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Pressure-induced transformations in an archetypal chalcogenide glass (GeSe2) have been investigated up to 157 GPa by X-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. Ge and Se K-edge XAS data allowed simultaneous tracking of the correlated local structural and electronic changes at both Ge and Se sites. Thanks to the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) signals of both edges, reliable quantitative information about the evolution of the first neighbor Ge-Se distribution could be obtained. It also allowed to account for contributions of the Ge-Ge and Se-Se bond distributions (chemical disorder). The low-density to high-density amorphous-amorphous transformation was found to occur within 10 to 30 GPa pressure range, but the conversion from tetrahedral to octahedral coordination of the Ge sites is completed above ~ 80 GPa. No convincing evidence of another high-density amorphous state with coordination number larger than six was found within the investigated pressure range. The number of short Ge-Ge and Se-Se "wrong" bonds was found to increase upon pressurization. Experimental XAS results are confirmed by MD simulations, indicating the increase of chemical disorder under high pressure. [ABSTRACT FROM AUTHOR]

We present a method to fabricate handcrafted thermoelectric devices on standard office paper substrates. The devices are based on thin films of WS2, Te, and BP (P‐type semiconductors) and TiS3 and TiS2 (N‐type semiconductors), deposited by simply rubbing powder of these materials against paper. The thermoelectric properties of these semiconducting films revealed maximum Seebeck coefficients of (+1.32 ± 0.27) mV K−1 and (−0.82 ± 0.15) mV K−1 for WS2 and TiS3, respectively. Additionally, Peltier elements were fabricated by interconnecting the P‐ and N‐type films with graphite electrodes. A thermopower value up to 6.11 mV K−1 was obtained when the Peltier element were constructed with three junctions. The findings of this work show proof‐of‐concept devices to illustrate the potential application of semiconducting van der Waals materials in future thermoelectric power generation as well as temperature sensing for low‐cost disposable electronic devices. [ABSTRACT FROM AUTHOR]

The search for novel lead-free materials with potential optoelectronic applications is a main research topic nowadays for an environment-friendly energy transition. Based on promising all-inorganic CsPbBr3 perovskite, we tested here the substitution of toxic Pb2+ with Cu2+ and explored the understudied CsCuBr3 halide. We present a mechanically-synthesized CsCuBr3 specimen obtained by ball milling, consisting of a well-crystallized and pure sample. Calorimetric measurements revealed a thermal event suggesting a structural phase transition around 422 K, immediately below the decomposition of the sample due to Br loss. A detailed structural analysis was carried out using neutron powder diffraction data from 20 to 420 K, focusing on the evolution of the orthorhombic phase (space-group: C2221) up to 400 K; this crystal arrangement consists of dimer units of face-sharing [CuBr6] octahedra containing Cu–Cu dimers with conspicuously short distances that account for the magnetic coupling between Cu2+ spins described before. Additionally, at 420 K, we identified a novel high-temperature phase as described in the hexagonal P6522 space group, where infinite chains of [CuBr4] square-planar units were observed. An additional study considering the Debye model was carried out, providing information on the relative Cs–Br and Cu–Br chemical bonds. From diffuse reflectance UV-Vis measurements, an optical gap of ∼1.8 eV was estimated for the orthorhombic phase at room temperature. Magnetic measurements indicate that the effective magnetization behavior versus temperature appears close to linear and seems to extrapolate to zero (smaller than 10−5 μB/Cu atom) at temperatures above 420 K, coinciding with onset of phase transition. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) layered metal halide perovskites have recently received a lot of attention due to their possible applications as photovoltaic and optoelectronic materials. Rubidium di-tin pentabromide, RbSn2Br5, is a promising lead-free alternative to its RbPb2Br5 counterpart. Its lack of toxic lead, improved stability, and tolerance to ambient conditions are a great step forward to be used in electronic devices. In contrast with lead-based halides, this sample has received less attention up to now. There have been no experimental studies on its transport parameters such as electronic conductivity, Seebeck coefficient, or thermal transport. Here, we describe how this material can be easily synthesized using a ball milling procedure, obtaining specimens with high crystallinity. TG measurements indicate total decomposition above ∼673 K, whereas DSC curves suggest melting and recrystallization at 562 K (heating run, endothermic) and 523 K (cooling run, exothermic), respectively. A structural analysis from room temperature up to 548 K from neutron powder diffraction (NPD) data allowed the determination of the Debye model parameters, providing information on the relative Rb–Br and Sn–Br chemical bonds. Synchrotron X-ray diffraction experiments confirmed a tetragonal structure (space group I4/mcm) and provided evidence on the presence of the Sn2+ lone electron pair (5s2) from a X–N study. Diffuse reflectance UV-vis spectroscopy yields an indirect optical gap of ∼3.08 eV, coincident with the literature and ab initio calculations. A maximum positive Seebeck coefficient of ∼2.3 × 104 μV K−1 is obtained at 440 K, which is one order of magnitude higher than those reported for other halide perovskites. We obtain an ultra-low thermal conductivity, below 0.2 W m−1 K−1 from 300 up to 550 K. [ABSTRACT FROM AUTHOR]

