No abstract

The new materials CsPbBi(3)Te(6) and CsPb(2)Bi(3)Te(7) were discovered through reactions of CsBi(4)Te(6) with PbTe, whereas the isostructural materials CsSnBi(3)Te(6) and CsSn(2)Bi(3)Te(7) were discovered through corresponding reactions with SnTe. The compounds can also be prepared from stoichiometric mixtures of Cs(2)Te, Pb (Sn), Bi, and Te. The crystal structures show a layered architecture of NaCl-type slabs alternating with layers of Cs atoms. This group of compounds offers a new quaternary system, Cs-M-Bi-Te (M = Pb and Sn), available for thermoelectric investigations, including fine-tuning of compositions and doping.

An attractive way to advance the design and prediction of new compositions and structures of solid-state compounds is to identify large homologies that are amenable to chemical control. Homologies allow the systematization of phases under a common conceptual umbrella and have the potential to capture the vast majority of existing and possible compounds. We have demonstrated this in the broad system A/M'/M''/Se (A = K, Rb, Cs, Sr, Ba; M' = Pb, Sn; M'' = Sb, Bi) by identifying the homologous "superseries" A(m)[M(1+l)Se(2+l)]2m[M(2l+n)Se(2+3l+n)]. This series generates a large variety of compounds with predictable composition and structure. All the compounds contain fundamental building units representing different fragments of the NaCl-type lattice. The three independent integers l, m, and n determine width, height, and shape of the building units and therefore cause structural evolution of the homologous series in three different dimensions. On the basis of this general formula one can design, in a modular fashion, new compounds that fit the structural evolution of the superseries, predicting simultaneously their structure and composition. Several new phases have been discovered with this approach. Here we give an overview of the character and predictive properties of the superseries and propose the classification of phases into homologies (when possible) which could serve as devices to predict new members.

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.

Silver colored ribbon-like crystals of K 12x Sn 52x Bi 11zx Se 22 (x#0.33) were prepared by direct combination of K 2 Se, Bi 2 Se 3 , Sn and Se at 800 ³C. A structure determination was carried out for K 0.66 Sn 4.82 Bi 11.18 Se 22 . Bi 2 Te 3type and NaCl-type building units are connected to form a new kind of three-dimensional anionic framework with K z ®lled tunnels. The building units are assembled from distorted, edge-sharing (Bi,Sn)Se 6 octahedra. Bi and Sn atoms are disordered over all metal sites of the chalcogenide network while the K site is only one third occupied. The charge transport measurements of K 0.66 Sn 4.82 Bi 11.18 Se 22 at room temperature reveal an electrical conductivity of 450 S cm 21 and a thermopower of 243 mV K 21 . The thermal conductivity is 1.5 W m 21 K 21 . An optical band gap of approximately 0.12 eV was determined at room temperature. K 0.66 Sn 4.82 Bi 11.18 Se 22 melts congruently at 680 ³C.