Modular Construction of A_1+xM_4-2xM‘_7+xSe₁₅ (A = K, Rb; M = Pb, Sn; M‘ = Bi, Sb):  A New Class of Solid State Quaternary Thermoelectric Compounds

Abstract

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+ x M4 - 2 x M‘7+ x Se15 (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+ x M4 - 2 x M‘7+ x Se15 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.

Structure and Thermoelectric Properties of the New Quaternary Bismuth Selenides A1−xM4−xBi11+xSe21 (A=K and Rb and Cs; M=Sn and Pb)—Members of the Grand Homologous Series Km(M6Se8)m(M5+nSe9+n)

“…[6, 7] K 2.5 Bi 8.5 Se 14 , [15] K 1x Sn 4À2x Bi 7x Se 15 , [8] and K 1Àx Sn 5Àx Bi 11x Se 22 . [13] The local environment of alkali metal ions, and the size of the NaCl-and Bi 2 Te 3 -type building units are very similar in K 1Àx Sn 4Àx Bi 11x Se 21 and K 1Àx Sn 5Àx Bi 11x Se 22 , see Figure 2.…”

“…According to Slacks phonon glass electron crystal concept [1a,b] the alkali atoms can reduce the thermal conductivity by undergoing ªrattlingº motion in tunnels or cages in the anionic framework. However, the low crystal symmetry and structural Because K 1.25 Pb 3.5 Bi 7.25 Se 15 [8] is an encouraging thermoelectric material, we investigated the system K/Sn/Bi/Se for analogues. In addition to the isostructural K 1x Sn 4À2x Bi 7x Se 15 , [8] we also discovered K 1Àx Sn 5Àx Bi 11x Se 22 [13] and K 1Àx Sn 4Àx Bi 11x Se 21 and the isostructural analogues Rb 1Àx Sn 4Àx Bi 11x Se 21 , Cs 1Àx Sn 4Àx Bi 11x Se 21 , K 1Àx Pb 4Àx Bi 11x Se 21 , Rb 1Àx Pb 4Àx Bi 11x Se 21 , and Cs 1Àx Pb 4Àx Bi 11x Se 21 .…”

“…However, the low crystal symmetry and structural Because K 1.25 Pb 3.5 Bi 7.25 Se 15 [8] is an encouraging thermoelectric material, we investigated the system K/Sn/Bi/Se for analogues. In addition to the isostructural K 1x Sn 4À2x Bi 7x Se 15 , [8] we also discovered K 1Àx Sn 5Àx Bi 11x Se 22 [13] and K 1Àx Sn 4Àx Bi 11x Se 21 and the isostructural analogues Rb 1Àx Sn 4Àx Bi 11x Se 21 , Cs 1Àx Sn 4Àx Bi 11x Se 21 , K 1Àx Pb 4Àx Bi 11x Se 21 , Rb 1Àx Pb 4Àx Bi 11x Se 21 , and Cs 1Àx Pb 4Àx Bi 11x Se 21 . We describe here the crystal structure, charge transport properties, and thermal conductivities of the new family A 1Àx M 4Àx Bi 11x Se 21 .…”

Alte und neue Konzepte für thermoelektrische Materialien

“…[120] Versuche, den ZT-Wert durch eine Substitution von Sb gegen Sn zu erhöhen, blieben erfolglos. [121] [139] ALn 1AEx Bi 4AEx S 8 [140] (A = K, Rb; Ln = La, Ce, Pr, Nd), BaLaBi 2 Q 6 [141] (Q = S, Se), a-,bAPbBi 3 Se 6 [142] (A = K, Rb, Cs), K 1Àx Sn 5Àx Bi 11+x Se 22 , [143] A 1+x M' 4À2x Bi 7+x Se 15 [144] (A = K, Rb; M' = Sn, Pb), [145] SnBi 4 Se 7 , [146] CdBi 2 S 4 , [147] CdBi 4 S 7 , [147] Cd 2.8 Bi 8.1 S 15 , [147] Cd 2 Bi 6 S 11 , [147] Ba 3 Bi 6.67 Se 13 [148] und Ba 3 MBi 6 Se 13 [148] (M = Sn, Pb) zu nennen. Derartige Verbindungen haben eine niedrige Wärmeleitfähigkeit, hohe Thermokraft und oftmals eine hohe elektrische Leitfähig-keit, [133,134,149,150] Eine der bemerkenswertesten Phasen ist b-K 2 Bi 8 Se 13 , das wegen seiner geringen Wärmeleitfähigkeit und seines relativ hohen Leistungsfaktors interessant ist.…”

Design in Solid State Synthesis Based on Phase Homologies: A1−xSn9−xBi11+xSe26 (A=K, Rb, Cs)—A New Member of the Grand Homologous Series Am[M6Se8]m[M5+nSe9+n] with M=Sn and Bi

“…Our approach, which has been outlined in detail elsewhere (3), is focused on complex quaternary and ternary bismuth chalcogenides incorporating alkali metals. The examples of CsBi 4 Te 6 (4), K 2 Bi 8 Se 13 (5,6), and K 1.25 Pb 3.5 Bi 7.25 Se 15 (7) show that these exploratory investigations can lead to novel thermoelectric materials with promising properties. Our investigations of the quaternary system K/Sn/Bi/Se led to various compounds, e.g., K 1+x Sn 3À2x Bi 7+x Se 14 (8) (15).…”