Antje Mrotzek 1, Tim Hogan 2 and Mercouri G. Kanatzidis 1, * 1

Transcription

1 Mat. Res. Soc. Symp. Proc. Vol Materials Research Society Search for New Thermoelectric Materials through Exploratory Solid State Chemistry. The Quaternary Phases A 1+x M 3-2x Bi 7+x Se 14, A 1-x M 3-x Bi 11+x Se 20, A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se 22 (A = K, Rb, Cs, M = Sn, Pb) and the Homologous Series A m [M 6 Se 8 ] m [M 5+n Se 9+n ]: Antje Mrotzek 1, Tim Hogan 2 and Mercouri G. Kanatzidis 1, * 1 Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, MI Department of Electrical Engineering, Michigan State University, East Lansing, MI Abstract The compound types A 1+x M 3-2x Bi 7+x Se 14, A 1-x M 3-x Bi 11+x Se 20, A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5- xbi 11+x Se 22 (A = K, Rb, Cs; M = Sn, Pb) form from reactions involving A 2 Se, Bi 2 Se 3, M and Se. The single crystal structures reveal that they are all structurally related so that they all belong to the homologous series A m [M 6 Se 8 ] m [M 5+n Se 9+n ] (M = di- and trivalent metal), whose characteristics are three-dimensional anionic frameworks with tunnels filled with alkali ions. The building units that make up these structures are derived from different sections of the NaCl lattice. In these structures, the Bi and Sn (Pb) atoms are extensively disordered over the metal sites of the chalcogenide network, giving rise to very low thermal conductivity. These phases are all narrow gap semiconductors with 0.25 < E g < 0.60 ev and many possess physico-chemical and charge transport properties suitable for thermoelectric investigations. Introduction Our approach to discovering new thermoelectric materials centers on complex quaternary and ternary bismuth chalcogenides with anisotropic frameworks. [1] [2],[3] The examples of CsBi 4 Te 6 [3],[4] and β-k 2 Bi 8 Se 13 show that such exploratory investigations can lead to promising new thermoelectric materials. Naturally, we enlarged our investigations to quaternary systems of the type A/M/Bi/Se (A = K, Rb, Cs, M= Pb, Sn) and identified new materials such as A 1+x M 3-2x Bi 7+x Se 14, [5] K 1+x Sn 4-2x Bi 7+x Se 15, [6] K 1-x Sn 4-x Bi 11+x Se 21, [7] K 1-x Sn 5-x Bi 11+x Se 22, [8], which have structures closely related to each other and to those of β-k 2 Bi 8 Se 13 and K 2.5 Bi 8.5 Se 14. [9] It turns out they are all members of a newly identified grand homologous series A m [M 6 Se 8 ] m [M 5+n Se 9+n ] [7] (M = di- and valent metal) in both compositional and structural sense. Their structures are composed of [M 5+n Se 9+n ] (NaCl 111 -type) and [M 6 Se 8 ] m (NaCl 100 -type) units of variable dimensions defined by n and m, which link to produce anionic frameworks with alkali metal (A m ) filled G5.1.1

