Modular Construction of A1+xM4-2xM‘7+xSe15 (A = K, Rb; M = Pb, Sn; M‘ = Bi, Sb): A New Class of Solid State Quaternary Thermoelectric Compounds
journal contributionposted on 10.02.2001, 00:00 by Kyoung-Shin Choi, Duck-Young Chung, Antje Mrotzek, Paul Brazis, Carl R. Kannewurf, Ctirad Uher, Wei Chen, Tim Hogan, Mercouri G. Kanatzidis
Five isostructural compounds, K1.25Pb3.5Bi7.25Se15 (I), K1.46Sn3.09Bi7.45Se15 (II), Rb1.45Pb3.1Sb7.45Se15 (III), K1.45Pb3.1Sb7.45Se15 (IV), and K2.15Pb1.7Sb8.15Se15 (V) were prepared by the molten flux method as silver rodlike air-stable crystals. They all crystallize in the monoclinic space group P21/m with a = 17.4481(8) Å, b = 4.1964(2) Å, c = 21.695(1) Å, β = 98.850(1)° for I, a = 17.454(5) Å, b = 4.201(1) Å, c = 21.760(6) Å, β = 98.550(5)° for II, a = 17.3160(7) Å, b = 4.1406(2) Å, c = 21.6401(8) Å, β = 99.139(1)° for III, a = 17.1204(6) Å, b = 4.1568(2) Å, c = 21.6362(8) Å, β = 98.706(1)° for IV, and a = 17.167(4) Å, b = 4.1494(9) Å, c = 21.684(5) Å, β = 98.664(3)° for V (Z = 2 for all compounds). The general formula A1+xM4-2xM‘7+xSe15 (A = K, Rb; M = Pb, Sn; M‘ = Bi, Sb) is derived from a large degree of variability in composition that is expressed in terms of mixed occupancy among A+, M2+, and M‘3+ atoms. The structure type has a three-dimensional framework assembled from NaCl- and Bi2Te3-type modular units. The framework features narrow tunnels filled with K+/Rb+ ions. The NaCl- and Bi2Te3-type units are composed of edge-shared distorted Bi3+/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/Sb, and K/Rb in the structure. K1.25Pb3.5Bi7.25Se15 is a potential new thermoelectric material with high power factor and low thermal conductivity at room temperature. The electrical conductivity and the thermopower for several A1+xM4-2xM‘7+xSe15 phases are reported. The optical band gaps were determined to be 0.53 (I), 0.39 (II), 0.36 (III), 0.45 (IV), and 0.60 (V) eV at room temperature. The compounds seem to melt congruently at 685 (I), 662 (II), 578 (III), 576 (IV), and 576 (V) °C.