1998 MRS Fall Meeting & Exhibit

Chairs

Mercouri Kanatzidis, Michigan State Univ 
Hylan Lyon, Marlow Industries Inc
Gerald Mahan, Univ of Tennessee
Terry Tritt, Clemson Univ

Symposium Support 

• DARPA
• General Motors Research & Development
• Keithley Instruments, Inc.
• MMR Technologies, Inc.
• Office of Naval Research
• Quantum Design, Inc. 

1998 Fall Exhibitor 

* Invited paper

SESSION Z1: GUIDANCE TO ADVANCED TEs 
Chair: Gerald D. Mahan 
Monday Morning, November 30, 1998 
Independence West (S)

10:00 AM OPENING REMARKS, Terry Tritt & Glen Slack. 
10:30 AM *Z1.1 
PROPERTIES OF NOVEL THERMOELECTRIC MATERIALS FROM FIRST PRINCIPLES CALCULACTIONS. David J. Singh , I.I. Mazin, J.L. Feldman, Complex Systems Theory Branch, Naval Research Laboratory, Washington, DC. 

Use of first principles methods based on density functional theory to investigate novel thermoelectric materials is illustrated for several empty and filled skutterudite compounds. Band structures and their relationship to transport properties especially as regards optimization of thermoelectric properties is discussed. Phonon models constructed from calculations and existing experimental data for CoSb₃ are presented. These are extended to the case of the filled skutterudite LaFe₄Sb₁₂and compared to neutron data. The calculations provide a new understanding of the low frequency modes associated with the strongly suppressed thermal conductivity of filled skutterudites. In particular, we find that the changes in phonon spectra due to La filling are assoiated with the strong interaction of nearly harmonic but low frequency La motions with those of nearby Sb atoms, and, to a lesser extent, Fe atoms, leading to strong scattering of the key phonons. 

11:00 AM *Z1.2 
ATOMIC DISPLACEMENT PARAMETERS: A USEFUL TOOL IN THE SEARCH FOR NEW THERMOELECTRIC MATERIALS? Brian C. Sales , Bryan C. Chakoumakos, David Mandrus, Solid State Division, ORNL, Oak Ridge, TN; Jeff W. Sharp, Marlow Industries, Dallas, TX. 

The atomic displacement parameter (ADP) measures the vibrational amplitude of an atom about its equilibrium position, although static disorder can also contribute to the ADP. In the description of a new crystal structure, crystallographers normally tabulate the ADP's for each atom in the structure. The effects that this parameter can have on various physical properties, however, has not been widely recognized. For instance, in the filled-skutterudite antimonides a large ADP for the rare earth atoms in the structure is indicative of local vibrational modes that strongly scatter heat carrying phonons. This results in an order of magnitude decrease in the thermal conductivity of the filled skutterudite relative to an unfilled analog compound. Evidence for these localized phonon modes has been obtained recently using heat capacity, resonant ultrasound spectroscopy and inelastic neutron scattering. The ideal thermoelectric material should possess the good electronic transport properties associated with crystalline semiconductors, but the poor thermal conductivity typical of a glass. Local vibrational modes in a ternary crystalline compound can result in just such an ``electron crystal-phonon glass'' solid. New ternary compounds with large ADP's and remarkably low thermal conductivities (< 0.6 W/m-K) will be introduced. Correlations between the ADP's and the thermoelectric properties of these compounds will be discussed. 

11:30 AM *Z1.3 
ELECTRONIC STRUCTURE OF COMPLEX BISMUTH CHALCOGENIDES AND OTHER NARROW-GAP THERMOELECTRIC MATERIALS. S.D. Mahanti , Dept of Physics and Astronomy, Michigan State University, East Lansing, MI. 