2 tunnels. The series has predictive properties so that it is possible to target and prepare new members. Here we discuss several materials for consideration in thermoelectric investigations with various compositions of the type A 1+x M 3-2x Bi 7+x Se 14, A 1-x M 3-x Bi 11+x Se 20, A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se 22 (A = K, Rb, Cs; M = Sn, Pb). Results and Discussion The family of materials discussed here were prepared by fusing at 800 C appropriate mixtures of A 2 Se (K, Rb, Cs, M (Pb, Sn), Se and Bi 2 Se 3 loaded in an evacuated carbon-coated silica tube. Generally, silver shiny, polycrystalline ingots composed of oriented long needles can be obtained. In this manner we synthesized many members of the families A 1+x M 3-2x Bi 7+x Se 14, A 1- xm 3-x Bi 11+x Se 20, A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se 22. A quantitative microprobe analysis using energy dispersive X-ray fluorescence spectroscopy was performed to obtain average compositions. Small variations of the ratio of the starting materials result in new compounds with gradually evolving structural features. The large collection of compounds discovered suggests strongly that the A 2 Q/MQ/Bi 2 Q 3 system is probably "infinitely adaptive". [10] That is a new structure type forms each time the composition changes rather than mixtures of two or more phases or solid solutions. Carving-up the NaCl Lattice. All the phases discussed here have one thing in common. All are made of structural fragments that can be thought of as having been excised out of a NaCl type lattice. These fragments represent various cuts of this lattice in different orientation and of different dimensions. Usually they vary in dimension along two directions of the NaCl lattice while the third dimension is infinite. This carving of the lattice results in either infinite slabs of various types or infinite rods. Figure 1 depicts the different ways the NaCl lattice can be sectioned to produce the building fragments observed in the compounds, discussed here. If the cut is made perpendicular to a certain direction (e.g. [100], [111]) the fragment may be called NaCl 100 type or NaCl 111 type respectively. Sometimes the NaCl type fragment is referred to as Bi 2 Te 3 type since the Bi 2 Te 3 structure also derives through excision of infinite twodimensional slabs cut perpendicular to the [111] direction of the NaCl lattice. Structure Description. The quaternary phases A 1+x M 3-2x Bi 7+x Se 14, A 1-x M 3-x Bi 11+x Se 20, A 1- xm 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se 22 all belong to the homologous series A m [M 6 Se 8 ] m [M 5+n Se 9+n ]. A 1-x M 3-x Bi 11+x Se 20 and A 1+x M 3-2x Bi 7+x Se 14 have n = 3 and m = 1 and 2, respectively, whereas A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se 22 have n = 4, m = 1 and n = 5, m = 1 respectively. Figures 2 and 3 show projections of their structures displaying distinct building units of the NaCl 111 -type and NaCl 100 -type. These units are linked side by side so as to form three-dimensional anionic frameworks with either fully or partially alkali ion filled tunnels. G5.1.2

3 NaCl100-type, m=2 NaCl100-type, m=1 NaCl111-type [100] Figure 1. The NaCl lattice viewed down the [110] direction. The boxed areas represent cuts that result in the building fragments observed in the homologous series. In principle, there are numerous ways to carve this archetypal lattice to produce an great abundance of potential building blocks and consequently new structural arrangements, not only for the A m [M 6 Se 8 ] m [M 5+n Se 9+n ] series but also for other homologies.. M 6 Se 8 -unit NaCl100-type m = 2 m = 1 M 8 Se 12 -unit NaCl111-type Figure 2. Comparison of the structures of A 1+x M 3-2x Bi 7+x Se 14 and A 1-x M 3-x Bi 11+x Se 20. The secondary NaCl 111 -type- and NaCl 100 -type building units are highlighted in both structures. Small white spheres: Se, large light-gray spheres with no surrounding bonds: A, dark-gray spheres: Bi, dark-gray spheres: M. G5.1.3

4 c a a NaCl-block Bi2Te3-block Figure 3. Comparison of the structures of A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se 22. Small white spheres: Se, large light-gray spheres with no surrounding bonds: A, dark-gray spheres: Bi, black spheres: M. The archetypal NaCl 111 -type (or Bi 2 Te 3 type) and NaCl 100 -type building units are highlighted in both structures. The structures depicted in Figure 2 possess the same [M 8 Se 12 ] NaCl 111 -type unit (see boxed area) which is four Bi-octahedra wide and two Bi-octahedra high and therefore resembles a cut out of a Bi 2 Te 3 -type layer. Condensation of these units via one octahedron edge results in a step-shaped layer of the formula [M 5+n Se 9+n ] (n = 3). The adoption of these stable structure types is accomplished through extensive mixed occupancy disorder between A and Bi, M and Bi, and even between all A, M, Bi atoms on selected key crystallographic sites. Those sites that are primarily occupied with Bi atoms have distorted octahedral geometry, the other sites involving A atoms can be of higher coordination. The distorted Bi-octahedra have interatomic distances ranging from Å for K 1.40 Sn 2.20 Bi 7.40 Se 14, Å for K 0.70 Sn 2.70 Bi Se 20, Å for Rb 0.36 Sn 2.36 Bi Se 20 and Å for Cs 0.46 Sn 2.46 Bi Se 20. The distances reflect the different extend of Sn/Bi disorder in the step-shaped layers of these compounds. The two structure types in Figure 2 differ in the size of the [M 6 Se 8 ]-units that link the stepped [M 5+n Se 9+n ]- layers (n = 3) via MSe interactions (M = Bi, Sn(Pb)) to a three-dimensional framework. In A 1- xm 3-x Bi 11+x Se 20 these blocks are three M octahedra wide and one octahedron high and therefore [7] very similar to those found in A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se [8] 22 (see Figure 3). All metal positions in this block show mixed Bi/M occupancy. In contrast, in A 1+x M 3-2x Bi 7+x Se 14 the corresponding [M 6 Se 8 ]-units are two M octahedra high in the direction perpendicular to the NaCl 111 -type-type layers. Here the outer surface of the NaCl 100 -type block shows considerable mixed K/Sn occupancy. For example in the specific case of K 0.70 Sn 2.70 Bi Se 20 we found 23% in the K1 site and 17% in the K2 sites that correspond to the Sn1 and Bi6 site respectively. This type of mixed occupancy significantly impacts the electrical properties of these materials. The phases of the type A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se 22 also bear a close structural relationship both among themselves and to those in Figure 2. The structures depicted in Figure G5.1.4

