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The full rare-earth (RE) chromites series (RE)CrO(3) with an orthorhombic distorted (Pnma) perovskite structure and the isostructural compound YCrO(3) can be synthesized through a simple microwave-assisted technique, yielding high-quality materials. Magnetization measurements evidence that the Néel temperature for antiferromagnetic Cr(3+)-Cr(3+) ordering strongly depends on the RE(3+) ionic radius (IOR), and a rich variety of different magnetic spin interactions exists. Dielectric spectroscopy on sintered pellets indicates electronic inhomogeneity in all samples as manifested by the presence of at least two dielectric relaxation processes associated with grain boundary and grain interior bulk contributions. X-ray diffraction, Raman spectroscopy, and temperature-dependent dielectric permittivity data do not indicate potential noncentrosymmetry in the crystal or concomitant ferroelectricity. Strong correlations between the magnetic and dielectric properties were not encountered, and microwave-synthesized (RE)CrO(3) may not be classified as magnetoelectric or multiferroic materials.

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

The reduction of the lattice thermal conductivity is one of the crucial steps in improving thermoelectric materials. In skutterudites, a well-known approach is to reduce the thermal conductivity by filling the structural cage with rare-earth atoms. In this work, we show that it is not just the amount of such filling itself but its nanoscale structuration that lowers the thermal conductivity. A straightforward synthesis procedure under high pressure yields Ce-and Yb-filled CoSb 3 skutterudites, with and without an inhomogeneous distribution of the filler atoms. The composition of the phases is evaluated from synchrotron Xray diffraction (SXRD) data; the highly nanostructured morphology is verified by high-resolution transmission electron microscopy (TEM). The filling fluctuation, i.e., the uneven distribution of filling atoms in the sample originating a phase segregation, brings about low lattice thermal conductivity, as a strong source of phonon scattering. This effect is prominent in the Ce-filled compound, where Ce is segregated into Ce-rich and Ce-poor regions, and the lattice contribution of the thermal conductivity κ L shows a concomitant reduction, approaching values as low as 1.6 W m −1 K −1 at 800 K. Although the level of filling is much higher in Yb x CoSb 3 , its lattice thermal conductivity remains larger. Overall, though, its power factor is enhanced due to charge transfer from the Yb-filler. We thus define a new paradigm for the design of filled skutterudites with exceptionally low thermal conductivities, based on the nanoscale mixing of two phases with different filling factors, spontaneously induced by high-pressure synthesis conditions, which can be considered as pseudoamorphous structures with significant reduction in κ L .