RESUMEN
Metal halide perovskites with a two-dimensional structure are utilized in photovoltaics and optoelectronics. High-crystallinity CsSn2Br5 specimens have been synthesized via ball milling. Differential scanning calorimetry curves show melting at 553 K (endothermic) and recrystallization at 516 K (exothermic). Structural analysis using synchrotron X-ray diffraction data, collected from 100 to 373 K, allows for the determination of Debye model parameters. This analysis provides insights into the relative Cs-Br and Sn-Br chemical bonds within the tetragonal structure (space group: I4/mcm), which remains stable throughout the temperature range studied. Combined with neutron data, X-N techniques permit the identification of the Sn2+ lone electron pair (5s2) in the two-dimensional framework, occupying empty space opposite to the four Sn-Br bonds of the pyramidal [SnBr4] coordination polyhedra. Additionally, diffuse reflectance UV-vis spectroscopy unveils an indirect optical gap of approximately â¼3.3 eV, aligning with the calculated value from the B3LYP-DFT method (â¼3.2 eV). The material exhibits a positive Seebeck coefficient as high as 6.5 × 104 µV K-1 at 350 K, which evolves down to negative values of -3.0 × 103 µV K-1 at 550 K, surpassing values reported for other halide perovskites. Notably, the thermal conductivity remains exceptionally low, between 0.32 and 0.25 W m-1 K-1.
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Thermoelectric materials offer a promising avenue for energy management, directly converting heat into electrical energy. Among them, AgSbTe2 has gained significant attention and continues to be a subject of research at further improving its thermoelectric performance and expanding its practical applications. This study focuses on Ag-deficient Ag0.7Sb1.12Te2 and Ag0.7Sb1.12Te1.95Se0.05 materials, examining the impact of compositional engineering within the AgSbTe2 thermoelectric system. These materials have been rapidly synthesized using an arc-melting technique, resulting in the production of dense nanostructured pellets. Detailed analysis through scanning electron microscopy (SEM) reveals the presence of a layered nanostructure, which significantly influences the thermoelectric properties of these materials. Synchrotron X-ray diffraction reveals significant changes in the lattice parameters and atomic displacement parameters (ADPs) that suggest a weakening of bond order in the structure. The thermoelectric characterization highlights the enhanced power factor of Ag-deficient materials that, combined with the low glass-like thermal conductivity, results in a significant improvement in the figure of merit, achieving zT values of 1.25 in Ag0.7Sb1.12Te2 and 1.01 in Ag0.7Sb1.12Te1.95Se0.05 at 750 K.
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Among thermoelectric materials, skutterudites are the most prominent candidates in the mid-temperature range applications. In the multiple-filled Sr0.2Yb0.2Co4Sb12 skutterudite, with Sr and Yb as fillers, we have enhanced the thermoelectric performance of CoSb3 through the reduction of lattice thermal conductivity and the optimization of carrier concentration and electrical conductivity. The high-pressure synthesis of the double-filled derivative promotes filling fraction fluctuation. This is observed by high angular resolution synchrotron X-ray diffraction, showing a phase segregation that corresponds to an inhomogeneous distribution of the filler atoms, located at the 2a positions of the cubic space group Im3Ì . In addition, scanning transmission electron microscopy (STEM) combined with EELS spectroscopy clearly shows a segregation of Sr atoms from the surface of the grains, which is compatible with the synchrotron X-ray powder diffraction results. Mean square displacement parameters analysis results in Einstein temperatures of â¼94 and â¼67 K for Sr and Yb, respectively, and a Debye temperature of â¼250 K. The strong effect on resonant and disorder scattering yields a significantly lower lattice thermal conductivity of 2.5 W m-1 K-1 at 773 K. Still, good weighed-mobility values were obtained, with high filling fraction of the Yb and Sr elements. This drives a reduced electrical resistivity of 2.1 × 10-5 Ω m, which leads to a peak zT of 0.26 at 773 K. The analysis and results performed for the synthesized (Sr,Yb)-double filled CoSb3, shed light on skutterudites for potential waste-heat recovery applications.
