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

Five isostructural compounds, K 1.25 Pb 3.5 Bi 7.25 Se 15 ( I ), K 1.46 Sn 3.09 Bi 7.45 Se 15 ( II ), Rb 1.45 Pb 3.1 Sb 7.45 Se 15 ( III ), K 1.45 Pb 3.1 Sb 7.45 Se 15 ( IV ), and K 2.15 Pb 1.7 Sb 8.15 Se 15 ( V ) were prepared by the molten flux method as silver rodlike air-stable crystals. They all crystallize in the monoclinic space group P 2 1 / 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 A 1+ x M 4 - 2 x M‘ 7+ x Se 15 (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 +, M 2+, and M‘ 3+ atoms. The structure type has a three-dimensional framework assembled from NaCl- and Bi 2 Te 3 -type modular units. The framework features narrow tunnels filled with K + /Rb + ions. The NaCl- and Bi 2 Te 3 -type units are composed of edge-shared distorted Bi 3+ /Sb 3+ octahedra. The Pb/Sn atoms are stabilized in 8-coordinated bicapped trigonal prismatic sites at the connecting points of NaCl-type blocks and Bi 2 Te 3 -type blocks. There exists a considerable amount of occupancy disorder among Pb/Sn, Bi/Sb, and K/Rb in the structure. K 1.25 Pb 3.5 Bi 7.25 Se 15 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 A 1+ x M 4 - 2 x M‘ 7+ x Se 15 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.