Iwami Higashi

The crystal structure of Mg51Zn20

Abstract The crystal structure of Mg51Zn20, a phase designated conventionally as “Mg7Zn3,” has been determined by the single-crystal X-ray diffraction method. It was solved by the examination of a Patterson synthesis, and refined by the ordinary Fourier and least-squares method; the R value obtained was 4.8% for 1167 observed reflections. The crystal is orthorhombic, space group Immm, with a = 14.083(3), b = 14.486(3), c = 14.025(3) A, and Z = 2. There are 18 independent atomic sites, Zn1Zn6, Mg1Mg10, A, and B, and the last two sites are statistically occupied by Zn and Mg atoms with the occupancies; 0.46(2)Zn7 + 0.52(2)Mg11 and 0.24(2)Zn8 + 0.74(2)Mg12, for A and B, respectively. The structure of the crystal is described as an arrangement of icosahedral coordination polyhedra, to which all the atomic sites but Zn3 site belong. In this arrangement the Zn atoms other than the Zn3 and Zn8(B) center the icosahedral coordination polyhedra with coordination number 12. The Zn3, Zn8 atoms, and all the Mg atoms except Mg11(A) are located at the centers of various coordination polyhedra with the coordination numbers from 11 to 15. The distances between neighboring atoms are 2.71–3.07, 2.82–3.65, and 2.60–3.20 A for ZnZn, MgMg, and ZnMg, respectively.

Crystal growth of borides and carbides of transition metals from molten aluminum solutions

Abstract Single crystals of borides and carbides of IVA, VA, VIA metals and Mn(VIIA) have been grown by using molten Al as a solvent. The binary crystals grown were TiB 2 , ZrB 2 , HfB 2 , VB, V 3 B 4 , NbB 2 , TaB, W 2 B 5 , TiC and TaC. The amounts of Al atoms incorporated from the solvent into these crystals were about 0.001, 0.1, 0.1, 0.1, 0.1, 0.05, 0.05, 0.1, 0.02 and 0.1 wt% respectively. The crystals had sizes of 1–5 mm at least in one direction. The molten solutions prior to cooling were kept at 1500–1550°C for 10 h except TaC, which was kept at 1300°C for50 h. In all cases where the transition metals never formed binary boride and carbide crystals, they formed ternary compounds of the M-Al-B or M-Al-C type.

Structure of B6O boron-suboxide by Rietveld refinement

The structures of the B6O samples prepared by oxidizing boron with ZnO at temperatures of 1350–1500°C in an argon atmosphere have been refined by the Rietveld method. The B6O samples were found to contain an amorphous phase from their X-ray diffraction profiles in addition to B6O diffraction peaks. The results indicate that the B6O samples, space group R¯3m, no.166, ahex = 0.5367(1) nm and chex = 1.2328(2) nm in the hexagonal unit cell, have oxygen deficiencies with 0.76(6) oxygen occupancy. A radial distribution function method applied to an extracted portion of the amorphous phase, suggests that each amorphous phase has nearly similar short-range order structure to that of the α-tetragonal boron type rather than to those of any other related boron phases.

Refinement of the structure of MgAlB14

The structure of MgAlB14 (space group, Imam; a = 5.848(1) A; b = 8.112(1) A; c = 10.312(1) A; Z = 4) was reinvestigated using single-crystal X-ray diffractometry. The crystal was grown from a high temperature AlB melt containing a small quantity of magnesium. A total of 1277 unique reflections (Mo Kα radiation; 2θ < 100°) was collected and used in a block diagonal leastsquares refinement to a final R value of 0.023. The boron framework in the crystal is the same as that reported by Matkovich and Economy. However, the distribution of the metal atoms is significantly different, i.e. no aluminium atoms are present at the magnesium site which is split into two positions separated by 0.39 A. The occupancies of the metals are 78% and 75% for the magnesium and aluminium sites respectively. The chemical composition determined by the structure analysis is Mg0.78Al0.75B14. Re-examination of the structure using the previously published intensity data indicates that the reported crystal probably had the same chemical composition and metal distributions as the present crystal.

Crystal structure of α-AlB12

Abstract The crystal structure of α-AlB12 (tetragonal; a = 10.158(2) A, c = 14.270(5) A, space group P41212 or P43212) has been determined by the single-crystal X-ray diffraction method. It was solved by the Fourier technique initially based on a partial B12 icosahedral structure, which was inferred from crystal chemical considerations. Refinement was made with the aid of a full-matrix least-squares program leading to a final R value of 3.0%. The structure is based on a three-dimensional framework consisting of B12 icosahedra, B19 units, and single B atoms; the B19 unit is a twinned icosahedron with a triangular composition plane and a vacant apex on each side. The chemical unit is Al3.2·2B12·B·B19 and its number in the unit cell is 4. The Al atoms are distributed statistically over five sites in the boron framework. The occupancies of the sites are 72, 49, 24, 15, and 2%, respectively.

