Radovan Černý

Lithium boro-hydride LiBH4. I. Crystal structure

Abstract The crystal structure of LiBH4 has been studied by synchrotron X-ray powder diffraction at room temperature and at 408 K. At room temperature it has orthorhombic symmetry [space group Pnma, a=7.17858(4), b=4.43686(2), c=6.80321(4) A]. The tetrahedral (BH4)− anions (point symmetry m) are aligned along two orthogonal directions and are strongly distorted with respect to bond lengths [B–H=1.04(2)–1.28(1) A] and bond angles [H–B–H=85.1(9)°–120.1(9)°]. As the temperature is increased the structure undergoes a first-order transition and becomes hexagonal (space group P63mc, a=4.27631(5), c=6.94844(8) A at T=408 K). The (BH4)− tetrahedra align along c, become more symmetric [point symmetry 3m, B–H=1.27(2)–1.29(2) A, H–B–H=106.4(2)°–112.4(9)°] and show displacement amplitudes that are consistent with dynamical disorder about their trigonal axis.The crystal structure of LiBH4 has been studied by synchrotron X-ray powder diffraction at room temperature and at 408 K. At room temperature it has orthorhombic symmetry [space group Pnma, a=7.17858(4), b=4.43686(2), c=6.80321(4) A]. The tetrahedral (BH4)− anions (point symmetry m) are aligned along two orthogonal directions and are strongly distorted with respect to bond lengths [B–H=1.04(2)–1.28(1) A] and bond angles [H–B–H=85.1(9)°–120.1(9)°]. As the temperature is increased the structure undergoes a first-order transition and becomes hexagonal (space group P63mc, a=4.27631(5), c=6.94844(8) A at T=408 K). The (BH4)− tetrahedra align along c, become more symmetric [point symmetry 3m, B–H=1.27(2)–1.29(2) A, H–B–H=106.4(2)°–112.4(9)°] and show displacement amplitudes that are consistent with dynamical disorder about their trigonal axis

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.

Structure and properties of complex hydride perovskite materials

Perovskite materials host an incredible variety of functionalities. Although the lightest element, hydrogen, is rarely encountered in oxide perovskite lattices, it was recently observed as the hydride anion H(-), substituting for the oxide anion in BaTiO3. Here we present a series of 30 new complex hydride perovskite-type materials, based on the non-spherical tetrahydroborate anion BH4(-) and new synthesis protocols involving rare-earth elements. Photophysical, electronic and hydrogen storage properties are discussed, along with counterintuitive trends in structural behaviour. The electronic structure is investigated theoretically with density functional theory solid-state calculations. BH4-specific anion dynamics are introduced to perovskites, mediating mechanisms that freeze lattice instabilities and generate supercells of up to 16 × the unit cell volume in AB(BH4)3. In this view, homopolar hydridic di-hydrogen contacts arise as a potential tool with which to tailor crystal symmetries, thus merging concepts of molecular chemistry with ceramic-like host lattices. Furthermore, anion mixing BH4(-)←X(-) (X(-)=Cl(-), Br(-), I(-)) provides a link to the known ABX3 halides.

Anisotropic diffraction peak broadening and dislocation substructure in hydrogen-cycled LaNi5 and substitutional derivatives

An erroneous equation and some consequently underestimated values of dislocation densities in the paper by Cerný et al. [J. Appl. Cryst. (2000), 33, 997–1005] are corrected.

Hydrogen cycling induced degradation in LaNi5-type materials

Defect generation in hydrogen cycled LaNi5 and substituted derivatives was studied in previous work by analysis of the X-ray line broadening. In the present work, the pulverization in the same samples is analyzed by granulometric measurements and scanning electron microscopy. Both phenomena correspond to irreversible degradation of the initial intermetallic compounds, as confirmed by first cycle hysteresis presence in pressure composition isotherms. They are analyzed in terms of the lattice expansion occurring at the discrete phase transition between α and β phases and measured by X-ray diffraction. This study shows that, in addition to this latter parameter, the limit of elasticity and the resistance to rupture must be considered.

