On the optical properties of thermoelectric alkali metal chalcogenide compounds
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0886-F08-01.1
On the optical properties of thermoelectric alkali metal chalcogenide compounds Ε. Hatzikraniotis(1), Th. Kyratsi(2,3), T. Zorba(1), K.M. Paraskevopoulos (1), M.G. Kanatzidis(2) (1)
Physics Department – Aristotle University of Thessaloniki, 54124 Thessaloniki, GREECE Department of Chemistry, Michigan State University, East Lansing, MI 48824 - USA (3) Department of Mechanical and Manufacturing Engineering, University of Cyprus, 1678 Nicosia, CYPRUS (2)
Abstract Solid solutions of K2Bi8-XSbXSe13 is an interesting series of complex bismuth-chalcogenide compounds, which are attractive for thermoelectric investigations. The highly anisotropic structure in these compounds results in needle-like morphology along the b- crystallographic axis, leaving structural tunnels where K+ ions reside. The complex electronic structure that arises from a large and low symmetry unit cell and the weakly bound K+ ions, lead to high Seebeck coefficient, highly anisotropic electrical properties and very low thermal conductivity. Reflectivity spectra in the FIR region are presented for several members in the Sb-rich (x ≥ 6) side. Optical investigations are carried out on crystalline and pelletized samples. The received spectra are analyzed, examined comparatively and discussed. Results indicate that upon Bi/Sb substitution in the Sb-rich region, phonons develop a mixed-mode behavior in the FIR spectral region. Introduction Investigations of ternary and quaternary compounds of alkali bismuth chalcogenides1 have shown that several compounds present promising thermoelectric properties. Alkali metals tend to create structural complexity in the crystal that can lead to complex electronic structure. The structure of β-K2Bi8Se132 include two different interconnected types of Bi/Se blocks, K ions positionally and compositionally disordered with Bi and loosely bound K atoms in tunnels. These features seem to be responsible for the low thermal conductivity of this compound (~1.3 W/m.K at room temperature). Recent work on thermal conductivity3 has shown the influence of the Bi/Sb partial substitution in K2Bi8-XSbXSe13 solid solutions; the low-temperature Umklapp peak of lattice thermal conductivity is gradually suppressed when the Sb concentration increases. For compositions x=5.6 and 7.2 the thermal conductivity peak is totally suppressed, and a temperature independent behavior appears, which is usually shown in ‘‘electron crystal–phonon glass’’ (ECPG) materials4. It is therefore interesting to examine the phonon spectra in this Sbrich range of compositions. In this work are reported for the first time the reflectivity spectra in the FIR region for several members in the Sb-rich (x ≥ 6.0) side. Optical investigations are carried out on crystalline and pelletized samples.
0886-F08-01.2
Figure 1: Crystal structure of K2Bi8-XSbXSe13 family of compounds. The structure is composed by interconnected rods of NaCl(100)- type and NaCl(111)-type local structure.
Experimental K2Bi8-XSbXSe13 solid solutions were synthesized with solid-state re
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