Considerable efforts are being undertaken to synthesize new materials with complex structure in the hope of discovering systems with enhanced thermoelectric characteristics. Since both electrical conductivity and thermoelectric coefficients, two of the three quantities which determine the figure of merit Z, depend sensitively on the electronic structure, we have carried out electronic band structure calculations in systems of potential thermoelectric interest. In this talk, I will focus on two classes of compounds, (i) ternary and multinary chalcogenides containing bismuth (BaBiTe₃ and CsBi₄Te₆), and (ii) rare earth nickel pnictides (RENiSb and RENiBi, where RE= Yb, Lu, Er). First principles calculations based on density functional theory within generalized gradient approximation have been used to obtain the electronic structure. These calculations were done using the general potential linearized augmented plane wave method (LAPW) implemented within WIEN97 code [1]. Many of these systems exhibit narrow-gap semiconducting or semimetallic behavior. The effect of changing the lattice constant on the band gap and density of states at the Fermi energy will be presented. The role of f-electrons on the nature of the states near the Fermi energy in the rare earth systems will be discussed. Transport properties such as conductivity and thermoelectric power calculated within Boltzman type theory will be discussed for a few selected systems. 

SESSION Z2: SKUTTERUDITES I 
Chair: George S. Nolas 
Monday Afternoon, November 30, 1998 
Independence West (S)

1:30 PM *Z2.1 
THE SYNTHESIS OF METASTABLE SKUTTERUDITES USING SUPERLATTICE REACTANTS. Heike Sellinschegg, Joshua R. Williams and David C. Johnson , Materials Science Institute and Dept. of Chemistry, University of Oregon, Eugene, OR; George Nolas, Research and Development Division, Marlow Industries, Dallas, TX. 

A series of kinetically stable crystalline skutterudites (M'_1-xM₄Sb₁₂ where M' = vacancy, RE, Hf, Al, Sn, Pb, Bi, In, Ga, Al and Y; M = Fe, Co) have been synthesized by controlled crystallization of elementally modulated reactants. Low angle diffraction data demonstrates that the elemental layers interdiffuse at temperatures below 150C. Nucleation of the skutterudite structure occurs with at large exotherm on annealing at temperatures near 200C regardless of the ternary metal. All of the metastable ternary compounds and the new binary compound were found to decompose exothermically on higher temperature annealing. The decomposition temperature ranged from 300C for the binary compound FeSb₃ to approximately 550C for the europium compound. The occupation of the ternary cation was found to depend on the composition of the initial reactant. Samples up to a half gram in size have been synthesized and hot-pressed into pellets. Results of electrical and thermal conductivity measurements of representative samples will be discussed. 

2:00 PM Z2.2 
FORMATION OF NEW FILLED-SKUTTERUDITES UNDER HIGH-PRESSURE. Hirotsugu Takizawa , Masayuki Ito, Kyota Uheda, Tadashi Endo, Tohoku Univ, Dept of Materials Chemistry, Sendai, JAPAN; Keiichi Miura, Chichibu Onoda Cement Corp, Sakura, Chiba, JAPAN. 

Various 14-group elements were inserted in the sukutterudite-type CoSb₃ host lattice under high-pressure and temperature conditions using the Belt-type high-pressure equipment. It was found that various elements could be inserted in the body-centered vacant site, resulting in the formation of new filled-skutterudites M_xCo₄Sb₁₂ (M: Si, Ge, Sn, Pb). The lattice parameter of resultant phase, M_xCo₄Sb₁₂, increased sequentially with increasing the metallic radius of inserted atom. The composition and the degree of space-filling were confirmed by density measurements. Rietveld refinement of the crystal structure revealed the rotation of linked CoSb₆ octahedra by atom insertion. Inserted atoms were weakly bonded to CoSb₆octahedra. Thermoelectric properties of M_xCo₄Sb₁₂ was evaluated in terms of the effects of atom insertion. 

2:15 PM Z2.3 
ANALYSIS OF ANTIMONY-TIN-BASED SKUTTERUDITES. S.B. Schujman , G.A. Slack, N.C. Nguyen, Rensselaer Polytechnic Institute, Department of Physics, Applied Physics and Astronomy, Troy, NY; G.S. Nolas, Marlow Industries, R&D Division, Dallas, TX; R.A. Young, Georgia Institute of Technology, School of Physics, Atlanta, GA; F. Mohammed, T. Tritt, Clemson University, Department of Physics, Clemson, SC. 