Effect of Fe excess on structural, magnetic and superconducting properties of single-crystalline Fe1+xTe1−ySey

Abstract Single crystals of Fe1+xTe1−ySey have been grown with a controlled Fe excess and Se doping, and the crystal structure has been refined for various compositions. The systematic investigation of magnetic and superconducting properties as a function of the structural parameters shows how the material can be driven into various ground states, depending on doping and the structural modifications. Our results prove that the occupation of the additional Fe site, Fe2, enhances the spin localization. By reducing the excess Fe, the antiferromagnetic ordering is weakened, and the superconducting ground state is favored. We have found that both Fe excess and Se doping in synergy determine the properties of the material and an improved 3-dimensional phase diagram is proposed.

Al3Li4(BH4)13: a complex double-cation borohydride with a new structure.

The new double-cation Al-Li-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (approximately 70 degrees C) combined with a high hydrogen density (17.2 wt%). It was synthesised by high-energy ball milling of AlCl(3) and LiBH(4). The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) A). The unexpected composition Al(3)Li(4)(BH(4))(13) can be rationalized on the basis of a complex cation [(BH(4))Li(4)](3+) and a complex anion [Al(BH(4))(4)](-). The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at approximately 70 degrees C, forming LiBH(4). The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane.

Crystallographic study of LaNi5−xSnx (0.2≤x≤0.5) compounds and their hydrides

The structural properties of tin substituted LaNi5 compounds and corresponding deuterides have been investigated by single crystal X-ray and neutron powder diffraction. Tin is found to substitute nickel exclusively on site 3g of space group P6/mmm. No other significant disorder occurs in the structure. At tin contents of x=0.4 and x=0.5, the crystal structures of the deuterides do not significantly deviate from those of other metal substituted LaNi5 deuterides. At a tin content of x=0.2, with higher deuterium content, however, a symmetry decrease to non-centrosymmetric space group P6mm is observed due to partial deuterium ordering.

Site Occupancies in the Battery Electrode Material LaNi3.55Mn0.4Al0.3Co0.75 as Determined by Multiwavelength Synchrotron Powder Diffraction

The battery electrode material LaNi 3.55 Mn 0.4 Al 0.3 Co 0.75 is a substitutional derivative of LaNi 5 in which La occupies site 1(a), and Ni, Mn, Al and Co sites 2(c) and 3(g) of space group P6/mmm. The distribution of the substituting elements Mn, Al and Co on the two latter sites was determined from synchrotron powder diffraction experiments by using the diffraction contrast produced by anomalous dispersion effects. The results of joint Rietveld refinement on three patterns collected close to the K edges of Ni, Co and Mn and on one pattern collected away from these edges showed that Al and Mn occupy almost exclusively site 3(g), while Co is distributed over sites 2(c) and 3(g) with a slight preference for site 3(g).

Hexamagnesium dicobalt undecadeuteride Mg6Co2D11: containing [CoD4]5− and [CoD5]4− complex anions conforming to the 18-electron rule

Abstract Mg6Co2D11 crystallizes in space group Pnma, a=8.1000(2) A , b=10.0643(2) A , c = 18.5664(4) A , V = 1513 A 3 , Z = 8. The partly disordered structure contains 14 symmetry-independent deuterium atoms, of which 9 are coordinated to cobalt in square-pyramidal [CoD5]4− and saddle-like [CoD4]5− complex anions, and 5 are bonded to magnesium only. The Co-D bond distances range between 1.52(1) A and 1.62(2) A. The limiting ionic formula can be written as 4Mg6Co2D11=5MgD219Mg2+2[CoD5]4−6[CoD4]5−. The 63-parameter structure was solved ab initio and refined from joint synchrotron and neutron high resolution powder diffraction data.