The nature of the thermoelectric materials Ag(1-x)Pb(m)SbTe(m+2) or LAST-m materials (LAST for Lead Antimony Silver Tellurium) with different m values at the atomic as well as nanoscale was studied with powder/single-crystal X-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy. Powder diffraction patterns of different members (m = 0, 6, 12, 18, infinity) are consistent with pure phases crystallizing in the NaCl-structure-type (Fmm) and the proposition that the LAST family behaved as solid solutions between the PbTe and AgSbTe2 compounds. However, electron diffraction and high resolution transmission electron microscopy studies suggest the LAST phases are inhomogeneous at the nanoscale with at least two coexisting sets of well-defined phases. The minority phase which is richer in Ag and Sb is on the nanosized length scale, and it is endotaxially embedded in the majority phase which is poorer in Ag and Sb. Moreover, within each nanodomain we observe extensive long range ordering of Ag, Pb, and Sb atoms. The long range ordering can be confirmed by single crystal X-ray diffraction studies. Indeed, data collections of five different single crystals were successfully refined in space groups of lower symmetry than Fmm including P4/mmm and Rm. The results reported here provide experimental evidence for a conceptual basis that could be employed when designing high performance thermoelectric materials and dispel the decades long belief that the systems (AgSbTe2)(1-x)(PbTe)x are solid solutions.

Thermoelectric (TE) power generation has come to be appreciated as an attractive means of low-cost conversion of waste heat to useful electrical energy with a small environmental impact. For a compound to qualify as an efficient thermoelectric material it should exhibit the highest TE figure of merit, ZT, possible at the temperature of operation, T. ZT is defined asand it involves the simultaneous manipulation of the TE power (absolute Seebeck coefficient) S, the electrical conductivity r, and the thermal conductivity j. The search for efficient TE materials mainly focuses on degenerate semiconductors since the underlying physics of these systems allow the coexistence of high thermopower values with high electrical conductivity to achieve high power factors: PF = S 2 r. The Seebeck coefficient is inversely related to the electrical conductivity according to the Boltzmann transport equation, and, as a result, maximization of one cannot be achieved without minimization of the other. An interesting alternative that has been recently suggested to achieve high power factors is the quantum-confinement effect; however, definite experimental verification of this is still lacking.[1]Another route to achieving high-performance TEs is through the minimization of the thermal conductivity. To this end, many suggestions have been made to increase ZT. These include the phonon-glass electron-crystal approach [2] (where loosely bound atoms rattle in cage structures [3] ) as in clathrates, [4] and the thin-film multilayer approach where the introduction of interfaces significantly reduces phonon propagation.[ [10] where compositional fluctuations at the nanoscopic level, resulting in a distinct type of nanostructuring, seem to play a key role in the previously reported very low thermal conductivity. [11] In contrast to the thin-film multilayers, bulk nanocomposite systems offer the advantages of large-scale industrial production and the sustenance of large thermal gradients for extended time. The challenge, therefore, lies in identifying equally efficient p-type materials so that they can be employed in the fabrication of TE modules.Here we report on the Ag(Pb 1 -y Sn y ) m SbTe 2 + m series and show that certain compositions exhibit high performance p-type TE properties (e.g., ZT ∼ 1.45 at 630 K) as a result of their very low thermal conductivity. We show as well that the Ag(Pb 1 -y Sn y ) m SbTe 2 + m systems are in fact bulk nanocomposites. We demonstrate that varying the m and y values, as well as the Ag and Sb concentrations, allows for control over a wide range of properties such as carrier concentration, TE power, and thermal conductivity. These exceptional properties, derived from specific compositions, outperform the standard state-of-the-art p-type systems like TAGS ((AgSbTe 2 ) 0.15 (GeTe) 0.85 , ZT ∼ 1.2 at 720 K [12] ), PbTe (ZT ∼ 0.7 at 740 K [13] ), and Zn 4 Sb 3 (ZT ∼ 1.3 at 670 K [14] ).The electronic-transport properties of the Ag(Pb 1 -y Sn y ) mSbTe 2 + m system can be tuned primarily through carefully control...