Since skutterudites were proposed as possible Phonon-Glass, Electron-Crystal materials, a lot of work has been done trying to fill the structural voids with foreign ``rattling'' atoms. In order to keep the electronic count per unit cell constant (and thus, the semiconducting properties of most of the compounds under study) partial replacement of either the cation or the anion in the original formula by an appropriate neighbor in the periodic table is an option. In the case of antimonides, replacing part of the Sb with Ge or Sn in order to compensate the extra charge introduced by void fillers has proved useful for compounds based on rare-earth filled IrSb₃. In the case of RhSb₃ we found that large quantities of Sn can be incorporated into the skutterudite structure of RhSb₃ without either filling voids or producing charge carriers. We have analyzed the stability of several cross-sections of the Rh-Sb-Sn ternary system and have found a wide range of compositions with the basic skutterudite structure as we vary the Sn content. In all the cases the Sn goes substitutionally into the Sb sites. Density measurements suggest the existence of metal vacancies, confirmed by Rietveld refinement with powder X-ray diffraction data. The possibility of Sn-induced mixed-valence of Rh on the anion sites is being investigated. 

2:30 PM Z2.4 SYNTHESIS OF A NEW THERMOELECTRIC MATERIAL, PB_XCO₄SB₁₂, USING MODULATED ELEMENTARY REACTANTS. Heike Sellinschegg , Doug Sillars, David C. Johnson, University of Oregon, Dept. of Chemistry and Materials Science Institute, Eugene, OR. 

Samples of Pb_XCo₄Sb₁₂, were synthesized using modulated elemental reactants. The lattice thermal conductivity of these skutterdites is extremely low which makes them extremely promising candidates for improved thermoelectric materials. It is believed that the low thermal conductivity is a result of the lead cation ''rattling'' around in the cage of the skutterudite structure and interacting with the phonons to significantly reduce their mean free paths. The lead filled skutterudites are thermodynamically unstable with respect to binary compounds and cannot be synthesized using conventional synthesis approaches. They can be prepared with the multilayer synthesis approach as discussed below: The initial reactants are made up of multiple repeats of a unit containing elemental layers of lead, cobalt and antimony. The elemental layers interdiffuse upon low temperature annealing and form amorphous reaction intermediates. Upon annealing at temperatures of about 150C crystallization of the skutterdite structure occurs. The compound is only kinetically stable, decomposing into a mixture of binary compounds upon annealing past a temperature of about 430C. With this method, the occupancy of the ternary cation can be controlled by varying the composition of the initial reactants. Samples have also been prepared partially replacing the cobalt by iron to control the number of carriers. The variation of the thermoelectric properties of these skutterudites as a function of lead occupancy will be discussed. 

SESSION Z3: CHALCOGENIDES I 
Chair: Jeff W. Sharp 
Monday Afternoon, November 30, 1998 
Independence West (S)

3:15 PM *Z3.1 
CHEVREL PHASES AS NEW THERMOELECTRIC MATERIALS. T. Caillat , G.J. Snyder, A. Borshchevsky, and J.-P. Fleurial, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA. 

Ternary chalcogenides of formula M_xMo₆X₈ (M= Cu, Ag, Ni, Fe, rare earth, etc.) and X = S, Se, or Te have been first identified by Chevrel in 1971 and are therefore often referred to as Chevrel phases. They have attracted considerable interest because of their superconducting properties with large critical magnetic fields. The crystal structures of the Chevrel phases present cavities which can greatly vary in size and can contain a large variety of atoms ranging from large ones such as Pb to small ones such as Cu. These atoms are not localized in the structure and, depending on their size, can move between different sites and may produce significant phonon scattering. Although most of the Chevrel phases studied until now were reported to be metallic, semiconducting Chevrel phases can be engineered by controlling the number of electrons per [Mo₆] cluster. This number can be controlled trough various substitution and filling of binary compounds. Because of their potential for low thermal conductivity as well as semiconducting properties through filling, these materials are attractive candidates for thermoelectric applications. We report on the synthesis and thermoelectric properties of several pseudo-binary and filled ternary compositions. Results obtained on filled compositions show that these materials possess very low thermal conductivity values (10 mW/cmK at 300K). The potential of these phases for thermoelectric applications will be discussed. 

