Quantitative relation between structure and thermal conductivity in type-I clathrates X8Ga16Ge30 (X = Sr, Ba) based on electrostatic-potential analysis

In order to quantitatively characterize the effect of motion of the guest atom in type-I clathrates X8Ga16Ge30 (X = Sr, Ba) on thermal conductivity, electrostatic potential in the crystal was visualized by synchrotron radiation x-ray diffraction. The obtained electrostatic potential clearly exhibited structural aspects of the guest atom, rattling, which is considered as a cause of suppression of thermal conductivity kappa. The parameterized degree of rattling "rattling factor," f(R), being defined as a volume ratio of the electrostatic interaction region of the guest atom to the spherical volume estimated by the covalent radius of the guest atom, was found to well classify kappa. The. monotonously decreases with increasing the f(R). Consequently, f(R) was judged as a better criterion to explore high-performance thermoelectric system of the clathrates.