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 .

Skutterudite‐type pnictides based on CoSb3 are promising semiconductor materials for thermoelectric applications. An exhaustive structural characterization by synchrotron X‐ray powder diffraction of different M‐filled CoSb3 (M = Y, K, Sr, La, Ce, Yb) skutterudites, with a panoply of M atoms with very different chemical nature, allows to better understand the effects of filling from a crystallo‐chemical point of view. These analyses focus on the correlation of chemical and structural features with the enhanced thermoelectric properties displayed by certain families of filled‐CoSb3 skutterudites. These are mainly determined by Sb positional parameters, yielding Oftedal plots that depend on the filling fraction, ionic state, and atomic radius of the filler. Together with the distortion of [Sb4] rings and [CoSb6] octahedra present in the skutterudite structure, these results are linked to the band‐convergence concept and its influence on the thermoelectric transport properties. Here, the structural changes observed in the different chemical compositions are relevant to understand the improved thermoelectric performance of single partially filled n‐type skutterudites.

Significant control over the properties of a high-carrier density superconductor via an applied electric field has been considered infeasible due to screening of the field over atomic length scales. Here, we demonstrate an enhancement of up to 30% in critical current in a back-gate tunable NbN micro- and nano superconducting bridges. Our suggested plausible mechanism of this enhancement in critical current based on surface nucleation and pinning of Abrikosov vortices is consistent with expectations and observations for type-II superconductor films with thicknesses comparable to their coherence length. Furthermore, we demonstrate an applied electric field-dependent infinite electroresistance and hysteretic resistance. Our work presents an electric field driven enhancement in the superconducting property in type-II superconductors which is a crucial step toward the understanding of field-effects on the fundamental properties of a superconductor and its exploitation for logic and memory applications in a superconductor-based low-dissipation digital computing paradigm.

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The development of new magnetic refrigerants demands an effective investigation of materials with a large magnetocaloric effect in a wide temperature range. Herein, we report on the structural, magnetic, and magnetocaloric properties of the two-site disordered double perovskite GdSrCoFeO 6 prepared by the modified solid-state synthesis method. Temperature-dependent synchrotron X-ray diffraction analysis revealed that GdSrCoFeO 6 crystallizes in the orthorhombic phase (Pnma), with Gd 3+ /Sr 2+ and Co 2+/3+ /Fe 3+/4+ ions randomly distributed on the A-and B-sites, respectively. An observed lattice parameter anomaly around 60 K indicates the occurrence of the magnetoelastic coupling, which coincides with the presence of ferro/ferrimagnetic (FM/FiM) ordering below T C ≈ 65 K from the magnetic measurements. These results match well with our first-principles calculation prediction of low-temperature magnetic (FM/FiM) and electronic (insulating/metal) transitions related to a combined effect of Co and Fe shortand long-range competitions, crossings of spin state at Co ions, and the hybridization degree between Gd-4f and Co-3d states. Additionally, a modified Arrott plot and Kouvel−Fisher analysis were used to establish the nature of the magnetic phase transition in GdSrCoFeO 6 , yielding the critical exponent β = 1.46(6)/1.45(6), γ = 1.48(5)/1.17(2), and δ = 2.01(3)/1.80(5), respectively. The specific heat analysis reveals two well-defined broad peaks (∼10 and ∼70 K), which match well with a Schottky anomaly (Gd-4f) and the magnetic transition of FM/FiM to paramagnetic order, respectively. The magnetocaloric effect (MCE) analysis reveals a maximum magnetic entropy change ΔS M max ≈ 13 J kg −1 K −1 (at ∼8 K) under a field of 0−7 T. These results evidence that the Schottky anomaly and the magnetoelastic coupling seem to be key factors for driving further enhancements to the MCE in GdSrCoFeO 6 , making it a possible candidate for cryogenic applications.