3:45 PM Z3.2 
THERMOELECTRIC PROPERTIES OF TERNARY AND QUATERNARY TRANSITION METAL TELLURIDES. C.-C. Wang, and R.J. Cava , Princeton University, Department of Chemistry and Materials Institute, Princeton NJ, and A.P. Ramirez, Bell Laboratories, Murray Hill, NJ. 

The thermoelectric properties of a wide range of complex late transition metal tellurides has been explored through a search program similar in method to that used to find new superconducting materials. Although the compounds in many cases had ordinary Seebeck coefficients, three new types of materials with potentially good thermoelectric properties have been found. These are based on the lanthanide transition metal tellurides of the LnAgTe₂ and LnCu_0.5Te₂ structure types and complex rocksalt compounds typified by AgInMnTe₃. All are small bandgap semiconductors, and members in each family with Seebeck coefficients larger than 100 V/K are reported. Effects of doping on the thermoelectric properties will be described. 

4:00 PM Z3.3 
FLUX SYNTHESIS OF NEW MULTINARY BISMUTH TELLURIDES AND THEIR THERMOELECTRIC PROPERTIES. Duck-Young Chung ¹, Kyoung-Shin Choi¹, Paul W. Brazis², Carl R. Kannewurf² and Mercouri G. Kanatzidis¹, ¹Dept. of Chemistry and Center for Fundamental Materials Research, Michigan State Univ., East Lansing, MI; ²Dept. of Electrical and Computer Engineering, Northwestern Univ., Evanston, IL. 

Since thermoelectric properties of solid state materials are directly dependent on their crystal structure, we are interested in ìbreaking downî the Bi₂Te₃ structural framework by introducing or combining with it other elements or binary compounds. The molten flux technique, is a very useful tool for performing such explorations. We present the synthesis and structure of the selected chalcogenide materials such as Rb_0.5Bi_1.83Te₃, CsPb₂Bi₃Te₇, and RbPb₂Bi₃Te₇. The last two compounds are isostructural, while Rb0.5Bi1.83Te3 is related to them in a very interesting fashion. These three compounds have low-dimensional structures with NaCl-type Bi/(Pb)/Te layers of varying thickness and alkali metals are stabilized between the layers via ionic interations. The thermal stability, melting behavior, electrical conductivity and thermopower of these compounds will be presented. Work supported by ONR. 

4:15 PM Z3.4 
TRANSPORT PROPERTIES OF DOPED CsBi₄Te₆ THERMOELECTRIC MATERIALS. Paul W. Brazis , Carl R. Kannewurf, Northwestern Univ, Dept of Electrical and Computer Engineering, Evanston, IL; Duck-Young Chung, Mercouri G. Kanatzidis, Dept of Chemistry, East Lansing, MI. 

In previous investigations we have introduced a variety of new chalcogenide-based materials with promising properties for thermoelectric applications. The chalcogenide CsBi₄Te₆ was previously reported to have a high ZT product with a maximum value at 250K. In order to improve this value, a series of doped CsBi₄Te₆ samples have been synthesized. This paper concentrates on material characterization studies through the usual transport measurements to determine Z and the carrier concentration. These data determine the optimum doping levels for several different dopants. 
(This work is supported by ONR grant N00014-98-1-0443.) 

4:30 PM Z3.5 
CHEVREL PHASES AS GOOD THERMOELECTRIC MATERIAL. C. Roche , P. Pecheur, M. Riffel, A. Jenny, H. Scherrer, S. Scherrer, Laboratoire de Physique des Matériaux, Ecole des Mines, Parc de Saurupt, Nancy, FRANCE. 