5 3 possess the same [M 8 Se 12 ] NaCl 111 -type unit (n=1). The structure of A 1-x M 4-x Bi 11+x Se 21 is composed of two distinct highlighted building units. The composition of the two phases differs only by one MSe per formula. In A 1-x M 4-x Bi 11+x Se 21 the Bi 2 Te 3 (i.e. NaCl 111 -type) building units in the structure are five Bi octahedra wide along the c-direction and they are joined in an offset fashion to form a stepped shaped layer. The connection point of these fragments is found at a single octahedral Bi site. In contrast, the same buildings units in A 1-x M 5-x Bi 11+x Se 22, which are also offset, are joined via an octahedron edge, Figure 3. In all four phases above the alkali metals are found in distorted tri-capped trigonal prismatic sites in the tunnels created by parallel arrangement of the two kinds of NaCl-type building units to a three-dimensional framework. Invariably in these structures the observed large atomic thermal displacement parameters (TDP)s are very large often 3-4 times the average value of the Bi/Se framework. This could be due to either rattling of the alkali atoms on their crystallographic sites or, more likely, positional disorder along the tunnel since the position is only about 1/3 occupied. Classification of the New Quaternary Selenides. Let us now examine how all the phases discussed here are members of the same homologous series A m [M 6 Se 8 ] m [M 5+n Se 9+n ] with specific n and m values. The close relationship of β-k 2 Bi 8 Se 13, A 1+x M 3-2x Bi 7+x Se 14 to A 1-x M 3-x Bi 11+x Se 20, A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se 22 becomes more apparent when we inspect the scheme depicted in Figure 4. The scheme places these and other phases in the same structural context and offers an easy way to predict new phases by combining known [M 6 Se 8 ] m - blocks and [M 5+n Se 9+n ]-layers. Besides varying n to obtain new compounds it would be worth exploring if members with m > 2 are also stable. For the three phases of A 1-x M 3-x Bi 11+x Se 20, A 1-x M 4-x Bi 11+x Se 21 and A 1-x M 5-x Bi 11+x Se 22 the NaCl 100 -type block remains the same and the latter two (i.e. n = 4 and n = 5) can be easily derived from A 1-x M 3-x Bi 11+x Se 20 by successively adding MSe equivalents to the [M 8 Se 12 ]-layers. The places in the scheme displaying question-marks are predicted phases which should exist and are worth targeting for synthesis. The A m [M 6 Se 8 ] m [M 5+n Se 9+n ] homology may capture only a fraction of the total number of phases possible, as we have observed compositions which not belong to the series. For example Cs 1-x Sn 1-x Bi 9+x Se 15 and Cs 1.5-3x Bi 9.5+x Se 15 crystallize in a structure type that does not belong to but is closely related to the members of the series A m [M 6 Se 8 ] m [M 5+n Se 9+n ]. These phases reveal a third dimension of structural evolution according to the formula A m [M 1+l Se 2+l ] 2m [M 1+2l+n Se 3+3l+n ]. 11 G5.1.5

