X-ray Characterization, Electronic Band Structure, and Thermoelectric Properties of the Cluster Compound Ag₂Tl₂Mo₉Se₁₁

Abstract

We report on a detailed investigation of the crystal and electronic band structures and of the transport and thermodynamic properties of the Mo-based cluster compound Ag2Tl2Mo9Se11. This novel structure type crystallizes in the trigonal space group R3̅c and is built of a three-dimensional network of interconnected Mo9Se11 units. Single-crystal X-ray diffraction indicates that the Ag and Tl atoms are distributed in the voids of the cluster framework, both of which show unusually large anisotropic thermal ellipsoids indicative of strong local disorder. First-principles calculations show a weakly dispersive band structure around the Fermi level as well as a semiconducting ground state. The former feature naturally explains the presence of both hole-like and electron-like signals observed in Hall effect. Of particular interest is the very low thermal conductivity that remains quasi-constant between 150 and 800 K at a value of approximately 0.6 W·m(-1)·K(-1). The lattice thermal conductivity is close to its minimum possible value, that is, in a regime where the phonon mean free path nears the mean interatomic distance. Such extremely low values likely originate from the disorder induced by the Ag and Tl atoms giving rise to strong anharmonicity of the lattice vibrations. The strongly limited ability of this compound to transport heat is the key feature that leads to a dimensionless thermoelectric figure of merit ZT of 0.6 at 800 K.

Structural and Electrical Properties Characterization of Sb1.52Bi0.48Te3.0 Melt-Spun Ribbons

“…Most of the recently-discovered families of thermoelectric materials reach their maximum ZT values at high temperatures, typically between 700 and 1200 K [4][5][6][7][8][9][10][11][12]. For thermoelectric applications near room temperature, solid solutions of bismuth telluride Bi 2 Te 3 with the isomorphous compounds Sb 2 Te 3 and Bi 2 Se 3 are still nowadays the best materials with ZT values around unity in both pand n-type compounds [2].…”

Electrical, Thermal, and Magnetic Characterization of Natural Tetrahedrites–Tennantites of Different Origin

“…In the latter research, several families of semiconductors have been identified in recent years, all with very low thermal conductivity whose temperaturedependence mimics that of amorphous alloys. Among others, these include Zintl phases, 1,2 Mobased cluster compounds, [3][4][5] chalcogenide-based compounds, [6][7][8] and such cage-like structures as clathrates. 9,10 Combined with semiconductor-like properties, the extremely low thermal conductivity results in high dimensionless thermoelectric figures of merit, ZT, defined as ZT = S 2 /(qj)T where T is the absolute temperature, S the Seebeck coefficient or thermopower, q the electrical resistivity, and j the total thermal conductivity, which is the sum of a lattice contribution j l and an electronic contribution j e .…”

Vibrational dynamics of the type-I clathrates A8Sn44□2 (A = Cs, Rb, K) from lattice-dynamics calculations, inelastic neutron scattering, and specific heat measurements

“…Understanding the underlying physical mechanisms responsible for the glass-like thermal conductivity (κ) exhibited by some crystalline materials has attracted considerable attention over the last few decades for both fundamental and practical reasons. Such unusual behavior has been observed in natural minerals, [1][2][3][4] complex crystal structures with a large number of atoms per unit cell, [5][6][7][8][9][10][11][12] and cage-like compounds, [13][14][15][16][17][18][19][20][21] with the type-I clathrates being one of the most prominent examples. 22 The crystal structure of these materials is composed of covalently bonded frameworks in the large cavities of which "guest" atoms reside [polyhedral cages; Fig.…”