , hydrogen atoms are located inside RE 3 Ni tetrahedra that share a common face, thereby forming a RE 3 Ni 2 trigonal bipyramid. This results in extraordinary short H–H separations of around 1.6 A. This feature is unique among well characterised metal hydride materials and is in striking contrast with the generally obeyed empirical rule of 2.0 A for H–H separations. On heating, the saturated materials release half of their hydrogen content at low temperatures, thereby statistically filling just one out of the two neighbouring tetrahedra. The remaining, more strongly bonded hydrogen, is released below 500°C under dynamic vacuum.

The High Resolution Powder Neutron Diffractometer PUS at the JEEP II Reactor at Kjeller in Norway

Abstract The performance of the new Powder Neutron Diffractometer PUS at the 2 MW JEEP II reactor shows the large potential for high resolution powder neutron diffraction even at a low flux reactor. The different components of the instrument are optimised with respect to flexibility and relationship between intensity and resolution. Wavelengths in the range 0.7–2.6 A are available from a focussing composite Ge monochromator. The detector system consists of position sensitive proportional counters. The temperatures available for experiments range from 7 to about 1600K. Nearly no malfunctions during the first 2½ years of operation proves the robustness of the complete system. Several experiments show the potential for studies of rather complex crystallographic problems, even with small sample quantities (<200mg), and time-resolved in-situ type experiments.

Structures of aluminium-based light weight hydrides

Abstract Light weight hydrides based on aluminium are among the most promising materials for hydrogen storage applications since the hydrogen content is up to 11 wt%. The crystal structures of many promising aluminium-based compounds have been determined and refined during the last years. This review gives an overview of structures of tetra-alanates (with AlH4 tetrahedra), hexa-alanates (with AlH6 octahedra) and different modifications of AlH3. The structure determinations are based on powder diffraction with X-rays, synchrotron X-rays and neutrons. In most structural work neutron diffraction has been used in order to determine accurate hydrogen/deuterium positions. In addition, density functional theory has been used both to predict new structures among these compounds and in combination with experimental work.

The identification of a hitherto unknown intermediate phase CaB2Hx from decomposition of Ca(BH4)2

Ca(BH4)2 is a promising material for hydrogen storage due to its high gravimetric capacity and expected suitable thermodynamic properties. Experimental reports indicate a more complex reaction pathway than the previously proposed route, including formation of unknown intermediate phases. In this work, the reaction scheme of a mixture of γ- and β-Ca(BH4)2 was investigated using temperature programmed desorption and high-resolution synchrotron radiation powder X-ray diffraction. An unknown intermediate Ca–B–H-containing phase has been identified as a CaB2Hx compound where x most probably equals 2. A structure model is proposed.

Structure and thermal properties of composites with RE-borohydrides (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb or Lu) and LiBH4

RECl3 (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb, Lu) and LiBH4 ball-milled in the molar ratio of 1 : 6 resulted in formation of numerous rare-earth metal (RE) borohydrides and LiCl in mixtures with excess LiBH4. The reaction products were characterized by powder X-ray diffraction and infrared spectroscopy, and their hydrogen sorption properties were investigated by thermogravimetric analysis combined with differential scanning calorimetry and in situ synchrotron radiation powder X-ray diffraction. La, Ce, Pr and Nd form LiRE(BH4)3Cl compounds which crystallize in the cubic space group I3m. Sm, Gd, Tb, Er and Yb form RE(BH4)3 phases crystallizing in the cubic space group Pa, with a possible polymorphic transition to a higher symmetry space group, Pmm. The smaller RE-elements Yb and Lu form tetrahedral [RE(BH4)4]− anionic complexes stabilized by Li+ cations crystallizing in the tetragonal space group P2c. Additionally Sm and Gd also exhibit a transition to the LiRE(BH4)3Cl phases observed for the largest lanthanides. LiSm(BH4)3Cl is reduced to Sm(BH4)2 upon heating, which takes a previously unreported orthorhombic structure in space group Pbcn. Eu did not form any of these RE-borohydride compounds, but infrared spectroscopy indicates formation of a new borohydride phase. Hydrogen desorption from the composite mixtures starts around 200 °C. Rehydrogenation of the La-system under p(H2) = 10 MPa at 300 °C showed a partial reversibility, with 18% recovery of the original hydrogen capacity, which increased to 36% at 415 °C. Rehydrogenation under p(H2) = 10 MPa at 400 °C showed a partial reversibility of 25% in the Er-system. Formation of REH2 during thermal decomposition destabilizes the remaining LiBH4 in the composites, thus releasing hydrogen at lower temperatures than for pure LiBH4.