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A new perovskite KOsO_{3} has been stabilized under high-pressure and high-temperature conditions. It is cubic at 500 K (Pm-3m) and undergoes subsequent phase transitions to tetragonal at 320 K (P4/mmm) and rhombohedral (R-3m) at 230 K as shown from refining synchrotron x-ray powder diffraction (SXRD) data. The larger orbital overlap integral and the extended wave function of 5d electrons in the perovskite KOsO_{3} allow to explore physics from the regime where Mott and Hund's rule couplings dominate to the state where the multiple interactions are on equal footing. We demonstrate an exotic magnetic ordering phase found by neutron powder diffraction along with physical properties via a suite of measurements including magnetic and transport properties, differential scanning calorimetry, and specific heat, which provide comprehensive information for a system at the crossover from localized to itinerant electronic behavior.
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Double perovskite oxides, characterized by their tunable magnetic properties and robust interconnection between the lattice and magnetic degrees of freedom, present an enticing foundation for advanced magnetic refrigeration materials. Herein, we delve into the influence of rare-earth elements on RSrCoFeO6 (R = Sm, Eu) disordered double perovskites by examining their structural, electronic, magnetic, and magnetocaloric properties. Temperature-dependent synchrotron X-ray diffraction analysis confirmed the stability of the orthorhombic phase (Pnma) across a wide temperature range. X-ray photoemission spectroscopy revealed that both Sm and Eu are in the 3+ state, whereas multiple states for Co2+/3+ and Fe3+/4+ are identified. The magnetic investigation and magnetocaloric effect (MCE) analysis brought to light the presence of a long-range antiferromagnetic (AFM) order with a second-order phase transition (SOPT) in both samples. The maximum magnetic entropy change ΔSMmax was approximately 0.9 J/kg K for both samples at applied field 0-7 T, manifesting prominently above Neel temperatures TN ≈ 93 K (Sm) and 84 K (Eu). Nevertheless, different relative cooling powers (RCP) of 112.6 J/kg (Sm) and 95.5 J/kg (Eu) were observed. A detailed analysis of the temperature-dependent lattice parameters shed light on a distinct magnetocaloric effect across the magnetic transition temperature, unveiling an anisotropic thermal expansion [αV = 1.41 × 10-5 K-1 (Sm) and αV = 1.54 × 10-5 K-1 (Eu)] wherein the thermal expansion axial ratio αbSm/αbEu = 0.61 became lower with increasing temperature, which suggests that the Eu sample experiences a greater thermal expansion in the b-axis direction. At the atomic bonding level, the evidence for magnetoelastic coupling around the magnetic transition temperatures TN was found through the anomalies along the average Co/Fe-O bond distance, formal valence, octahedral distortion, as well as an anisotropic lattice expansion.
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Transition-metal chalcogenides with intercalated layered structures are interesting systems in material physics due to their attractive electronic and magnetic properties, with applications in the fields of magnetic refrigerators, catalysts, and thermoelectrics, among others. In this work, we studied in detail the structural, electronic, and magnetic properties of (Fe,Ti)-based sulfides with formula FexTi2S4 (x = 0.24, 0.32, and 0.42), prepared as polycrystalline materials under high-pressure conditions. They present a layered Heideite-type crystal structure, as assessed by synchrotron X-ray diffraction. A local structure analysis using Fe K-edge extended X-ray-absorption fine structure (EXAFS) data unveiled a conspicuous contraction of the main Fe-S bond in Fe0.24Ti2S4 at the vicinity of the magnetic transition 60-80 K. We suggest that this anomaly is related to magnetoelastic coupling effects. The EXAFS analysis allowed extraction of the Einstein temperatures (θE), i.e., the phonon contribution to the specific heat, for the two bond pairs Fe-S(1) [θE ≈318 K; 290 K (C/T)] and Fe-Ti(1) [θE ≈218 K; 190 K (C/T)]. In addition to the structural and local vibrational measurements, we probed the magnetic properties using magneto-calorimetry, magnetometry under applied pressure, magnetoresistance (MR), and Hall effect measurements. We observed the appearance of a broad peak in the specific heat around 120 K in the x = 0.42 compound that we associated with an antiferromagnetic ordering electronic transition. We found that the antiferromagnetic transition temperature is pressure and composition sensitive and reduces at 1.2 GPa by â¼12 and â¼3 K, for the members with x = 0.24 and x = 0.42, respectively. Similarly, the saturation magnetization in the ordered phase depends on both pressure and iron content, reducing its value by 50, 90, and 30% for x = 0.24, 0.32, and 0.42, respectively. We observed clear jumps in the magnetic hysteresis loops, MR, and anomalous Hall effect (AHE) below 2 K at fields around 2-4 T. We associated this observation with the metamagnetic transitions; from the Berry-curvature a decoupling parameter of SH = 0.12 V-1 is determined. Comparison of the results on the temperature-dependent magnetization, MR, and AHE elucidates a strong inelastic scattering contribution to the AHE at higher temperatures due to the cluster spin-glass phase.
