Palladium-Doped CsAgBiBr with 1300 nm Near-Infrared Photoresponse.

Lead-free halide double perovskite (HDP) CsAgBiBr has set a benchmark for research in HDP photoelectric applications due to its attractive optoelectronic properties. However, its narrow absorption range is a key limitation of this material. Herein, a novel dopant, palladium (Pd), is doped into CsAgBiBr and significantly extends the absorption to ≈1400 nm.

Understanding Antiferromagnetic Coupling in Lead-Free Halide Double Perovskite Semiconductors.

Solution-processable semiconductors with antiferromagnetic (AFM) order are attractive for future spintronics and information storage technology. Halide perovskites containing magnetic ions have emerged as multifunctional materials, demonstrating a cross-link between structural, optical, electrical, and magnetic properties. However, stable optoelectronic halide perovskites that are antiferromagnetic remain sparse, and the critical design rules to optimize magnetic coupling still must be developed.

Hypervalent hydridosilicate in the Na-Si-H system.

Hydrogenation reactions at gigapascal pressures can yield hydrogen-rich materials with properties relating to superconductivity, ion conductivity, and hydrogen storage. Here, we investigated the ternary Na-Si-H system by computational structure prediction and synchrotron diffraction studies of reaction mixtures NaH-Si-H at 5-10 GPa. Structure prediction indicated the existence of various hypervalent hydridosilicate phases with compositions NaSiH (m = 1-3) at comparatively low pressures, 0-20 GPa.

Formation and Polymorphism of Semiconducting KSiH and Strategy for Metallization.

KSiH, crystallizing in the cubic KPtCl structure type (3̅), features unusual hypervalent SiH complexes. Here, the formation of KSiH at high pressures is revisited by in situ synchrotron diffraction experiments, considering KSiH as a precursor. At the investigated pressures, 8 and 13 GPa, KSiH adopts the trigonal (NH)SiF structure type (3̅1) upon formation.

Lattice Dynamics and Electron-Phonon Coupling in Double Perovskite CsNaFeCl.

Phonon-phonon and electron/exciton-phonon coupling play a vitally important role in thermal, electronic, as well as optical properties of metal halide perovskites. In this work, we evaluate phonon anharmonicity and coupling between electronic and vibrational excitations in novel double perovskite CsNaFeCl single crystals. By employing comprehensive Raman measurements combined with first-principles theoretical calculations, we identify four Raman-active vibrational modes.

NaFeH and NaCoH: Hydrogen-Rich First-Row Transition Metal Hydrides from High Pressure Synthesis.

The formation of ternary hydrogen-rich hydrides involving the first-row transition metals TM = Fe and Co in high oxidation states is demonstrated from in situ synchrotron diffraction studies of reaction mixtures NaH-TM-H at 10 GPa. NaFeH and NaCoH feature pentagonal bipyramidal FeH and octahedral CoH 18-electron complexes, respectively. At high pressure, high temperature (300 < ≤ 470 °C) conditions, metal atoms are arranged as in the face-centered cubic Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes undergo reorientational dynamics.

Anharmonicity and Ultralow Thermal Conductivity in Lead-Free Halide Double Perovskites.

The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs_{2}AgBiBr_{6}, the prototypical compound in this class and show that the lattice dynamics of Cs_{2}AgBiBr_{6} is highly anharmonic, largely in regards to tilting of AgBr_{6} and BiBr_{6} octahedra. Using an energy- and temperature-dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch.

Layered Zinc Hydroxide Dihydrate, Zn(OH)·2HO, from Hydrothermal Conversion of ε-Zn(OH) at Gigapascal Pressures and its Transformation to Nanocrystalline ZnO.

Layered zinc hydroxides (LZHs) with the general formula (Zn) (OH) (A ) ·HO (A = Cl, NO , ac, SO , etc) are considered as useful precursors for the fabrication of functional ZnO nanostructures. Here, we report the synthesis and structure characterization of the hitherto unknown "binary" representative of the LZH compound family, Zn(OH)·2HO, with A = OH, = 5, = 2, and = 2. Zn(OH)·2HO was afforded quantitatively by pressurizing mixtures of ε-Zn(OH) (wulfingite) and water to 1-2 GPa and applying slightly elevated temperatures, 100-200 °C.

Lead-Free Halide Double Perovskite Cs AgBiBr with Decreased Band Gap.

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs AgBiBr , shows attractive optical and electronic features, making it promising for high-efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics.

Na-Ni-H Phase Formation at High Pressures and High Temperatures: Hydrido Complexes [NiH] the Perovskite NaNiH.

The Na-Ni-H system was investigated by synchrotron diffraction studies of reaction mixtures NaH-Ni-H at around 5, 10, and 12 GPa. The existence of ternary hydrogen-rich hydrides with compositions NaNiH and NaNiH, where Ni attains the oxidation state II, is demonstrated. Upon heating at ∼5 GPa, face-centered cubic () NaNiH forms above 430 °C.

Localized versus itinerant character of 4f-states in cerium oxides.

The electronic structure of cerium oxide is investigated here using a combination of ab initio one-electron theory and elements from many-body physics, with emphasis on the nature of the 4f electron shell of cerium ions. We propose to use the hybridization function as a convenient measure for the degree of localization of the 4f shell of this material, and observe that changing the oxidation state is related to distinct changes in the hybridization between the 4f shell and ligand states. The theory reveals that CeO has essentially itinerant 4f states, and that in the least oxidized form of ceria, CeO, the 4f states are almost (but not fully) localized.