6 m = 2 m = 1? M 6 Se 10 -layer (n = 1) (e.g. KSn 2 Bi 11 Se 19 ) β K 2 Bi 8 Se 13 (m = 2; n = 1) + 2 MSe KSn 3 Bi 7 Se 14 (m = 2; n = 3) M 8 Se 12 -layer (n = 3) + 1 MSe KSn 3 Bi 11 Se 20 (m = 1; n = 3)? (e.g. K 2 Sn 7 Bi 14 Se 29 ) M 9 Se 13 -layer (n = 4) + 1 MSe KSn 4 Bi 11 Se 21 (m = 1; n = 4) KSn 4 Bi 7 Se 15 (m = 2; n = 5) M 10 Se 14 -layer (n = 5) KSn 5 Bi 11 Se 22 (m = 1; n = 5) Figure 4. The homologous series A m [M 6 Se 8 ] m [M 5+n Se 9+n ]. A member generating scheme illustrating successive additions of MSe units to a M 5 Se 9 layer. Small white spheres: Se, large light-gray spheres: K, middle-gray spheres: M. G5.1.6

7 Charge Transport Properties. By far the majority of the phases depicted in Figure 4 await electrical and thermoelectric characterization and further investigation. Only a small number of members has been examined so far and these only in a preliminary sense. Table 1 summarizes room temperature values for thermopower, electrical conductivity and band gaps for several phases. All materials are n-type semiconductors. The electrical conductivity was measured on polycrystalline ingots of several A 1-x M 3- xbi 11+x Se 20 compounds and not surprisingly, was found to be strongly influenced by the preparation conditions. If the molten mixture of the starting materials was rapidly cooled, high electrical conductivity of ~1700 S/cm and 1350 S/cm was observed respectively for K 1-x Sn 3- xbi 11+x Se 20 and Rb 1-x Sn 3-x Bi 11+x Se 20. For a slowly cooled sample of K 1-x Sn 3-x Bi 11+x Se 20 a lower room temperature conductivity at 520 S/cm. For slowly cooled samples, in going from K to a Cs analog the electrical conductivity seems to decrease (370 S/cm for Cs 1-x Sn 3-x Bi 11+x Se 20 ). Generally, if all other conditions are the same, the Pb analogs K 1-x Pb 3-x Bi 11+x Se 20 (700 S/cm) and Cs 1-x Pb 3-x Bi 11+x Se 20 (690 S/cm) possess higher electrical conductivity than the corresponding Sn compounds. A similar Sn vs Pb trend tends to persist A 1+x M 3-2x Bi 7+x Se 14, see Table 1. The high conductivity of quenched samples is attributed to the generation of a large number of carrier producing defects in the structure, whose exact nature is not known but can be speculated to be M/Se anti-site defects or Se vacancies. As prepared these compounds possess moderate negative Seebeck coefficients with a nearly linear dependence, see Table 1 and Figure 5. The negative values indicate n-type behavior with electrons as the dominant charge carriers. With rising temperature from 300 to 400 K the absolute value of the negative Seebeck coefficient increases from -45 to -62 µv/k for K 1+x Sn 3-2x Bi 7+x Se 14 and -32 to 52 µv/k for K 1+x Pb 3-2x Bi 7+x Se 14, respectively. The Cs analogs have slightly higher thermopower at room temperature with -48 µv/k (Cs 1+x Sn 3-2x Bi 7+x Se 14 ) and - 56 µv/k (Cs 1+x Pb 3-2x Bi 7+x Se 14 ). Such moderately low values are reasonable considering the high electrical conductivity of these samples. Slowly cooled samples of K 1-x Sn 3-x Bi 11+x Se 20 exhibit a linearly rising absolute Seebeck value from -68 µv/k at 300K to -107 µv/k at 400K. However, we observed much lower thermopower for the quenched sample. The low values starting from -16 to -28 µv/k correspond to the higher electrical conductivity observed for this sample, see Table 1. Quenched Rb 1-x Sn 3-x Bi 11+x Se 20 behaves similarly, it is highly doped and exhibits low thermopower of -26 µv/k at 300 K that rises to -35 µv/k at 400 K. Annealing these samples for several hours or days can increase the thermopower significantly to values up to 150 to 200 µv/k range. This suggests the quenching process creates a large number of structural defects in these structures. By comparison, the slowly cooled sample of Cs 1-x Sn 3-x Bi 11+x Se 20 possesses higher thermopower (-69 µv/k increasing to -92 µv/k) consistent with its lower electrical conductivity of 370 S/cm. For K 1-x Pb 4-x Bi 11+x Se 21, Rb 1-x Sn 4-x Bi 11+x Se 21, Rb 1-x Pb 4-x Bi 11+x Se 21 and Cs 1-x Pb 4-x Bi 11+x Se 21 the electrical conductivity of polycrystalline samples as a function of temperature are displayed in G5.1.7