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Despite the great success of hybrid CH3NH3PbI3 perovskite in photovoltaics, ascribed to its excellent optical absorption properties, its instability toward moisture is still an insurmountable drawback. All-inorganic perovskites are much less sensitive to humidity and have potential interest for solar cell applications. Alternative strategies have been developed to design novel materials with appealing properties, which include different topologies for the octahedral arrangements from three-dimensional (3D, e.g., CsPbBr3 perovskite) or two-dimensional (2D, e.g., CsPb2Br5) to zero-dimensional (0D, i.e., without connection between octahedra), as the case of Cs4PbX6 (X = Br, I) halides. The crystal structure of these materials is complex, and their thermal evolution is unexplored. In this work, we describe the synthesis of Cs4PbBr6-xIx (x = 0, 2, 4, 6) halides by mechanochemical procedures with green credentials; these specimens display excellent crystallinity enabling a detailed structural investigation from synchrotron X-ray powder diffraction (SXRD) data, essential to revisit some features in the temperature range of 90-298 K. In all this regime, the structure is defined in the trigonal R3Ì c space group (#167). The presence of Cs and X vacancies suggests some ionic mobility into the crystal structure of these 0D halides. Bond valence maps (BVMs) are useful in determining isovalent surfaces for both Cs4PbBr6 and Cs4PbI6 phases, unveiling the likely ionic pathways for cesium and bromide ions and showing a full 3D connection in the bromide phase, in contrast to the iodide one. On the other hand, the evolution of the anisotropic displacement parameters is useful to evaluate the Debye temperatures, confirming that Cs atoms have more freedom to move, while Pb is more confined at its site, likely due to a higher covalency degree in Pb-X bonds than that in Cs-X bonds. Diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy shows that the optical band gap can be tuned depending on iodine content (x) in the range of 3.6-3.06 eV. From density functional theory (DFT) simulations, the general trend of reducing the band gap when Br is replaced by I is well reproduced.
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AgSbTe2 intermetallic compound is a promising thermoelectric material. It has also been described as necessary to obtain LAST and TAGS alloys, some of the best performing thermoelectrics of the last decades. Due to the random location of Ag and Sb atoms in the crystal structure, the electronic structure is highly influenced by the atomic ordering of these atoms and makes the accurate determination of the Ag/Sb occupancy of paramount importance. We report on the synthesis of polycrystalline AgSbTe2 by arc-melting, yielding nanostructured dense pellets. SEM images show a conspicuous layered nanostructuration, with a layer thickness of 25-30 nm. Neutron powder diffraction data show that AgSbTe2 crystalizes in the cubic Pm-3m space group, with a slight deficiency of Te, probably due to volatilization during the arc-melting process. The transport properties show some anomalies at ~600 K, which can be related to the onset temperature for atomic ordering. The average thermoelectric figure of merit remains around ~0.6 from ~550 up to ~680 K.
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All-inorganic lead halide perovskites like CsPbBr3, CsPbI3, or RbPbI3 are good replacements for the classical hybrid organic-inorganic perovskites like CH3NH3PbI3, susceptible to fast degradation in the presence of humid air. They also exhibit outstanding light absorption properties suitable for solar energy applications. Here, we describe the synthesis of RbPbI3 by mechanochemical procedures with green credentials, avoiding toxic or expensive organic solvents; this specimen exhibits excellent crystallinity. We report neutron powder diffraction data, essential to revisit some subtle structural features around room temperature (200-400 K). In all these regimes, the orthorhombic Pnma crystal structure is characterized by the presence along the b direction of the crystal of double rows of edge-sharing PbI6 octahedra. The lone electron pairs of Pb2+ ions have a strong stereochemical effect on the PbI6 octahedral distortion. The relative covalency of Rb-I versus Pb-I bonds shows that the Pb-I-related motions are more rigid than Rb-I-related vibrations, as seen in the Debye temperatures from the evolution of the anisotropic displacements. The optical gap, measured by diffuse reflectance UV-vis spectroscopy, is â¼2.51 eV and agrees well with ab initio calculations. The thermoelectric Seebeck coefficient is 3 orders of magnitude larger than that of other halide perovskites, with a value of â¼117,000 µV·K-1 at 460 K.
