A5InPb8 (A = K, Rb): an apparent zintl phase with lead tetrahedra interbridged by mu6-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 A5InPb8 (A = K, Rb). These crystallize in the trigonal space group R3m (No. 166, Z = 3) with cell dimensions of a = 6.8835(6) and 6.885(1) A and c = 37.591(5) and 37.64(2) A for K5InPb8 and Rb5InPb8, respectively. The structure contains clusters built of pairs of Pb4 tetrahedra that are interbridged by a mu6-In 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 A) 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 K5InPb8 is metallic (rho298 approximately 42 microohms cm, (delta rho/delta T)/rho approximately 1.4(2) x 10(-1) K(-1)) which is in agreement with the expectations according to calculations on the anion network.