A5InPb8 (A = K, Rb):: An apparent Zintl phase with lead tetrahedra interbridged by μ6-In atoms

Reaction of elemental In, Pb, and K, or Rb within welded Ta containers at 900 degrees C followed by subsequent annealing at 350 degrees C gives the new phases A(5)lnPb(8) (A = K, Rb). These crystallize in the trigonal space group R (3) over barm(No. 166, Z = 3) with cell dimensions of a = 6.8835(6) and 6.885(1) angstrom and c = 37.591(5) and 37.64(2) angstrom for K(5)lnPb(8) and Rb(5)lnPb(8), respectively. The structure contains clusters built of pairs of Pb-4 tetrahedra that are interbridged by a mu(6)-ln atom. The InPb8 units, which in the isolated case would behave as ideal 40-electron Wade's rule clusters, are weakly interlinked into sheets in the ab plane by long (3.5 angstrom) intercluster Pb-Pb interactions. According to the EHTB calculations, these cause a broadening of the valence band and thus generate a number of new states at the Fermi level. Compound K5lnPb8 is metallic (rho(298) approximate to 42 mu Omega cm, (delta rho/delta T)/rho 1.4(2) x 10(-1) K-1) which is in agreement with the expectations according to calculations on the anion network.