Modular construction of A(1+x)M(4-2x)M ' Se-7+x(15) (A = K, Rb; M = Pb, Sn; M ' = Bi, Sb): A new class of solid state quaternary thermoelectric compounds

Five isostructural compounds, K1.25Pb3.5Bi7.25Se15 (I), K1.46Sn3.09Bi7.45Se 15 (II), Rb1.45Pb3.1- Sb7.45Se15 (III), K1.45Pb3.1Sb7.45Se15 (IV), and K2.1 5Pb1.7Sb8.15Se15 (V) were prepared by the molten flux method as silver rodl ike air-stable crystals. They all crystallize in the monoclinic space group P2(1)/m with a = 17.4481(8) Angstrom, b = 4.1964(2) Angstrom, c = 21 695(1 ) Angstrom, beta = 98.850(1)degrees for I, a 17.454(5) Angstrom, b = 4.201( 1) Angstrom, c = 21.760(6) Angstrom, beta = 98.550(5)degrees for II, a = 17 .3160(7) Angstrom, b = 4.1406(2) Angstrom, c = 21.6401(8) Angstrom, beta = 99.139(1)degrees for III, a = 17.1204(6) Angstrom, b = 4.1568(2) Angstrom, c = 21.6362(8) Angstrom, beta = 98.706(1)degrees for IV, and a = 17.167(4) Angstrom, b = 4.1494(9) Angstrom, c = 21.684-(5) Angstrom, beta = 98.664(3) degrees for V (Z = 2 for all compounds). The general formula A(1+x)M(4-2x)M 'Se-7+x(15) (A = K, Rb; M = Pb, Sn; M' = Bi, Sb) is derived from a large de gree of variability in composition that is expressed in terms of mixed occu pancy among A(+), M2+, and M'(3+) atoms. The structure type has a three-dim ensional framework assembled from NaCl- and Bi2Te3-type modular units. The framework features narrow tunnels filled with K+/Rb+ ions. The NaCl- and Bi 2Te3-type units are composed of edge-shared distorted Bi3t/Sb3+ octahedra. The Pb/Sn atoms are stabilized in 8-coordinated bicapped trigonal prismatic sites at the connecting points of NaCl-type blocks and Bi2Te3-type blocks. There exists a considerable amount of occupancy disorder among Pb/Sn, Bi/S b, and K/Rb in the structure. K1.25Pb3.5- Bi7.25Se15 is a potential new the rmoelectric material with high power factor and low thermal conductivity at room temperature. The electrical conductivity and the thermopower for seve ral A(1+x)M(4-2x)M'Se-7+x(15) phases are reported. The optical band gaps we re determined to be 0.53 (I), 0.39 (II), 0.36 (III), 0.45 (IV), and 0.60 CV ) eV at room temperature. The compounds seem to melt congruently at 685 (I) , 662 (II), 578 (III), 576 (TV), and 576 (V) degreesC.