Koichiro Suekuni

Phonon-glass electron-crystal thermoelectric clathrates : Experiments and theory

Type-I clathrate compounds have attracted a great deal of interest in connection with the search for efficient thermoelectric materials. These compounds constitute networked cages consisting of nano-scale tetrakaidecahedrons (14 hedrons) and dodecahedrons (12 hedrons), in which the group 1 or 2 elements in the periodic table are encaged as the so-called rattling guest atom. It is remarkable that, though these compounds have crystalline cubic-structure, they exhibit glass-like phonon thermal conductivity over the whole temperature range depending on the states of rattling guest atoms in the tetrakaidecahedron. In addition, these compounds show unusual glass-like specific heats and THz-frequency phonon dynamics, providing a remarkable broad peak almost identical to those observed in topologically disordered amorphous materials or structural glasses, the so-called Boson peak. An efficient thermoelectric effect is realized in compounds showing these glass-like characteristics. This decade, a number of experimental works dealing with type-I clathrate compounds have been published. These are diffraction experiments, thermal and spectroscopic experiments in addition to those based on heat and electronic transport. These form the raw materials for this article based on advances this decade. The subject of this article involves interesting phenomena from the viewpoint of not only physics but also from the view point of the practical problem of elaborating efficient thermoelectric materials. This review presents a survey of a wide range of experimental investigations of type-I clathrate compounds, together with a review of theoretical interpretations of the peculiar thermal and dynamic properties observed in these materials.

High-performance thermoelectric mineral Cu12−xNixSb4S13 tetrahedrite

X-ray structural analysis and high-temperature thermoelectric properties measurements are performed on polycrystalline samples of artificial mineral Cu12−xNixSb4S13 tetrahedrite. Analysis of the atomic displacement parameter manifests low-energy vibration of Cu(2) out of CuS3 triangle plane. The vibration results in low lattice thermal conductivity of less than 0.5 W K−1 m−1. By tuning of the Ni composition x and decrease of electronic thermal conductivity, dimensionless thermoelectric figure of merit for x = 1.5 achieves 0.7 at 665 K, which is a considerably high value among p-type Pb-free sulfides. Because the tetrahedrite is an environmentally friendly material, it constitutes a good thermoelectric material for use in support of a sustainable society.

Thermoelectric Properties of Mineral Tetrahedrites Cu10Tr2Sb4S13 with Low Thermal Conductivity

We have investigated thermoelectric properties of synthesized mineral Cu10Tr2Sb4S13 (Tr = Mn, Fe, Co, Ni, Cu, and Zn) tetrahedrites, which have a cubic and complex crystal structure. The mother phase Tr = Cu shows metal–semiconductor transition and anomalous hysteresis. Through various Tr substitutions, the thermopower was increased and thermal conductivity was decreased. Results show that Tr = Ni had the largest dimensionless figure of merit Z T of 0.15 at 340 K. The main advantage for the large Z T is the quite low lattice thermal conductivity. Because of the large Z T and the environmentally friendly components, tetrahedrites are anticipated as a good thermoelectric material.

Glasslike versus crystalline thermal conductivity in carrier-tuned Ba8 Ga16 X30 clathrates (X=Ge,Sn)

The present controversy over the origin of glasslike thermal conductivity observed in certain crystalline materials is addressed by studies on single-crystal x-ray diffraction, thermal conductivity

High-performance thermoelectric minerals: Colusites Cu26V2M6S32 (M = Ge, Sn)

\ensuremath{\kappa}(T)

Systematic study of electronic and magnetic properties for Cu12–xTMxSb4S13 (TM = Mn, Fe, Co, Ni, and Zn) tetrahedrite

, and specific heat

Enhancement of thermoelectric efficiency in type-VIII clathrate Ba8Ga16Sn30 by Al substitution for Ga

{C}_{p}(T)

First-principles study of type-I and type-VIII Ba8Ga16Sn30 clathrates

of carrier-tuned

Research Update: Cu–S based synthetic minerals as efficient thermoelectric materials at medium temperatures

{\mathrm{Ba}}_{8}{\mathrm{Ga}}_{16}{X}_{30}