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Visualization of the relation between thermal conductivity and the “rattling” of a guest atom trapped in a cage structure (Press Release)

Release Date
20 Apr, 2012
  • BL02B1 (Single Crystal Structure Analysis)
- A new atomic-motion approach to material development for electric energy generation -

Japan Synchrotron Radiation Research Institute (JASRI)
WPI-AIMR, Tohoku University
Shimane University

The researchers at Japan Synchrotron Radiation Research Institute (JASRI), RIKEN, Advanced Institute for Materials Research (WPI-AIMR, Tohoku University), and Shimane University, in collaboration with the University of Tokyo, have conducted a study on cage compounds -a group of compounds viewed as promising candidates for use in elements utilized to generate electric energy -and succeeded in visualizing the atomic motions that play a functional role. Electric-energy generation leveraging waste heat is gaining expectations as a promising technology for sustainable energy technology, and cage compounds are considered capable of providing elemental material.

A temperature difference across a stub of material gives rise to a small end-to-end potential difference. This phenomena, called the “thermoelectric effect,”(*1) can be applied to power generation using waste heat, thus gathering high expectations for use in sustainable energy technologies. One of the key properties for a “thermoelectric material(*2) -i.e. one that exhibits the thermoelectric effect -to gain high conversion efficiency is to maintain a temperature at one end of the material that is different from the temperature at the other end. In other words, the material should be a poor heat conductor but a good electric conductor. As a good electric conductor generally tends to exhibit good thermal conduction, upgrading power-generation property is no easy task.

Compounds classed as “clathrates(*3) have a cage-like network structure with a foreign atom trapped within it, which makes “rattling(*4) motions in the confined space. The rattling motion has the effect of suppressing lattice vibration (i.e. reduced thermal conduction) while it contributes to increased electric conduction. These two properties make this class of compounds a highly-promising candidate for good thermoelectric materials. However, a detailed understanding of how the rattling motion hinders thermal conduction has been out of reach. A clear understanding of the role played by the rattling motion has been strongly desired, because it is expected to pave the way to the development of high-performance thermoelectric materials, enabling utilization of the knowledge of atomic motions in design.

The research group conducted high-precision structural analysis on three types of clathrate compounds, each with significantly distinct properties in terms of thermal conduction, using the synchrotron radiation X-ray available at SPring-8.(*5) Potential distribution within the crystal was closely examined based on the structural information. The results clearly indicated, for the first time in the world, that thermal conductivity decreases uniformly as the extent of the impact exerted by intra-cage rattling increases.

The success of this approach indicates the possibility of forecasting thermal conductivity characteristics if only a tiny single crystal of a candidate material, a size roughly of 0.01mm, is available. The approach is expected to provide a direct evaluation method conducive to informed design and development of atomic-motion-leveraged thermoelectric materials that are to be used in electricity-generating elements for the utilization of waste heat.

The research reported here was conducted by the research group led by Mr. Akihiko Fujiwara (Chief researcher, JSARI), Mr. Masaki Takada (senior researcher, RIKEN), Prof. Katsumi Tanigaki (WPI-AIMR, Tohoku University), and Prof. Hiroshi Tanaka (Shimane University), and the results were published on the online publication of “Physical Review B” (a journal of American Physical Society) on April 17, 2012.

"Quantitative relation between structure and thermal conductivity in type-I clathrates X8Ga16Ge30 (X = Sr, Ba) based on electrostatic-potential analysis"
Akihiko Fujiwara, Kunihisa Sugimoto, Che-Hsiu Shih, Hiroshi Tanaka, Jun Tang, Yoichi Tanabe, Jingtao Xu, Satoshi Heguri, Katsumi Tanigaki, and Masaki Takata
Physical Review B 85, 144305 (2012), published online 17 April 2012


Fig.1. Three different cage structures of type 1 clathrate compounds: the guest atom and its rattling range (the extent of the region within which the guest atom exerts influence) are shown in pale purple.
Fig.1. Three different cage structures of type 1 clathrate compounds:
the guest atom and its rattling range (the extent of the region within
which the guest atom exerts influence) are shown in pale purple.

Although the cage structures, consisting of Ga and Ge atoms (green), are almost the same, the spatial extent of the guest atom movements differs noticeably for each compound. In comparison with case (a), where a Ba guest is trapped in a cage structure with a high degree of completeness, the extent of the guest motion is larger and wilder in the two cases to the right: (b), where the Ba atom is trapped in a cage structure with defects (lattice atoms are missing) only in a very small portion (1.5%) of the structure, and (c), where the guest is replaced with Sr, which is smaller than Ba. The research succeeded in representing the region of influence (red-purple) exerted by the guest in a visual perspective, and revealed that it is directly correlated with the reduction of thermal conductivity.

Fig.2. Contribution from rattling to thermal conduction.
Fig.2. Contribution from rattling to thermal conduction.

The rattling factor is defined as the normalized volume of the rattling region in reference to the typical size of the guest atom. The figure clearly indicates, for the first time in the world, that thermal conductivity decreases as the rattling factor increases.

*1 Thermoelectric effect

A generic term representing a variety of effects involved in mutual interactions between thermal flow (thermal energy) and electric current (electric energy). The term encompasses three separately-identified effects: the Seebeck effect (generation of a potential difference between two points that are maintained at two distinctive temperature levels), the Peltier effect (generation of a temperature difference between two points that are maintained at two different potential levels), and the Thomson effect (absorption/evolution of heat when an electric current flows between two points maintained at different temperatures). The Seebeck effect is harnessed for the generation of electrical energy using the thermoelectric effect (thermoelectric generation).

*2 Thermoelectric material
A generic term for materials with significant thermoelectric effect. These materials are used for electric energy generation (Seebeck effect), or, for cooling and heating applications (Peltier effect). The thermoelectric voltage induced in response to one degree centigrade of temperature difference is called “thermopower” (or Seebeck coefficient), and provides a measure for representing thermoelectric properties inherent to the material.

*3 Clathrate
A class of compounds in which an atom or molecule is trapped within the cage structure of a crystal lattice (also called inclusion compounds). The cage structure is called a host, and the atom/molecule trapped within is called a guest. The guest-host linkage is generally very weak, allowing wild movements of the trapped component within the cage.

*4 Rattling
Rattling originally means moving around wildly and making a series of loud sounds, thus the word “rattle” is often used to mean a baby’s toy that makes a series of short loud sounds. Because the cage structure of clathrates and the wild behavior of the guest contained within it are suggestive of the toy (rattle), the behavior of the guest atom is named “rattling.”

*5 SPring−8
SPring-8 is a facility that generates the world's highest-performance synchrotron radiation. It is located in Harima Science Garden City in Hyogo prefecture and is owned by RIKEN. JASRI is responsible for its operation, management, and support for users. The name “SPring-8” is derived from “Super Photon ring-8 GeV.” Synchrotron radiation is the narrow and extremely powerful light that is obtained when the direction of electrons accelerated to close to the speed of light is bent using electromagnets. Research in a wide range of fields, including nanotechnology, biotechnology, and their industrial applications, has been carried out using the synchrotron radiation at SPring-8.

For more information, please contact:
 Dr. Akihiko Fujiwara (JASRI)

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