Raman effect in icosahedral boron-rich solids

Abstract We present Raman spectra of numerous icosahedral boron-rich solids having the structure of α-rhombohedral, β-rhombohedral, α-tetragonal, β-tetragonal, YB66, orthorhombic or amorphous boron. The spectra were newly measured and, in some cases, compared with reported data and discussed. We emphasize the importance of a high signal-to-noise ratio in the Raman spectra for detecting weak effects evoked by the modification of compounds, accommodation of interstitial atoms and other structural defects. Vibrations of the icosahedra, occurring in all the spectra, are interpreted using the description of modes in α-rhombohedral boron by Beckel et al. The Raman spectrum of boron carbide is largely clarified. Relative intra- and inter-icosahedral bonding forces are estimated for the different structural groups and for vanadium-doped β-rhombohedral boron. The validity of Badgers rule is demonstrated for the force constants of inter-icosahedral B–B bonds, whereas the agreement is less satisfactory for the intra-icosahedral B–B bonds.

Boron-rich crystals in A1-M-B (M = Li, Be, Mg) systems grown from high-temperature aluminum solutions

Abstract Crystals of icosahedral B12 compounds grown from high-temperature A1 solutions have been studied. The crystals grown are A1LiB14, A1MgB14 and A1~1.1Be~0.6B22. They are obtained as platelets or irregularly shaped polyhedra with maximum dimensions of 5 to 7 mm. The crystallography and the mechanical properties of these materials are presented.

Structural investigation of Cr2B3, Cr3B4, and CrB by single-crystal diffractometry

Abstract The crystal structures of Cr2B3, Cr3B4, and CrB, of which Cr2B3 is a new compound, were investigated by single-crystal X-ray diffractometry. Cr2B3 crystallizes in the orthorhombic space groupCmcm witha = 3.0264(5), b = 18.115(4), c = 2.9542(4)A˚, Z = 4. The intensity data were collected on a four-circle diffractometer with graphite-monochromatizedMoKα radiation. The structure was solved by the Patterson method and refined with a full-matrix least-squares program to anR value (= Σ|ΔF|Σ|Fo|) of 0.021 for 1310 reflections. The structures of Cr3B4 and CrB were refined starting from published data; theR values were 0.032 (Cr3B4) and 0.022 (CrB). The structure of Cr2B3 can be described by stacking sheets of the AlB2 structure type in the same manner as in the cases of Cr3B4 and CrB. The fusion of two neighboring sheets is performed by a 5Cr tetragonal pyramid and a Cr-B direct bond at their boundary. The essential difference between the structures of these compounds is only in the widths of the sheets. The relations of the widths of the sheets to the bond distances and angles of boron atoms are discussed.

Optical and electronic properties of the orthorhombic MgAIB14-type borides

Abstract A systematic investigation of the orthorhombic borides LiAlB 14 , MgAlB 14 and ErAlB 14 was carried out. In all cases several indirect allowed optical interband transitions with phonon emission were derived from transmission measurements in the absorption edge range. For LiAlB 14 the temperature dependence between 22 and 293 K was determined. The results are compatible with the interband photoconductivity and the electroabsorption. Despite the identical basic structures of the icosahedral boron network, the interband transition energies of the orthorhombic borides depend on the chemical composition. As for other icosahedral boron-rich solids, there are edge tails with considerable absorption levels extending to lower energies, at least in MgAlB 14 and ErAlB 14 , obviously evoked by the metal atoms. ErAlB 14 is a one-dimensional conductor or a one-dimensional high-conductivity semiconductor with a distinct plasma edge close to 2000 cm −1 . The IR phonon spectra in the spectral range between 2000 and 1200 cm −1 show the vibrations of the light single atoms. The phonon spectrum at 1200 cm −1 or less, belonging essentially to the intra-icosahedral vibrations, is closely related to the α-rhombohedral boron structural group. Phonon quenching as a result of electron-phonon interaction on the icosahedra occurs as the atomic number of the metal atoms is increased. The carrier type depends on the transfer of electrons from the metal atoms to the structure. In the case of MgAlB 14 the Seebeck coefficient is of the order of −6500 μV K −1 making it of interest for thermoelectric applications.

Crystal structure of CuB23

Abstract The crystal structure of CuB23 has been determined by the single-crystal X-ray diffraction method. The final R value was 5.5%. The crystal is rhom-bohedral, space group R 3 m , with the hexagonal lattice constants, a = 10.985(1) and c = 23.925(2) A . The boron framework of the crystal is basically the same as that in β-rhombohedral boron. The Cu atoms are distributed statistically over three kinds of holes in the boron framework, and in one of the holes the Cu site is split into two at an interval of 0.4(1)A. Comparisons with CuB √ 28 reported in the literature are also made, especially concerning the occupancies of the Cu sites.