Best thermoelectric materials have a high figure of merit Z. In order to increase this figure of merit, we can try to lower the thermal conductivity . This is the idea of Slack as he proposes an ideal thermoelectric material the Phonon-Glass and Electron-Crystal (PGEC). A good thermoelectric material, according to this model, must have a lattice thermal conductivity close to the minimum, a narrow band gap, high carriers-mobility, and low electronegativity difference between constituting atoms. To obtain thermal conductivity close to the minimum, the material must have some weakly bond atoms with low coordination number. Chevrel phases have been mostly studied for their superconducting properties. Their general formula is M_yMo₆X₈ where M is a cation and X a chalcogen atom (S, Se, Te). These compounds have an open lattice with large voids. The arrangement of the host structure Mo₆X₈ is the same for all compounds. Each building block is formed by a Mo octahedron surrounded by 8 chalcogen atoms which form nearly a cube.. The stacking of these clusters leads to a rhombohedral structure with an angle close to 90 and leaves a certain number of cavities where cations can be inserted. ``Big'' cations (such as Pb, Sn...) have a fixed position and compounds with big cations are stoichiometric. ``Small'' cations are statistically distributed between 12 positions and form non-non-stoichiometric compounds. These small cations are weakly bound and constitute good scattering centres for phonons. The lattice thermal conductivity of such compounds is then expected to be low. 
The concentration of small cations isn't fixed by geometrical but by electronic factors. Indeed Chevrel phases are characterised by the number of valence electrons by Mo atoms (called Cluster Valence Electron Number - cluster VEC). Stable phases have a VEC between 3,5 and 4. As said before most of Chevrel phases are metallic, but some of them are semiconducting. Band structure calculations allowed us to find some of them. We performed calculations on several compounds, SnMo₆Se₈, Cu₂Mo₆Se₈, Zn₂Mo₆Se₈, Cd₂Mo₆Se₈, using LMTO-TB method (calculations in direct space on a limited number of neighbours). SnMo₆Se₈ and Cu₂Mo₆Se₈ were found metallic whereas Zn₂Mo₆Se₈ and Cd₂Mo₆Se₈ are semiconducting. 

SESSION Z4: POSTER SESSION 
Chairs: Mercouri G. Kanatzidis and Hylan B. Lyon 
Monday Evening, November 30, 1998 
8:00 P.M. 
Grand Ballroom (S)

Z4.1 
THERMOPOWER OF IRON DISILICIDES SYNTHESIZED UNDER THE LASER ACTION. Mikhail Nishchenko , Stanislav Likhtorovich, Inst for Metal Physics, Dept of Electronic Structure, Kiev, UKRAINE. 

The features of the electron density distribution in iron disilicide's stoichiometric semiconducting beta phase, nonstoichiometric Fe(1-x)Si(2) metallic alpha phase with high concentration (x=0.13 - 0.23) of structural vacancies in Fe sublattice, and the metastable alpha phase produced by the laser-alloying of Fe and Si were studied using the positron annihilation and Mossbauer spectroscopies. The electrons of various symmetries are found to be redistributed between the atomic species as a result of the beta-to- alpha phase transition. Namely, quasifree 3p(Si) electrons move toward the vacancy site due to the breaking of Fe-Si covalent bond; the most itinerant 4s(Fe) electrons screen the positive charge formed at Si atoms; and 3d(Fe) electrons contribute to the Fe-Si covalent bond. The state of iron disilicide obtained under the laser action is an intermediate one between those of the metallic and semiconducting phases. Its quadruple splitting (0.50-0.54 mm/s) and isomer shift of 0.16 to 0.21 mm/s (referred to bcc Fe) are lower than those characteristic of the alpha phase (0.64 and 0.30 mm/s, respectively). The resonance lines' width is decreased, the s electron density at Fe-57 nucleus is increased, and concentration of the positron-trapping centers is reduced while the alpha-phase tetragonal lattice persists. The above-mentioned implies the weakening of the metallic component of Fe-Si bond and strengthening of the covalent one caused by deacreased concentration of structural vacancies. The laser-alloyed iron disilicides exhibit thermopower values of up to 1 mV/K at room temperature. 

Z4.2 
NANOPHASE METAL-CONDUCTING POLYMER COMPOSITES: A NEW APPROACH TO AN OLD PROBLEM (EXPERIMENTAL RESULTS). George H. Johnson , Robert A. Martin and W. Leo Johnson III, Integrated Cryoelectronics, Inc., Yorklyn, DE. 

