Thermal Expansion and Rattling Behavior of Gd-Filled Co4Sb12 Skutterudite Determined by High-Resolution Synchrotron X-ray Diffraction

Abstract

In this work, Gd-filled skutterudite GdxCo4Sb12 was prepared using one step method under high pressure in a piston-cylinder-based press at 3.5 GPa and moderate temperature of 800 °C. A detailed structural characterization was performed using synchrotron X-ray diffraction (SXRD), revealing a filling fraction of x = 0.033(2) and an average <Gd–Sb> bond length of 3.3499(3) Å. The lattice thermal expansion accessed via temperature-dependent SXRD led to a precise determination of a Debye temperature of 322(3) K, from the fitting of the unit-cell volume expansion using the second order Grüneisen approximation. This parameter, when evaluated through the mean square displacements of Co and Sb, displayed a value of 265(2) K, meaning that the application of the harmonic Debye theory underestimates the Debye temperature in skutterudites. Regarding the Gd atom, its intrinsic disorder value was ~5× and ~25× higher than those of the Co and Sb, respectively, denoting that Gd has a strong rattling behavior with an Einstein temperature of θE = 67(2) K. As a result, an ultra-low thermal conductivity of 0.89 W/m·K at 773 K was obtained, leading to a thermoelectric efficiency zT of 0.5 at 673 K.

Multiscale Length Structural Investigation and Thermoelectric Performance of Double-Filled Sr0.2Yb0.2Co4Sb12: An Exceptional Thermal Conductivity Reduction by Filler Segregation to the Grain Boundaries

“…The Einstein temperatures for Sr and Yb were estimated as 94 and 67 K, respectively. For Yb, the θ E value agrees quite well with those reported for other rare-earth filler elements, such as Gd (∼67 K) 36 and Eu (∼68 K). 30 In this way, we may attest that Sr and Yb atoms possess an enormous rattling effect at the skutterudite voids (2a), which can explain or at least compose the entire scenario for the exceptionally low thermal conductivity reported in double-filled Sr 0.2 Yb 0.2 Co 4 Sb 12 .…”

“…Oftedal ( y + z ) Sb positional parameters against filling fraction, with red circles for Sr 0.2 Yb 0.2 Co 4 Sb 12‑δ filler-rich and filler-poor phases, and gray circles for other single-filled skutterudites described in refs , , and .…”

“…These results demonstrated that the covalent environment became more rigid and, likely, more covalent when the fillers are incorporated into the open cavities of the skutterudite structure. The increase of bonding strength is a consequence of the electron transfer from the filler to the caging structure, which is stabilized under high pressure by Sb vacancies for the unfilled CoSb 3 , and those vacancies are reduced upon filling. ,,, …”

“…Averaging the Debye temperatures by the Co and Sb atomic masses, we can estimate the Debye temperature for the covalent framework of the skutterudite around ∼250 K, which has a reasonable agreement with pristine and other filled skutterudites. 30,36,56…”

“…As individual fillers, they have previously shown significant enhancement of the thermoelectric performance of CoSb 3 skutterudite. Both single-filled derivatives have demonstrated reduced lattice thermal conductivity and high filling fraction compared to most other filler elements. ,,,,− Additionally, high-pressure conditions during the preparation of Sr-CoSb 3 have resulted in filling fraction fluctuations, leading to the formation of a mixture of two differently filled phases with distinct unit-cell parameters. In this study, we have synthesized the multiple-filled Sr 0.2 Yb 0.2 Co 4 Sb 12 skutterudite.…”