Thermoelectric MxCo4Sb12 skutterudites are well-known to exhibit a reduced thermal conductivity thanks to the rattling effect of the M-filler at the large cages occurring in the framework, centered at the 2a sites of the I m 3 ‾ space group. A novel Eu-filled skutterudite has been synthesized under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. The structural refinement from high-angular resolution synchrotron X-ray diffraction (SXRD) patterns unveils an unusual position for Eu filler atoms. By difference Fourier synthesis they are found at 12d sites, conforming statistically occupied octahedra within the mentioned cages around 2a positions. The Debye temperature was estimated by averaging the isotropic displacements by the atomic masses, leading to θ D of 273(2) K. Oftedal plots concerning the y and z Sb fractional positions, the unit-cell parameter a and M filling fraction include the novel Eu specimen in the trend observed for other filled materials prepared under high-pressure, including rare-earths, alkali or alkali-earth elements, all accepted as rattlers in filled skutterudites. A total thermal conductivity (κ) of 0.82 W m−1 K−1 is measured at 773 K for Eu0.02(1)Co4Sb12, below that of other filled skutterudites, which is promoted by the enhanced phonon scattering of Eu located at 12d sites. FE-SEM images showed large, homogeneous grains, well compacted after the high-pressure synthesis. [ABSTRACT FROM AUTHOR]

The structural ordering process in complex perovskites has a pivotal role for tuning many physical properties for broad applications, ranging from microwave technology, proton‐conduction, multiferroicity, and so on. Therefore, the characterization of order type in these materials is essential for designing new devices with high‐performance. Here, the coexistence of B‐site 1:1 and 1:2 order types in mixed ordered A3CaNb2O9 (A = Ba, Sr) perovskite was investigated by combining Raman spectroscopy and high‐resolution synchrotron X‐ray powder diffraction. High‐wavenumber interval 700–825 cm−1 exhibits two bands concerning the symmetric breathing modes of [NbO6] octahedra in A3CaNb2O9, which were ascribed to the 1:1 and 1:2 domain regions in coexistence. This model was fully corroborated using two phases for describing the synchrotron X‐ray pattern of the Ba3CaNb2O9 sample. Therefore, the Raman spectroscopy can be indeed applied as a rapid tool for probing the achievement of ordered, partially ordered, or disordered structures in complex perovskites. For the first time, BaLaCaNbO6 was synthesized and structural characterized, being indexed by the monoclinic unit cell belonging to the space group I2/m. [ABSTRACT FROM AUTHOR]

Rubidium di-lead pentabromide, RbPb2Br5, belongs to a family of layered lead-containing halides, with the common formula APb2X5 (where A = K, Rb, Cs; X = Cl, Br). The optical properties of RbPb2Br5 and rare-earth doped specimens are promising as low-phonon energy materials for tunable middle infrared and long-wavelength infrared laser sources, with suitable stability and resistance to wet conditions. In contrast to CsPb2Br5, the Rb counterpart has been barely studied and deserves further attention. Up to now, there have been no experimental reports on the transport properties such as the electronic conductivity, Seebeck coefficient or thermal transport. We describe here that this material can be prepared by ball milling in a straightforward way, yielding specimens with superior crystallinity. A structural investigation using synchrotron X-ray powder diffraction (SXRD) data combined with neutron powder diffraction (NPD) in a wide temperature range, from 15 to 573 K, was essential to evaluate the thermal evolution and to determine the Debye constants, yielding information on the relative Rb–Br and Pb–Br chemical bonds. In combination with DSC and TG measurements, no phase transitions were observed. Furthermore, an analysis of SXRD and NPD data (XRD-NPD) at room temperature reveals the directions of electron lone pair of Pb2+ ions lead atoms: its stereochemical effect is obvious in the [PbBr8] octahedral distortions. Diffuse reflectance UV-Vis spectroscopy yields an optical gap of 3.36 eV, close to that determined for a single-crystal material. Photoluminescence measurements indicate a lack of overlap between the excitation and emission spectra, due to the considerable Stokes shift, which prohibits self-absorption and thus enables applications in photovoltaics and biomedicine. The experimental information about the chemical bonds and band gap was studied via first-principles calculations. A maximum positive Seebeck coefficient of 3 200 μV K−1 is obtained at 560 K, which is one order of magnitude higher than those reported for other halide perovskites. [ABSTRACT FROM AUTHOR]