Developing advanced thermoelectric (TE) materials can impact conversion technologies of waste heat to electrical power. It is well expected that by fabricating TE materials in a nanostructured form their properties can be significantly enhanced. 1 Efficient TE materials must exhibit a large TE figure of merit, ZT, defined as ZT ) σS 2 /κ, where S is the TE power (absolute Seebeck coefficient), σ is the electrical conductivity, and κ is the thermal conductivity. The Seebeck coefficient is generally inversely related to the electrical conductivity and as a result maximization of ZT is difficult.One modality in achieving high performance is through the minimization of the thermal conductivity using nanostructures. Superlattice thin film structures grown by molecular beam epitaxy (MBE) PbSe 0.98 Te 0.02 /PbTe 2-4 have achieved ZT values > 3. 5 These MBE-grown materials contain pyramidal-shaped "nanodots" of PbSe with uniform size (∼20 nm) embedded inside a matrix of PbTe. This assembly possesses record low values of thermal conductivity (∼0.3-0.4 W/(m‚K)) while at the same time retains a high power factor. Therefore, it is of significant interest to devise general, convenient, low cost synthetic methodologies for incorporating similar nanometer scale inclusions into bulk semiconductor materials in an effort to mimic the high ZT superlattice structures. Examples of materials with naturally occurring nanostructuring and enhanced TE properties have been reported. 6,7 Here we describe the intentional preparation of nanometer sized inclusions of Sb, Bi, and InSb in bulk PbTe using a general liquid matrix encapsulation technique. We observe that nanocrystals with large contrast in average mass (e.g., Sb and InSb) with that of the PbTe medium achieve stronger scattering of acoustic phonons than nanocrystals with minimal contrast. We also find that the reduction in lattice thermal conductivity is not monotonic with increasing concentration of nanoparticles, but there is an optimum concentration beyond which the lattice thermal conductivity actually increases. These results are in agreement with theoretical expectations and recent reports that embedded nanocrystals of ErAs promote strong scattering of acoustic phonons in a InGaAs matrix. 8 There is a plethora of published work on the preparation of stable free-standing semiconductor nanocrystals capped with surfactants, 9 embedded in polymers 10 or glasses. 11 However, there is relatively little effort devoted to preparing nanocrystals inside solid matrices or bulk crystals. 12-14 In general, bulk crystals with nanocrystals embedded in them represent a fascinating set of nanostructured materials whose scope extends beyond the field of thermoelectrics especially when the properties of guest/matrix are chosen for specific functions.To achieve nanoscale matrix encapsulation of a minority phase A inside a majority phase B, we choose the former to have very low or no solubility in the solid state but to be completely soluble in the liquid state. We choose the major phase B to ha...

The series of Pb(9.6)Sb(0.2)Te(10)(-)(x)Se(x) compounds with different Se content (x) were prepared, and their structure was investigated at the atomic and nanosized regime level. Thermoelectric properties were measured in the temperature range from 300 to 700 K. The Pb(9.6)Sb(0.2)Te(10)(-)(x)Se(x) series was designed after the refinement of the single-crystal structure of Pb(3.82)Sb(0.12)Te(4) (Pb(9.6)Sb(0.3)Te(10); S.G. Pmm) by substituting isoelectronically in anion positions Te by Se. The Pb(9.6)Sb(0.2)Te(10)(-)(x)Se(x)() compounds show significantly lower lattice thermal conductivity (kappa(L)) compared to the well-known PbTe(1)(-)(x)Se(x) solid solutions. For Pb(9.6)Sb(0.2)Te(3)Se(7) (x = 7), a kappa(L) value as low as 0.40 W/m.K was determined at 700 K. High-resolution transmission electron microscopy of several Pb(9.6)Sb(0.2)Te(10)(-)(x)Se(x) samples showed widely distributed Sb-rich nanocrystals in the samples which is the key feature for the strong reduction of the lattice thermal conductivity. The reduction of kappa(L) results in a significantly enhanced thermoelectric figure of merit of Pb(9.6)Sb(0.2)Te(10)(-)(x)Se(x) compared to the corresponding PbTe(1)(-)(x)Se(x) solid solution alloys. For Pb(9.6)Sb(0.2)Te(3)Se(7) (x = 7), a maximum figure of merit of ZT approximately 1.2 was obtained at approximately 650 K. This value is about 50% higher than that of the state-of-the-art n-type PbTe. The work provides experimental validation of the theoretical concept that embedded nanocrystals can promote strong scattering of acoustic phonons.