OPTICAL AND ELECTRONIC-PROPERTIES OF THE ORTHORHOMBIC MGALB14-TYPE BORIDES

A systematic investigation of the orthorhombic borides LiAlB14, MgAlB1 4 and ErAlB14 was carried out. In all cases several indirect allowed o ptical interband transitions with phonon emission were derived from tr ansmission measurements in the absorption edge range. For LiAlB14 the temperature dependence between 22 and 293 K was determined. The result s are compatible with the interband photoconductivity and the electroa bsorption. Despite the identical basic structures of the icosahedral b oron network, the interband transition energies of the orthorhombic bo rides depend on the chemical composition. As for other icosahedral bor on-rich solids, there are edge tails with considerable absorption leve ls extending to lower energies, at least in MgAlB14 and ErAlB14, obvio usly evoked by the metal atoms. ErAlB14 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 sing le atoms. The phonon spectrum at 1200 cm-1 or less, belonging essentia lly to the intra-icosahedral vibrations, is closely related to the alp ha-rbombohedral boron structural group. Phonon quenching as a result o f electron-phonon interaction on the icosahedra occurs as the atomic n umber of the metal atoms is increased. The carrier type depends on the transfer of electrons from the metal atoms to the structure. In the c ase of MgAlB14 the Seebeck coefficient is of the order of -6500 muV K- 1 making it of interest for thermoelectric applications.