Lanxian Shen

Preparation and electrical transport properties of VIII-type Sn-based single-crystal clathrates (Eu/Ba)8Ga16Sn30

In this work, the type-VIII Sn-based single-crystal clathrates with different stoichiometric ratios were prepared, and the structure of the samples was investigated. The results indicate that Eu atoms tend to replace Ba atoms. The introduced Eu atoms lead to a decrease in the materials’ melting point and the concentration of carriers, whilst the carrier mobility is increased. Despite the above, there was no obvious change on the electrical conductivity, and the lattice constant of samples were decreased. The Seebeck coefficient of the samples was found to be negative and its absolute value decreased with increasing Eu. The ZT values were found to be almost 0.46 at 550 k when the Eu initial ratio was equal to 0.25.

Structural and electronic properties of type I and VIII Sr8Ga16Sn30 clathrates under compression

Based on the ab initio density functional theory under generalised gradient approximation, we calculated the total energy and band structure for the type I and VIII Sr8Ga16Sn30 under hydrostatic pressure, the results reveal that the type VIII Sr8Ga16Sn30 is the stable phase and the type I Sr8Ga16Sn30 is the metastable phase, and there is no phase transformation between the type I and VIII Sr8Ga16Sn30 under hydrostatic pressure. The type I and VIII Sr8Ga16Sn30 are indirect semiconductors with band gaps of 0.125 and 0.009 eV respectively. With increasing pressure, the band gaps are increased, and there is no significant change for the dispersion of band near the Fermi lever. The stability of Sr8Ga16Sn30 is lower than that of Ba8Ga16Sn30; therefore, it is more difficult in the synthesis of Sr8Ga16Sn30 compared to that of Ba8Ga16Sn30.

Structural stabilities and electrical properties of Ba8Ga16−x Cu x Sn30 single crystals under high temperatures

Single crystalline samples of type-I and type-VIII Ba8Ga16−x Cu x Sn30 (x = 0, 1) clathrates are prepared by the Sn-flux method. Effects of Cu-doping on stability and electrical properties of Ba8Ga16Sn30 single crystal are explored by first-principle and experiment. All samples are heated to different high temperatures and maintained at these temperatures for 120 min and then cooled to room temperature to explore their structural stabilities. Results from DTA and powder x-ray diffraction analysis indicate that type-I Ba8Ga16Sn30 structure is transformed into type-VIII phase after the sample has been heated to 185 °C. Type-VIII BGS is stable during heating and cooling, but type-VIII Ba8Ga15CuSn30 decomposes into Sn and Ba(Ga/Sn)4 during cooling. Meanwhile, the electrical properties of type-I samples are measured, their electrical conductivities are enhanced, and the Seebeck efficient is reduced with Cu substitution. The type-I samples after phase transformations show the electrical characteristics of type-VIII samples.