8 Figure 6 and the thermopower in Figure 7. Rb 1-x Pb 4-x Bi 11+x Se 21 showed a high conductivity with values starting around 1050 S/cm at 80K and falling to 530 S/cm at 300K. In comparison, the Sn analog is less conductive with an electrical conductivity of 450 S/cm at 80K and 160 S/cm at 400 K. The observed temperature dependencies of the conductivity are similar in all analogs, consistent with the behavior of a degenerately doped narrow band gap semiconductor. Table 1. Charge transport properties and band gaps for some members of the A m [M 6 Se 8 ] m [M 5+n Se 9+n ] series Compound σ 300K (S/cm) S 300K (µv/k) E g (ev) β-k 2 Bi 8 Se K 1+x Sn 3-2x Bi 7+x Se c) K 1+x Pb 3-2x Bi 7+x Se Cs 1+x Sn 3-2x Bi 7+x Se Cs 1+x Pb 3-2x Bi 7+x Se K 1+x Sn 4-2x Bi 7+x Se a) K 1-x Pb 3-x Bi 11+x Se a) Rb 1-x Sn 3-x Bi 11+x Se c) a) Rb 1-x Pb 3-x Bi 11+x Se b) Cs 1-x Sn 3-x Bi 11+x Se b) Cs 1-x Pb 3-x Bi 11+x Se c) a) K 1-x Sn 3-x Bi 11+x Se c) b) K 1-x Sn 3-x Bi 11+x Se Rb 1-x Sn 4-x Bi 11+x Se Cs 1-x Sn 4-x Bi 11+x Se K 1-x Pb 4-x Bi 11+x Se c) a) Rb 1-x Pb 4-x Bi 11+x Se b) Rb 1-x Pb 4-x Bi 11+x Se Cs 1-x Pb 4-x Bi 11+x Se K 1-x Sn 4-x Bi 11+x Se c) K 1-x Sn 5-x Bi 11+x Se c) a) quenched b) slowly cooled c) could not be determined reliably due to the high absorption from mid gap states. Variable temperature thermopower data from polycrystalline ingots of K 1-x Pb 4-x Bi 11+x Se 21, Rb 1-x Sn 4-x Bi 11+x Se 21, Rb 1-x Pb 4-x Bi 11+x Se 21 and Cs 1-x Pb 4-x Bi 11+x Se 21 show negative Seebeck coefficients and nearly linear dependence, see Figure 7. With rising temperature the Seebeck coefficient varies from - 14 µv/k at 75K to - 72 µv/k at 400K for Rb 1-x Sn 4-x Bi 11+x Se 21 and Cs 1- xpb 4-x Bi 11+x Se 21. We observed smaller thermopower for K 1-x Pb 4-x Bi 11+x Se 21 and Rb 1-x Pb 4-x Bi 11+x Se 21 G5.1.8

9 starting from - 4 µv/k at 75K to - 35 µv/k at 400K and from - 12 µv/k at 75K to - 45 µv/k at 300K, respectively. The change from a lighter alkali metal to a heavier one results in higher thermopower for A 1-x Pb 4-x Bi 11+x Se 21. However, due to different doping levels in the samples from preparation to preparation a definitive trend could not be found. We expect these compounds to be sensitive to doping and work along these lines is in progress. 0 Seebeck Coefficient (µv/k) KSn3Bi11Se20 KPb3Bi11Se20 RbSn3Bi11Se20 quenched CsSn3Bi11Se20 CsPb3Bi11Se20 KSn3Bi7Se14 KPb3Bi7Se14 KSn3Bi11Se20 quenched Temperature (K) Figure 5. Seebeck coefficient data as a function of temperature for selected members of the A 1+x M 3-2x Bi 7+x Se 14 to A 1-x M 3-x Bi 11+x Se 20 families. Electrical Conductivity / S/cm Electrical Conductivity / S cm K1-xPb4-xBi11+xSe21 Rb1-xSn4-xBi11+xSe21 Rb1-xPb4-xBi11+xSe21 Cs1-xPb4-xBi11+xSe Temperature / K Figure 6. Electrical conductivity data as a function of temperature for selected members of the family A 1-x M 4-x Bi 11+x Se 21. G5.1.9