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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
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In this review, we describe different families of metastable materials, some of them with relevant technological applications, which can be stabilized at moderate pressures 2-3.5 GPa in a piston-cylinder press. The synthesis of some of these systems had been previously reported under higher hydrostatic pressures (6-10 GPa), but can be accessed under milder conditions in combination with reactive precursors prepared by soft-chemistry techniques. These systems include perovskites with transition metals in unusual oxidation states (e.g., RNiO3 with Ni3+, R = rare earths); double perovskites such as RCu3Mn4O12 with Jahn-Teller Cu2+ ions at A sites, pyrochlores derived from Tl2Mn2O7 with colossal magnetoresistance, pnictide skutterudites MxCo4Sb12 (M = La, Yb, Ce, Sr, K) with thermoelectric properties, or metal hydrides Mg2MHx (M = Fe, Co, Ni) and AMgH3 (A: alkali metals) with applications in hydrogen storage. The availability of substantial amounts of sample (0.5-1.5 g) allows a complete characterization of the properties of interest, including magnetic, transport, thermoelectric properties and so on, and the structural characterization by neutron or synchrotron X-ray diffraction techniques.
RESUMEN
Skutterudite-type compounds based on â¡Co4Sb12 pnictide are promising for thermoelectric application due to their good Seebeck values and high carrier mobility. Filling the 8a voids (in the cubic space group Im3Ì ) with different elements (alkali, alkali earth, and rare earth) helps to reduce the thermal conductivity and thus increases the thermoelectric performance. A systematic characterization by synchrotron X-ray powder diffraction of different M-filled Co4Sb12 (M = K, Sr, La, Ce, and Yb) skutterudites was carried out under high pressure in the range â¼0-12 GPa. The isothermal equations of state (EOS) were obtained in this pressure range and the Bulk moduli (B0) were calculated for all the filled skutterudites, yielding unexpected results. A lattice expansion due to the filler elements fails in the description of the Bulk moduli. Topochemical studies of the filler site environment exhibited a slight disturbance and an increased ionic character when the filler is incorporated. The mechanical properties by means of Bulk moduli resulted in being sensitive to the presence of filler atoms inside the skutterudite voids, being affected by the covalent/ionic exchange of the Co-Sb and Sb-Sb bonds.
RESUMEN
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 3s2. 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-1K-1 in the range 323-673 K was recorded in our polycrystalline BP, which is below the reported values for single-crystals in literature.
RESUMEN
PbTe-based alloys have the best thermoelectric properties for intermediate temperature applications (500-900 K). We report on the preparation of pristine PbTe and two doped derivatives (Pb0.99Sb0.01Te and Ag0.05Sb0.05Pb0.9Te, so-called LAST18) by a fast arc-melting technique, yielding nanostructured polycrystalline pellets. XRD and neutron powder diffraction (NPD) data assessed the a slight Te deficiency for PbTe, also yielding trends on the displacement factors of the 4a and 4b sites of the cubic Fm-3m space group. Interestingly, SEM analysis shows the conspicuous formation of layers assembled as stackings of nano-sheets, with 20-30 nm thickness. TEM analysis shows intra-sheet nanostructuration on the 50 nm scale in the form of polycrystalline grains. Large numbers of grain boundaries are created by this nanostructuration and this may contribute to reduce the thermal conductivity to a record-low value of 1.6 Wm-1K-1 at room temperature. In LAST18, a positive Seebeck coefficient up to 600 µV K-1 at 450 K was observed, contributing further towards improving potential thermoelectric efficiency.
RESUMEN
Documents from the 17th Meeting, English and Spanish, are bound together in one volume
Meeting of the Advisory Committee on Medical Research, 17. Pan American Health Organization; 2-5 May 1978
Asunto(s)
Investigación , Preparaciones Farmacéuticas , Utilización de Medicamentos , Apoyo a la Investigación como Asunto , Organización Panamericana de la Salud , Costa RicaRESUMEN
Documents from the 17th Meeting, English and Spanish, are bound together in one volume
Reunion del Comite Asesor sobre Investigaciones Medicas, 17. Organización Panamericana de la Salud; 2-6 mayo 1978