Crystal Chemistry of Borides
The inorganic structural chemistry of boron and its compounds can be characterized by a remarkable complexity which is due to the preferentially formed one-, two- and three-dimensional arrangements of covalently bonded boron atoms. This is mainly driven by the need of saturation of the valence requirements of the constituting electron-deficient boron atoms. The topological dimension of these aggregates can be correlated with the element-to-boron ratio. This relation has been particularly explored for compounds composed of metal atoms (transition elements, rare earth metals, actinides) and boron. There, depending on the metal content the boron substructure evolves from isolated boron atoms to boron pairs, fragments of chains and infinite chains followed by formation of two-dimensional frameworks.
SrPd4B and isotypic BaPd4B crystallize with a new structure type, which shows a similar arrangement of trigonal prisms consisting of palladium atoms and centred by boron as those observed in the borides from homologous series of derivatives of the CaCu5 type. Both synthesised borides are diamagnetic in the whole temperature range and show low metallic conductivity and no superconductivity above 0.35 K.
Figure 1. Boron-centred trigonal prisms in the CeCo3B2, CeCo4B and SrPd4B structure types: a) common fragments consisting of two edgesharing three-capped trigonal prisms [BT6R3] b) arrangement of the fragments [BT6R3] highlighted in Figure a and c) spatial arrangements of trigonal prisms [BT6] centred by boron atoms.
Thus, they are metals with a low electronic density of states at the Fermi level, which is confirmed for SrPd4B by theoretical calculations (1.7 eV–1 f.u.–1 at EF). From the electronic band structure no itinerant magnetism can be expected, even not for realistic amount of doping.
Figure 2. Total and partial (atom resolved) electronic density of states for SrPd4B (top). Orbital resolved DOS per atom for palladium and boron (bottom).
Gumeniuk R., Schmitt M., Schnelle W., Burkhardt U., Rosner H., Leithe-Jasper A. Z. Anorg. Allg. Chem. 636 (2010) 954.
Further increase of the boron fraction gives rise to the formation of three-dimensional covalently bonded boron frameworks. Here the type and degree of connectivity of boron polyhedra as well as the occupation of the interstitial sites by metal atoms become the structure governing elements.
Figure 3. Electron localizability indicator (ELI-D, Y) in YRhB4: (a) crystal structure view along ; (b) isosurface of Y = 1.73 visualize positions of ELI-D attractors revealing the two-center bonding within the boron network
I.Veremchuk, T. Mori, Yu. Prots, W.Schnelle, A. Leithe-Jasper, M. Kohout, Yu. Grin, J. Solid State Chem. 181 (2008) 1983-1991
Figure 4. Temperature dependence of the inverse magnetic susceptibility for TmRhB4 and YbRhB4
Figure 5. X-ray absorption spectrum at the Yb LIII edge of YbRhB4 in comparison with the reference Yb2O3 indicating the 4f13 state of ytterbium.
There, depending on the metal content the
boron substructure evolves from isolated
boron atoms to boron pairs, fragments of
chains and infinite chains followed by
formation of two-dimensional frameworks.
Figure 6. Isosurface of the electron localization function (η=0.75) of AlB2 (blue spheres, aluminum; green, boron; unit cell is given by black lines; view tilt from )
U. Burkhardt, V. Gurin, F. Haarmann, H. Borrmann, W. Schnelle, A. Yaresko, Yu. Grin, J. Solid State Chem. 177 (2004) 389-394
Figure 7. Crystal orientation EBSD maps and pattern quality images of sections of a Spark Plasma Sintered TiB2 sample
J. Schmidt, M. Boehling, U. Burkhardt, Yu. Grin, Science and Technology of Materials 8 (2007) 376-382