Mysterious SiB: Identifying the Relation between α- and β-SiB.

Binary silicon boride SiB has been reported to occur in two forms, as disordered and nonstoichiometric α-SiB , which relates to the α-rhombohedral phase of boron, and as strictly ordered and stoichiometric β-SiB. Similar to other boron-rich icosahedral solids, these SiB phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood.

Exploring the Mg-Cr-H System at High Pressure and Temperature via in Situ Synchrotron Diffraction.

The complex transition metal hydride MgCrH has been previously synthesized using high pressure conditions. It contains the first group 6 homoleptic hydrido complex, [Cr(II)H]. Here, we investigated the formation of MgCrH by in situ studies of reaction mixtures of 3MgH-Cr-H at 5 GPa.

Low viscosity of the Earth's inner core.

The Earth's solid inner core is a highly attenuating medium. It consists mainly of iron. The high attenuation of sound wave propagation in the inner core is at odds with the widely accepted paradigm of hexagonal close-packed phase stability under inner core conditions, because sound waves propagate through the hexagonal iron without energy dissipation.

Phase Stability of Dynamically Disordered Solids from First Principles.

Theoretical studies of phase stability in solid materials with dynamic disorder are challenging due to the failure of the standard picture of atoms vibrating around fixed equilibrium positions. Dynamically disordered solid materials show immense potential in applications. In particular, superionic conductors, where the disorder results in exceptionally high ionic conductivity, are very promising as solid state electrolytes in batteries and fuel cells.

Unraveling Hidden Mg-Mn-H Phase Relations at High Pressures and Temperatures by in Situ Synchrotron Diffraction.

The Mg-Mn-H system was investigated by in situ high pressure studies of reaction mixtures MgH-Mn-H. The formation conditions of two complex hydrides with composition MgMnH were established. Previously known hexagonal MgMnH (h-MgMnH) formed at pressures 1.

Oxygen diffusion in ceria doped with rare-earth elements.

We examine the effects of the dopant type and the dopant distribution on the ion diffusion in ceria doped with rare-earth elements (Pr, Nd, Pm, Sm, Eu, and Gd). Diffusion is simulated by means of a Kinetic Monte Carlo method using input transition rates derived from diffusion barriers calculated in the framework of density functional theory (DFT). Based on diffusion simulations, we discuss the characteristics of the dopants in terms of the diffusion barriers, and study oxygen ion trajectories for different dopants and distributions.

Synthesis of TiAuC, TiAuC and TiIrC by noble metal substitution reaction in TiSiC for high-temperature-stable Ohmic contacts to SiC.

The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of TiAuC and TiAuC phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into TiSiC single-crystal thin films with simultaneous out-diffusion of Si.

Highly Efficient Free Energy Calculations of the Fe Equation of State Using Temperature-Dependent Effective Potential Method.

Free energy calculations at finite temperature based on ab initio molecular dynamics (AIMD) simulations have become possible, but they are still highly computationally demanding. Besides, achieving simultaneously high accuracy of the calculated results and efficiency of the computational algorithm is still a challenge. In this work we describe an efficient algorithm to determine accurate free energies of solids in simulations using the recently proposed temperature-dependent effective potential method (TDEP).

Pressure-induced hydrogen bond symmetrization in iron oxyhydroxide.

Under high pressures the hydrogen bonds were predicted to transform from a highly asymmetric soft O-H···O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms. Despite four decades of research on hydroxyl containing compounds, pressure induced hydrogen bond symmetrization remains elusive. Following single crystal x-ray diffraction, Mössbauer and Raman spectroscopy measurements supported by ab initio calculations, we report the H-bonds symmetrization in iron oxyhydroxide, FeOOH, resulting from the Fe(3+) high-to-low spin crossover at above 45 GPa.

Hemispherical anisotropic patterns of the Earth's inner core.

It has been shown that the Earth's inner core has an axisymmetric anisotropic structure with seismic waves traveling approximately 3% faster along polar paths than along equatorial directions. Hemispherical anisotropic patterns of the solid Earth's core are rather complex, and the commonly used hexagonal-close-packed iron phase might be insufficient to account for seismological observations. We show that the data we collected are in good agreement with the presence of two anisotropically specular east and west core hemispheres.

Optimization of ionic conductivity in doped ceria.

Oxides with the cubic fluorite structure, e.g., ceria (CeO2), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations.

Laves-phase structural changes in the system CaAl2-xMgx.

Compounds CaAl(2)(-)(x)Mg(x) (0 < or = x < or = 2) were synthesized and structurally characterized by X-ray diffraction experiments. With increasing Mg content x the sequence of Laves phase structures MgCu(2) --> MgNi(2) --> MgZn(2) is revealed. The homogeneity ranges of the underlying phases were determined to be 0 < or = x < 0.

The Origin of the Distorted Close-Packed Elemental Structure of Indium.

Why does indium adopt a tetragonally distorted fcc structure, and not a fcc structure like its lighter homologue aluminum? This question has puzzled chemists and physicists for a long time, and has now been addressed by means of ab initio full-potential calculations in the framework of density functional theory. It was found that the different tendency of aluminum and indium to form s-p hybridized bands is responsible for the different ground-state structures (shown schematically).