Simultaneous structure and carrier tuning of dimorphic clathrateBa8Ga16Sn30

Abstract

We report structural, transport, and thermal properties of carrier-tuned Ba 8 Ga 16 Sn 30 single crystals with the type-1 clathrate structure ͑␤ phase͒, demonstrating that Ba 8 Ga 16 Sn 30 is a unique thermoelectric clathrate material wherein both the structure type and the carrier type are tunable. The results are compared with the properties of the better known type-8 structure ͑␣ phase͒ and of A 8 Ga 16 Ge 30 ͑A =Sr,Eu͒. Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of 520Ϯ 3°C. Refinements of single-crystal x-ray diffraction data indicate that the Ba͑2͒ guest ion in the tetrakaidecahedron occupies the off-center 24k sites which are 0.43-0.44 Å away from the centered 6d site. The temperature-linear coefficient of the specific heat is 29 mJ/ mol K 2 for both n-and p-type carriers in the ␤ phase, four times larger than that for the ␣ phase, suggesting contributions from tunneling of the Ba͑2͒ guest ions between off-center minima. Analysis of specific heat with a soft-potential model ͑SPM͒ gives a characteristic energy of 20 K for the Ba͑2͒ vibration, significantly lower than 50 K for the ␣ phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities L for the ␤ phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in L ͑T͒ is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the ␤ phase is significantly larger than that for Sr 8 Ga 16 Ge 30 .

Electronic Structures and Thermoelectric Properties of Sb-Doped Type-VIII Clathrate Ba₈Ga₁₆Sn₃₀

“…17,18) Suekuni et al prepared p-and n-type type-I Ba 8 Ga 16 Sn 30 (BGS-I) and BGS-VIII, and studied the thermoelectric properties. 19) Koga et al calculated the band structure and density of states for BGS-VIII with different Ga content and found repulsive GaGa force between the nearest neighbor position. 20) This feature has been confirmed in type-I clathrates theoretically and experimentally.…”

NMR relaxation and rattling phonons in the type-I Ba8Ga16Sn30clathrate

“…For type-I Ba 8 Ga 16 Sn 30 , the pure elements were mixed together based on the nominal composition followed by an initial arc melting in argon environment. Annealing in an evacuated quartz tube at 900 o C for 50 hours was then applied, followed by a controlled slow cooling to 500 o C in 80 hours 23,24 . X-ray diffraction (XRD) measurements were performed on a Bruker D8 X-ray Powder Diffractometer, and wavelength dispersion spectroscopy (WDS) measurements were done in a Cameca SX50 spectrometer.…”

“…The clathrate samples were prepared using the self-flux method, following a technique reported previously 23 . Because of the existence of a type-l/type-VIII dimorphism in Ba 8 Ga 16 Sn 30 , a carefully controlled annealing process is needed during the sample making procedure.…”

NMR Studies of Ba8Ga16Sn30Clathrates

“…1(b)), while β-BGS is a cubic (Pm − 3n, No.223) consists of six tetrakaidecahedra X 24 and two X 20 ( Fig.1(d)). It is reported that the guest Ba ions in α-BGS are on-center for the 8c-site in the X 20 cage and exhibit on-center harmonic-like vibrations, 8) whereas those in β-BGS are rotating among the off-center positions in the tetrakaidecahedron. 8) Especially, type I Ba 8 Si 46 shows superconductivity.…”

“…[1][2][3][4][5][6][7][8] These compounds have the unique caged crystal structure where a "guest" ion enclosed in an over-sized cage can vibrate within an anharmonic potential, so called "rattling". Theoretically, the interaction between the rattling and conduction electrons are regarded as a multi-channel Kondo effect and gives rise to the heavy fermion behavior.…”