10 Seebeck coefficient (µv/k) K 1-x Pb 4-x Bi 11+x Se Rb 1-x Sn 4-x Bi 11+x Se 21 Rb 1-x Pb 4-x Bi 11+x Se Cs 1-x Pb 4-x Bi 11+x Se Temperature (K) Figure 7. Seebeck coefficient data as a function of temperature for selected members of the family A 1-x M 4-x Bi 11+x Se 21. Very little work has been done yet on the K 1-x Sn 5-x Bi 11+x Se 22 samples. The electrical conductivity of an ingot of K 0.66 Sn 4.82 Bi Se 22 was measured to be 450 S/cm at room temperature. The thermopower of compacted samples of K 0.66 Sn 4.82 Bi Se 22 increases steadily up from 13 µv/k at 80 K to 98 µv/k at 600 K. Thermal Cond. / W m -1 K -1 2 Rb 1-xPb 4-xBi 11+xSe Temperature / K Thermal Cond. / W m -1 K -1 2 K 1-xSn 4-xBi 11+xSe Temperature / K Thermal Cond. / W m -1 K -1 2 Cs1-xPb4-xBi11+xSe Temperature / K Figure 8. Variable temperature thermal conductivity data from polycrystalline ingots of Rb 1-x Pb 4- xbi 11+x Se 21, K 1-x Sn 4-x Bi 11+x Se 2, and Cs 1-x Pb 4-x Bi 11+x Se 21. Thermal Conductivity. A common feature of all these phases is that the thermal conductivity is extraordinarily low. The low symmetry, large lattice constants and extensive mass fluctuation disorder are inherent in these phases, and almost guarantee an extremely high thermal resistance. The temperature dependence of the thermal conductivity was measured on polycrystalline ingots of K 1-x Pb 4-x Bi 11+x Se 21, Rb 1-x Sn 4-x Bi 11+x Se 21, Rb 1-x Pb 4-x Bi 11+x Se 21 and Cs 1-x Pb 4- G5.1.10

11 xbi 11+x Se 21. Besides K 1-x Pb 4-x Bi 11+x Se 21 which exhibited a slightly higher thermal conductivity (1.6 W/m K at 300K), all samples possess very low thermal conductivity around 1 Wm -1 K -1 at room temperature, Figure 8. The observed values for A 1-x M 4-x Bi 11+x Se 21 are among the lowest reported for materials with potential thermoelectric interest. Optimized Bi 2 Te 3 alloys have a thermal conductivity of ~1.5 W/m K, which is about 50% higher than those of A 1-x M 4-x Bi 11+x Se 21 (A = K, Rb, Cs; M = Sn, Pb). Because of their very low thermal conductivity it seems worthwhile to study systematically A/A or Pb/Sn solid solutions in order to enhance the thermoelectric properties. The thermal conductivity of K 0.66 Sn 4.82 Bi Se 22 is also low and it increases with rising temperature from 0.8 W/m K (80 K) to 1.4 W/m K (270 K). Absorption, α/s Energy gaps. The infrared absorption spectra of A 1+x M 3-2x Bi 7+x Se 14 and A 1-x M 3-x Bi 11+x Se 20 (A = K, Rb, Cs; M = Sn, Pb) were recorded at room temperature in the range of 0.1 to 0.7 ev. The optical band gaps of K 1+x Sn 3-2x Bi 7+x Se 14, Rb 1-x Sn 3-x Bi 11+x Se 20 and Cs 1-x Pb 3-x Bi 11+x Se 20 could not be determined reliably. However, for K 1+x Pb 3-2x Bi 7+x Se 14, Cs 1+x M 3-2x Bi 7+x Se 14, K 1- E g ~ 0.30 ev Energy, ev Figure 9. Infrared absorption spectrum of K 1- xsn 3-x Bi 11+x Se 20 obtained at room temperature. The energy band gap is indicated in the spectrum. xm 3-x Bi 11+x Se 20, Rb 1-x Pb 3-x Bi 11+x Se 20 and Cs 1-x Sn 3- xbi 11+x Se 20 we were able to observe optical band gaps between ~ 0.3 ev (see Figure 9) and ~0.6 ev. The narrow band gaps of these quaternary selenides are consistent with the observed charge transport behavior described above. Concluding Remarks The exploration of the system A 2 Q/MQ/Bi 2 Q 3 (A = K, Rb, Cs; M = Sn, Pb) leads to the suggestion that it may be "infinitely adaptive". The existence of the grand homologous series A m [M 6 Se 8 ] m [M 5+n Se 9+n ] that defines a large family of materials seems clear at this stage. All members of the homologous series are constructed from the basic NaCl and the NaCl 100 -type modules whose size vary according to n and m. The phases are n-type narrow band gap semiconductors with extremely low thermal conductivity. This broad class of materials promises to be a great new source of potentially useful TE materials. The pursuit of additional members of the series is currently under way. Acknowledgment. Financial support from the Office of Naval Research and DARPA is gratefully acknowledged. G5.1.11