Electro-active (conducting) polymers are known to have inherently high Seebeck coefficients, exceeding 1000 V/K. Incorporation of a nanophase metal, at low volume percent concentration, into a conducting polymer gives nanophase composites with very much higher electrical conductivity (increasing with volume percent concentration from 10^-7 S/cm to 10⁺¹ S/cm) and useful thermoelectric performance with very low thermal conductivity (about 10^-3 W/cmK). Specifically, we report upon nanophase particles of silver (n-Ag) incorporated into a special conducting polymer, regio-regular poly (3-octylthiophene) (RPOT). The resulting n-Ag/RPOT composites, at volume concentrations in a distinct range below 10 volume percent, show substantial thermoelectric effects with high electrical conductivity while maintaining high thermal resistivity. Data will be presented on the thermoelectric properties as a function of n-Ag volume percent concentration and thermoelectric performance data for simple devices. When the n-Ag particles and the RPOT polymer chains are effectively mixed in n-Ag/RPOT composites, the resulting material can have thermoelectric device performance for which a figure-of-merit, ZT, exceeding one is achievable. 

Z4.3 
THEORETICAL MODELING OF THERMOELECTRICITY IN Bi NANOWIRES. Xiangzhong Sun , Zhibo Zhang, MIT, Dept of Physics, Cambridge, MA; G. Chen, UCLA, Dept of Mechanical and Aerospace Engineering, Los Angeles, CA; M.S. Dresselhaus, MIT, Dept of Electrical Engineering and Computer Science and Dept of Physics, Cambridge, MA; J.Y. Ying, MIT, Dept of Chemical Engineering, Cambridge, MA. 

Theoretical models based on the basic band structure of bulk Bi, suitably modified for 1D quantum wires, suggest that Bi nanowires (with diameters < 100 nm) should have interesting thermoelectric properties associated with the quantum confinement-induced semimetal-semiconductor phase transition. These interesting properties are predicted to occur in this nanowire diameter range, with the detailed behavior depending on the crystalline orientation of the nanowires. The theoretical predictions are compared with experiments on ultra-fine Bi nano-wires (10-120 nm diameter) with packing densities as high as 7 x 10¹⁰/cm² that were fabricated by pressure injection of molten Bi into the evacuated channels of an anodic alumina template. The resulting Bi nano-wires are shown to be single crystals (with the same structure and lattice constant as bulk Bi) and all the nano-wires are similarly oriented along the wire axis. To make the Bi nanowires n-type and p-type, Te and Sn/Pb doping, respectively, is used. By carefully tailoring the Bi wire diameter and carrier concentration, substantial enhancement in the thermoelectric figure of merit is expected. 

Z4.4 
GROWTH OF BISMUTH TELLURIDE THIN FILMS BY HOT WALL EPITAXY, THERMOELECTRIC PROPERTIES. J.C.Tedenac1, S. Dal Corso, S.Charar, B.Liautard, LPMC - UMR, Montpellier, FRANCE; Medcos, Montpellier, FRANCE, UPRESA . Eugene Bataillon, Montpellier, FRANCE. 

It is well known that bismuth telluride compound (Bi₂Te₃), its isomorphs (Bi₂Se₃ and Sb₂Te₃) and their alloys have the optimum bandgap (0.13eV to 0.21eV) for efficient solid state cooling applications around 300K. Recently an interesting proposal has been made by many authors. They argued that the use of quantum well structures can enhance the figure of merit ZT as a result of the improvement of carrier charge density of state and the reduction of the thermal conductivity. However, for the production of such structures it is necessary to establish the optimum growth conditions and doping levels of thin films based on Bi₂Te₃ and its isomorphs. In this paper we reportthe growth characteristics of Bi₂Te₂ ternary alloys thin films elaborated by the Hot Wall Epitaxy (HWE) technique. Bismuth telluride based ternary alloys have been deposited as thin films on SiO₂ substrate using H.W.E.. The Hot Wall Epitaxy technique used have been demonstrated to be a suitable technique in chalcogenides growth. It has an important advantage in the growth of films of high quality : these are formed in closed chamber, that make possible to keep substrate at relatively high temperature Ts without selective loss of individual components from condensate. Experimental procedures, such as substrate and source materials preparations, have been described in our previous publications. Thin films obtained are well oriented (00l) and have block single-crystal structure. The electrical properties of these thin films have been studied.