Barium aluminides [formula omitted][formula omitted

Abstract

Three aluminides of the series Ba x Al 5 ( x = 3 , 3.5 , 4 ) were synthesized from stoichiometric ratios of the elements in Ta crucibles. The crystal structure of the new compound Ba 7 Al 10 was determined using single crystal X-ray data (space group R 3 ¯ m , a = 604.23 ( 9 ) , c = 4879.0 ( 12 ) pm , Z = 3 , R 1 = 0.0325 ). The compound exhibits Al Kagomé (3.6.3.6.) nets in which half of the triangles form the basis of trigonal bipyramids Al 5 . The apical Al are thus three-bonded assuming a charge of - 2 ( 27Al-NMR chemical shift δ = 660 pm ), whereas the Al atoms of the basal triangle (i.e. of the Kagomé net) are four-bonded and thus of formal charge - 1 ( δ = 490 ppm ) . The total charge of the anion is thus exactly compensated by the Ba cations, i.e. the compound can be interpreted as an electron precise Zintl phase, exhibiting a distinct pseudo-band gap at the Fermi level of the calculated tDOS. According to the total formula, the structure displays a combination the stacking sequences of Ba 3 Al 5 and Ba 4 Al 5 , the structures of which have been redetermined with current methods (both hexagonal with space group P 6 3 / mmc ; Ba 3 Al 5 : a = 606.55 ( 7 ) , c = 1461.8 ( 2 ) pm , Z = 2 , R 1 = 0.0239 ; Ba 4 Al 5 : a = 609.21 ( 7 ) , c = 1775.8 ( 3 ) pm , Z = 2 , R 1 = 0.0300 ). These three compounds with slightly different electron counts but similar polyanions allow to compare the bond lengths, the electronic structures and the overall bonding situation in dependence of positive or negative deviation of the electron count in relation to the novel formally electron precise Zintl compound Ba 7 Al 10 .