The local atomic structure of skutterudite-type compounds derived from CoSb3 plays a pivotal role in tuning their electronic and thermal properties in thermoelectric applications. For instance, the shape of the occurring [Sb4] rings has direct consequences on the band convergence and, then, the possible enhancement of the thermoelectric efficiency. In this work, both local and electronic structures of the CoSb3 skutterudite were evaluated by the X-ray absorption technique. Extended X-ray-absorption fine-structure oscillations at the Sb K-edge were fitted in good agreement to the body-centered cubic phase. The edge shift values were taken referencing the Co and Sb foils. The standard samples were used, namely, CoO (Co2+), Co3O4 (Co2.5+), Sb2O3 (Sb3+), and Sb2O5 (Sb5+). Based on the valence state dependence of the edge shift, the valences of Co and Sb in CoSb3 were estimated as +0.8(5) for Co and −2.2(3) for Sb, which suggests a partial charge transfer from the metal to the pnictide element. From the bonding distances of Co–Sb, Sb–Sb (short), and Sb–Sb (long), the lattice parameter and fractional coordinates (y, z) were evaluated and, then, compared to those extracted from synchrotron X-ray diffraction. From temperature-dependent X-ray absorption spectroscopy data at 80–350 K, the Einstein temperatures and local coefficients of thermal expansion of those pair-bonds were properly estimated. Comparing these values with those obtained from diffraction, we have established the boundaries of both short- and long-range order techniques for structural characterization of skutterudite-based thermoelectrics.

Black phosphorus (BP) allotrope has an orthorhombic crystal structure with a narrow bandgap of 0.35 eV. This material is promising for 2D technology since it can be exfoliated down to one single layer: the well-known phosphorene. In this work, bulk BP was synthesized under high-pressure conditions at high temperatures. A detailed structural investigation using neutron and synchrotron X-ray diffraction revealed the occurrence of anisotropic strain effects on the BP lattice; the combination of both sets of diffraction data allowed visualization of the lone electron pair 3s 2 . Temperature-dependent neutron diffraction data collected at low temperature showed that the a axis (zigzag) exhibits a quasi-temperature-independent thermal expansion in the temperature interval from 20 up to 150 K. These results may be a key to address the anomalous behavior in electrical resistivity near 150 K. Thermoelectric properties were also provided; low thermal conductivity from 14 down to 6 Wm −1 K −1 in the range 323−673 K was recorded in our polycrystalline BP, which is below the reported values for singlecrystals in literature.

SnSe has been recently reported as an attractive thermoelectric material, with an extraordinarily high, positive, Seebeck coefficient. Here, we describe the synthesis, structural, microscopic, and thermoelectric characterization of Sn1–xSbxSe intermetallic alloys prepared by a straightforward arc-melting technique. Sb-doped tin selenide was synthesized as strongly nanostructured polycrystalline pellets. Neutron diffraction studies reveal that Sb is placed at the Sn sublattice in the crystal structure, showing concentrations as high as 30%, and generates a significant number of Sn vacancies, while the increase of the interlayer distances favors the nanostructuration. The material is nanostructured both out-of-plane in nanometer-scale layers and in-plane by ∼5 nm undulations of these layers. This nanostructuring, along with an increased amount of Sn vacancies, accounts for a reduction of the thermal conductivity, which is highly desirable for thermoelectric materials. The phonon mean free path is estimated to be on the order of 2 nm from low temperature, thermal conductivity, and specific heat, in accordance with the nanostructuration observed by high-resolution transmission electron microscopy. The thermal conductivity of SnSe is characterized by three independent techniques to assure a room temperature value of Sn0.8Sb0.2Se of κ ∼ 0.6 W/m K. The freshly prepared Sb-doped compounds exhibit an abrupt change in the type of charge carriers, leading to large, negative Seebeck coefficients, although the arc-melt synthesized pellets remain too resistive for thermoelectric applications. Cold-pressed pellets evolve to be p-type at room temperature, but reproducibly turn n-type around 500 K, with increased electrical conductivity and maximum observed figure of merit, ZT ∼ 0.3 at 908 K.