High-Pressure Synthesis, Crystal Structure, and Magnetic and Transport Properties of a Six-Layered SrRhO₃

A SrRhO₃ polytype with six-layered (6M) structure was synthesized under high pressure and high temperature. The crystal structure was obtained by refining X-ray powder diffraction with the monoclinic space group <i>C</i>2/<i>c</i> with lattice parameters <i>a</i> = 5.5650(1) Å, <i>b</i> = 9.5967(2) Å, <i>c</i> = 14.0224(4) Å, and β = 92.846(2)°. It is isostructural with SrIrO₃ synthesized under ambient pressure and consists of dimers of the face-shared Rh(2)­O₆ octahedra connected by their vertices to the corner-shared Rh(1)­O₆ octahedra along the <i>c</i> axis with a stacking of SrO₃ layers in the sequence of <i>CCHCCH</i>, where <i>C</i> and <i>H</i> denote cubic and hexagonal closed packing, respectively. With increasing pressure, the 6M SrRhO₃ transforms to an orthorhombic perovskite (Pv) phase, having <i>a</i> = 5.5673(1) Å, <i>b</i> = 5.5399(2) Å, <i>c</i> = 7.8550(2) Å in the space group <i>Pbnm</i>. A pressure–temperature phase diagram shows that the 6M-Pv phase boundary moves to lower temperatures with increasing pressure. Both the 6M and the Pv phases of SrRhO₃ were characterized by magnetic susceptibility, resistivity, and thermopower; they are all metals with an enhanced and temperature-dependent magnetic susceptibility; no long-range magnetic order has been found. The polytype structures are normally found in ABO₃ oxides with the geometric tolerance factor <i>t</i> > 1. SrRhO₃ represents another example (in addition to SrIrO₃) where the polytype 6M structure can be stabilized with a <i>t</i> < 1

Visualization of the Diffusion Pathway of Protons in (NH₄)₂Si_0.5Ti_0.5P₄O₁₃ as an Electrolyte for Intermediate-Temperature Fuel Cells

We demonstrate that (NH₄)₂Si_0.5Ti_0.5P₄O₁₃ is an excellent proton conductor. The crystallographic information concerning the hydrogen positions is unraveled from neutron-powder-diffraction (NPD) data for the first time. This study shows that all the hydrogen atoms are connected though H bonds, establishing a two-dimensional path between the [(Si_0.5Ti_0.5)­P₄O₁₃^2–]<i>_n</i> layers for proton diffusion across the crystal structure by breaking and reconstructing intermediate H–OP bonds. This transient species probably reduces the potential energy of the H jump from an ammonium unit to the next neighboring NH₄⁺ unit. Both theoretical and experimental results support an interstitial-proton-conduction mechanism. The proton conductivities of (NH₄)₂Si_0.5Ti_0.5P₄O₁₃ reach 0.0061 and 0.024 S cm^–1 in humid air at 125 and 250 °C, respectively. This finding demonstrates that (NH₄)₂Si_0.5Ti_0.5P₄O₁₃ is a promising electrolyte material operating at 150–250 °C. This work opens up a new avenue for designing and fabricating high-performance inorganic electrolytes

Optimizing Thermoelectric Properties through Compositional Engineering in Ag-Deficient AgSbTe₂ Synthesized by Arc Melting

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

Structural Features and Optical Properties of All-Inorganic Zero-Dimensional Halides Cs₄PbBr_6–<i>x</i>I<i>_x</i> Obtained by Mechanochemistry

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 R3̅c 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

Magnetic Interactions in the Double Perovskites R₂NiMnO₆ (R = Tb, Ho, Er, Tm) Investigated by Neutron Diffraction

R₂NiMnO₆ (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic <i>P</i>2₁/<i>n</i> space group, with an almost complete order between Ni²⁺ and Mn⁴⁺ cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for La₂NiMnO₆, with a ferromagnetic structure involving Mn⁴⁺ and Ni²⁺ moments. This spin alignment can be rationalized taking into account the Goodenough–Kanamori rules. The magnetic ordering temperature (<i>T</i>_CM) decreases abruptly as the size of the rare earth decreases, since <i>T</i>_CM is mainly influenced by the superexchange interaction between Ni²⁺ and Mn⁴⁺ (Ni²⁺–O–Mn⁴⁺ angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below <i>T</i>_CM

LaMn₃Ni₂Mn₂O₁₂: An A- and B‑Site Ordered Quadruple Perovskite with A‑Site Tuning Orthogonal Spin Ordering

A new oxide, LaMn₃Ni₂Mn₂O₁₂, was prepared by high-pressure and high-temperature synthesis methods. The compound crystallizes in an AA′₃B₂B′₂O₁₂-type A-site and B-site ordered quadruple perovskite structure. The charge combination is confirmed to be LaMn³⁺₃Ni²⁺₂Mn⁴⁺₂O₁₂, where La and Mn³⁺ are 1:3 ordered at the A and A′ sites and the Ni²⁺ and Mn⁴⁺ are also distributed at the B and B′ sites in an orderly fashion in a rocksalt-type manner, respectively. A G-type antiferromagnetic ordering originating from the A′-site Mn³⁺ sublattice is found to occur at <i>T</i>_N ≈ 46 K. Subsequently, the spin coupling between the B-site Ni²⁺ and B′-site Mn⁴⁺ sublattices leads to an orthogonally ordered spin alignment with a net ferromagnetic component near <i>T</i>_C ≈ 34 K. First-principles calculations demonstrate that the A′-site Mn³⁺ spins play a crucial role in determining the spin structure of the B and B′ sites. This LaMn₃Ni₂Mn₂O₁₂ provides a rare example that shows orthogonal spin ordering in the B and B′ sites assisted by ordered A-site magnetic ions in perovskite systems