Abstract: Theoretical and experimental methods were employed to investigate the vibrational modes of BiOBr. Wavenumber prediction and assignments of the infrared and Raman modes were performed using density functional theory and lattice dynamics calculations. The theoretical models were directly compared with the experimental results of particles fabricated using the microwave‐assisted hydrothermal method and results reported in literature. The simulation results of the structure parameters, band gap, and vibrational modes are in agreement with the corresponding experimental values. The proposed models and calculations provided detailed insights into the vibrational modes and enabled to determine propagated assignment errors in results reported in the literature. [ABSTRACT FROM AUTHOR]

All-inorganic perovskites exhibit outstanding light absorption properties in the visible range suitable for solar energy applications. We focus on the synthesis of CsPb 2 (Br,I) 5 using mechanochemical procedures. Synchrotron X-ray diffraction (SXRD) data are essential for determining the crystallographic evolution in the 295-753 K temperature range. From room temperature to 573 K, the crystal structure is refined in a two-dimensional (2D) tetragonal framework (space group: I 4/ mcm ) consisting of layers of face sharing [Pb(Br,I) 8 ] polyhedra. Above a transition temperature of T t = 630 and 621 K, identified from differential scanning calorimetry (DSC) curves for CsPb 2 Br 5 and CsPb 2 Br 4 I, respectively, a cubic three-dimensional (3D) corner-sharing perovskite structure is identified, showing substantial Cs and (Br,I) deficiency. A more ionic character for Cs-halide versus Pb-halide is derived from the evolution of the anisotropic atomic displacements. An ultralow thermal conductivity of 0.35-0.18 W m -1 K -1 is related to the low Debye temperatures. From high-pressure SXRD studies, the change in B 0 can be attributed to a basic expansion of the unit-cell volume caused by the chemical pressure from iodine within the CsPb 2 Br 5 framework. UV-vis-NIR spectroscopy revealed optical gaps of approximately 2.93 and 2.50 eV, respectively, which agrees with ab initio calculations.

Despite the great success of hybrid CH 3 NH 3 PbI 3 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., CsPbBr 3 perovskite) or two-dimensional (2D, e.g., CsPb 2 Br 5 ) to zero-dimensional (0D, i.e., without connection between octahedra), as the case of Cs 4 PbX 6 (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 Cs 4 PbBr 6- x I x ( 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 R 3̅ 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 Cs 4 PbBr 6 and Cs 4 PbI 6 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.

All-inorganic lead halide perovskites like CsPbBr 3 , CsPbI 3 , or RbPbI 3 are good replacements for the classical hybrid organic-inorganic perovskites like CH 3 NH 3 PbI 3 , 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 RbPbI 3 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 PbI 6 octahedra. The lone electron pairs of Pb 2+ ions have a strong stereochemical effect on the PbI 6 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 ∼2.51 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 ∼117,000 μV·K -1 at 460 K.

Skutterudite-type compounds based on □Co 4 Sb 12 pnictide are promising for thermoelectric application due to their good Seebeck values and high carrier mobility. Filling the 8 a voids (in the cubic space group Im 3̅) 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 Co 4 Sb 12 (M = K, Sr, La, Ce, and Yb) skutterudites was carried out under high pressure in the range ∼0-12 GPa. The isothermal equations of state (EOS) were obtained in this pressure range and the Bulk moduli ( B 0 ) 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.