Low thermal conductivity in La-filled cobalt antimonide skutterudites with an inhomogeneous filling factor prepared under high-pressure conditions

Abstract

La-filled skutterudites LaxCo4Sb12 (x = 0.25, 0.5) have been synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. The structural properties of the reaction products were characterized by synchrotron x-ray powder diffraction, clearly showing an uneven filling factor of the skutterudite phases, confirmed by transmission electron microscopy. The non-homogeneous distribution of La filling atoms is adequate to produce a significant decrease in lattice thermal conductivity, mainly due to strain field scattering of high-energy phonons. Furthermore, the lanthanum filler primarily acts as an Einstein-like vibrational mode having a strong impact on the phonon scattering. Extra-low thermal conductivity values of 2.39 W m -1 K -1 and 1.30 W m -1 K -1 are measured for La0.25Co4Sb12 and La0.5Co4Sb12 nominal compositions at 780 K, respectively. Besides that, lanthanum atoms contributed to increase the charge carrier concentration in the samples. In the case of La0.25Co4Sb12, there is an enhancement of the power factor and an improvement of the thermoelectric properties. Journal Name ARTICLEThis journal is

Substantial thermal conductivity reduction in mischmetal skutterudites MmxCo4Sb12 prepared under high-pressure conditions, due to uneven distribution of the rare-earth elements

“…In fact, these lattice thermal conductivities are indeed low, even compared to La-and Ce-filled CoSb 3 prepared under the high-pressure conditions (B2.5 and 2.8 W m À1 K À1 at RT, respectively), which display an analogous phase separation, but no elemental segregation. 15,16 As for samples prepared by other means, Nolas et al 25 13,14,22 As suggested before, the low lattice thermal conductivity observed in the present high-pressure Mm-filled compounds is not only attributed to the resonant scattering introduced by the filler element, but also to the phase segregation into Ce-and Mm-filled skutterudites. Increased disorder and strain scattering is induced by the uneven distribution of Ce and La atoms in the Mm-filled phase.…”

“…4 shows the transmission electron microscopy image of a pair of submicrometer-size grains and their chemical mapping. The annular 15 On the other hand, the concentration of Ce, in addition to being homogeneous and higher than the concentration of La, is also higher than that obtained in a pure Ce x Co 4 Sb 12 specimen. 16 This suggests that the Ce filling factor can be increased when La is added to Ce as a doping element for CoSb 3 .…”

“…4 Recently, we studied the structure and thermoelectric transport properties of R x CoSb 3 (R = La, Ce, or Yb) prepared under high pressure. 15,16 A significant reduction of the thermal conductivity was found as a consequence of the uneven filler distribution in R-poor and R-rich phases at a nanometric scale. 15 This type of reduction was previously attributed to filling fraction fluctuation and consequent strain-field and long-range acoustic phonon scattering by Wei et al in La-filled Fe 4 Co 12 skutterudite.…”

“…15,16 A significant reduction of the thermal conductivity was found as a consequence of the uneven filler distribution in R-poor and R-rich phases at a nanometric scale. 15 This type of reduction was previously attributed to filling fraction fluctuation and consequent strain-field and long-range acoustic phonon scattering by Wei et al in La-filled Fe 4 Co 12 skutterudite. 17 In this work, we use mischmetal (Mm) as the source of filler elements, instead of pure rare-earth metals.…”

“…This segregation of different filler elements contributes to further reduce the lattice thermal conductivity, from those measured for nominal La 0.5 Co 4 Sb 12 , Yb 0.5 Co 4 Sb 12 and Ce 0.5 Co 4 Sb 12 prepared under high-pressure. 15,16 This could be a good starting point to enhance even more the thermoelectric properties in filled skutterudites.…”