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

Abstract: 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.

• The paper demonstrates that type-I clathrate compounds achieve glass-like thermal conductivity while maintaining high electrical conductivity, essential for efficient thermoelectrics.
• The paper employs diffraction, spectroscopy, and thermal transport measurements alongside theoretical models like the two-level system to analyze the dynamic behavior of rattling guest atoms.
• The findings suggest that optimizing guest-host interactions in clathrates can significantly enhance the thermoelectric figure of merit (ZT) for practical waste heat recovery applications.

Overview of "Phonon-glass electron-crystal thermoelectric clathrates: Experiments and theory"

The study conducted by Takabatake et al. investigates the potential of type-I clathrate compounds as efficient thermoelectric materials. These compounds are characterized by a crystalline framework composed of group 13 or 14 elements, encapsulating smaller group 1 or 2 elements as guest atoms within their nano-scale cages. The research focuses on understanding the unique thermal and electronic properties of these clathrates, which exhibit phonon-glass-like behavior while maintaining electron-crystal characteristics.

Key Findings

• Phonon-glass Behavior: Despite having a crystalline structure, the clathrate compounds show glass-like thermal conductivity across a wide temperature range due to the dynamics of the so-called "rattling" guest atoms. These vibrations significantly reduce the phonon thermal conductivity (κ_ph), resembling that of amorphous materials, and contribute to a broad Boson peak observed in the specific heat (C_V).
• Thermoelectric Efficiency: The efficiency of thermoelectric materials, quantified by the dimensionless figure of merit (ZT), is dependent on high electrical conductivity (σ) and low thermal conductivity (κ_tot). The study indicates that the phonon-glass behavior of clathrates facilitates low κ_tot while maintaining high σ, enhancing their potential thermoelectric efficiency.
• Experimental and Theoretical Investigations: The authors utilize a range of experimental methods, including diffraction experiments, spectroscopic analyses, and thermal transport measurements, to probe the structural and dynamic properties of clathrates. Theoretical interpretations using models like the two-level system for tunneling states and double-well potentials are applied to describe the observed behaviors and to simulate thermal conductivities and specific heats.

Implications

The dual characteristics of phonon-glass and electron-crystal in type-I clathrates present a promising avenue for the development of thermoelectric materials, which can efficiently convert waste heat to electricity. This capability is especially relevant in applications where environmental and human-friendly materials are preferred over traditional, often toxic, thermoelectric compounds like Bi-Te and Pb-Te alloys.

From a theoretical perspective, the clathrates challenge conventional models of thermal and electronic transport due to their unique structural dynamics. Their study provides insights into manipulating lattice vibrations and electronic structures, potentially influencing future materials design focused on maximizing ZT values for energy recovery technologies.

Future Directions

The continued exploration of clathrates could involve:

• Synthesis and Discovery: Developing new clathrate compositions with adjusted guest atom positions to fine-tune thermal and electronic transport properties.
• Advanced Modeling: Enhancing models to better simulate the interaction between guest atoms and host frameworks, capturing the complex anharmonic effects and their impact on thermal and electrical conductivities.
• Scalable Applications: Investigating the practical implementation of these materials in thermoelectric devices, ensuring scalability and economic feasibility while maintaining performance efficiency.

In conclusion, this research underscores the potential of clathrate compounds in broadening the spectrum of efficient thermoelectric materials. The combination of experimental insights and theoretical frameworks forms a critical foundation for advancing the technology of phonon-glass electron-crystal systems.