Barium aluminides BaxAl5(x=3,3.5,4)

Three aluminides of the series BaxAl5(x=3,3.5,4) were synthesized from stoichiometric ratios of the elements in Ta crucibles. The crystal structure of the new compound Ba7Al10 was determined using single crystal X-ray data (space group R3-barm, a=604.23(9), c=4879.0(12)pm, Z=3, R1=0.0325). The compound exhibits Al Kagome (3.6.3.6.) nets in which half of the triangles form the basis of trigonal bipyramids Al5. The apical Al are thus three-bonded assuming a charge of -2 (27Al-NMR chemical shift δ=660pm), whereas the Al atoms of the basal triangle (i.e. of the Kagome net) are four-bonded and thus of formal charge -1(δ=490ppm). 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 Ba3Al5 and Ba4Al5, the structures of which have been redetermined with current methods (both hexagonal with space group P63/mmc; Ba3Al5: a=606.55(7), c=1461.8(2)pm, Z=2, R1=0.0239; Ba4Al5: a=609.21(7), c=1775.8(3)pm, Z=2, R1=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 Ba7Al10. - Al5 layers of Kagome nets in the new binary electron precise Zintl compound Ba3.5Al5, also found in Ba3Al5 and Ba4Al5.