12 References [1] (a) Kanatzidis, M. G.; DiSalvo, F. J. ONR Quarterly Review 1996, XXVII, (b) Chung, D.-Y.; Iordanidis, L.; Choi, K.-S.; Kanatzidis, M. G. Bull. Kor. Chem. Soc 1998, 19, (c) Kanatzidis, M. G. Semicond. and Semimetals, Academic Press 2001, 69, [2] Chung, D.-Y.; Hogan, T.; Brazis, P. W.; Rocci-Lane, M.; Kannewurf, C. R.; Bastea, M.; Uher, C.; Kanatzidis, M. G. Science 2000, 287, [3] (a) Kanatzidis, M. G.; Chung, D.-Y.; Iordanidis, L.; Choi, K.-S.; Brazis, P.; Rocci, M.; Hogan, T.; Kannewurf, C. Mat. Res. Soc. Symp. Proc. 1998, 545, (b) Brazis, P. W.; Rocci-Lane, M. A.; Ireland, J. R.; Chung, D.-Y.; Kanatzidis, M. G.; Kannewurf, C. R. Proc. Of the XVIII th Int. Conf. On Thermoelectrics (ITC 99), Baltimore, USA 1999, [4] (a) Kanatzidis, M. G.; McCarthy, T. J.; Tanzer, T. A.; Chen, L.-H.; Iordanidis, L.; Hogan, T.; Kannewurf, C. R.; Uher, C.; Chen, B. Chem. Mater. 1996, 8, (b) Chen, B.; Uher, C.; Iordanidis, L.; Kanatzidis, M. G. Chem. Mater. 1997, 9, [5] Choi, K.-S.; Chung, D.-Y.; Mrotzek, A.; Brazis, P.; Kannewurf, C. R.; Uher, C.; Chen, W.; Hogan, T.; Kanatzidis, M. G. Chem. Mater. 2001, 13, [6] Choi, K.-S.; Chung, D.-Y.; Mrotzek, A.; Brazis, P. W.; Kannewurf, C. R.; Uher C.; Chen W.; Hogan, T.; Kanatzidis, M. G., Chem. Mater. 2001, 13, [7] Mrotzek, A.; Chung, D-Y.; Hogan, T.; Kanatzidis, M. G. J. Mater. Chem. 2000, 10, [8] Mrotzek, A.; Chung, D-Y.; Ghelani, N.; Hogan, T.; Kanatzidis, M. G. Chem. Eur. J. 2001, 7, [9] Chung, D.-Y.; Choi, K.-S.; Iordanidis, L.; Schindler, J. L.; Brazis, P. W.; Kannewurf, C. R.; Chen, B.; Hu, S.; Uher, C.; Kanatzidis, M. G. Chem. Mater., 1997, 9, [10] (a) Anderson, J. S., J. Chem. Soc., Dalton Trans. 1973, 10, (b) Swinnea, J. S.; Steinfink, H., J. Solid State Chem. 1982, 41, (c) Mercurio, D.; Parry, B. H.; Frit, B.; Harburn, G.; Williams, R. P.; Tilley, R. J. D., J. Solid State Chem. 1991, 92, [11] Mrotzek, A.; Iordanidis L.; Kanatzidis, M. G., Chem. Commun. 2001, G5.1.12