Pressure Tuning of Crystalline As 2 Te 3
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FF4.6.1
Pressure Tuning of Crystalline As2Te3
T. J. Scheidemantel
1,2
, J. F. Meng 2 , J. V. Badding
2
1
Department of Physics, The Pennsylvania State University, University Park, PA, USA, 16802 2 Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, PA, USA, 16802
Abstract We report the pressure dependence of the thermoelectric power of As2 Te3 . Pressures up to 10 GP a were induced using a Mao-Bell diamond anvil cell. The absolute value of the thermoelectric power dropped from S ≈ 230µV /K at ambient pressure to S ≈ 75 GP a near 5 GP a. At 6 GP a it then increased rapidly to S ≈ 220µV /K. This behavior is indicative of a structural phase transition as suggested by previously published high pressure phase diagrams.
Introduction The efficiency of a thermoelectric refrigerator or power generator depends on its geometry and on the figure of merit Z, Z=
S2 , ρκ
(1)
where S is the Seebeck coefficient or thermoelectric power, ρ is the electrical resisitivity, and κ is the thermal conductivity which has two components, an electronic contribution and a lattice contribution.
FF4.6.2
Heavily doped alloys of semiconductors can have high figures of merit and hence much effort is aimed at improving their transport properties to enhance Z. The best bulk, room temperature thermoelectric materials are alloys containing Bi2 Te3 , Sb2 Te3 , Bi2 Se3 , and Sb2 Se3 [1, 2]. Sb2 Se3 is orthorhombic (P nma), while the remaining three binary compounds have a rhombohedral structure with spacegroup R3m. Solid solutions containing these constituents also have this same rhombohedral structure over a broad range of concentrations, and their thermoelectric properties have been investigated intensively. Other group V-VI compounds such as arsenic telluride have not been studied as much, likely because it is not isostructural with Bi2 Te3 . Several methods, such as the synthesis of new materials, the design of quantum structures, and combinatorial synthesis techniques, are all employed in the search for new or improved thermoelectric materials [3, 4, 5, 6, 7, 8]. Pressure tuning offers an alternative means to search for improvements in transport properties. If an improved property is observed under pressure, it provides a target for synthesis at ambient pressure. Pressure can be changed more rapidly than a new material can be synthesized, allowing the phase space of a material’s interaction parameters to be rapidly explored [9]. Few investigations have been performed on crystalline As2 Te3 as a thermoelectric material candidate [10, 11, 12]. Arsenic telluride has a monoclinic structure with spacegroup C2/m (α-As2 Te3 ) [13, 14]. α-As2 Te3 has a moderately high thermoelectric power, a low thermal conductivity for a crystalline material, but a poor electrical conductivity. These result in a figure of merit at room temperature that is much lower than currently used thermoelectric materials, ZT ∼ 0.16 [10, 11]. A high pressure phase of arsenic telluride (β−As2 Te3 ) is of the Bi2 Te3 -struct
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