AuPb₂I₇: A Narrow Bandgap Au³⁺ Iodide Semiconductor

The unusual Au³⁺ ternary halide AuPb₂I₇ has been isolated from reactions of AuI, PbI₂, and I₂. AuPb₂I₇ crystallizes in the triclinic <i>P</i>1̅ space group as micron-scale needles with cell dimensions <i>a</i> = 4.5170(3) Å, <i>b</i> = 7.3847(4) Å, <i>c</i> = 12.2970(7) Å, α = 76.374(4)°, β = 83.711(4)°, γ = 72.987(3)° at room temperature with ρ = 6.538 g/cm³ and has no structural phase transition down to 100 K. The title compound has a unique three-dimensional structure composed of [Pb₂I₇]^3– pseudolayers extending in [010] bridged by square planar Au³⁺ at an oblique angle in the [001] direction. The pseudolayers are composed of ¹/_∞[Pb₂I₂]²⁺ chains propagating down [100] linked by square planar I^– ions through [010]. AuPb₂I₇ has a bandgap of 1.17 eV and is stable in air for several days, before degrading to PbI₂, Au⁰, and I₂. Density functional theory calculations show that AuPb₂I₇ is an indirect bandgap semiconductor where the bandgap stems predominantly from Au–I metal–ligand charge transfer

On the Stereochemical Inertness of the Auride Lone Pair: Ab Initio Studies of AAu (A = K, Rb, Cs)

The “lone” 6s electron pair often plays a key role in determining the structure and physical properties of compounds containing sixth-row elements in their lower oxidation states: Tl⁺, Pb²⁺, and Bi³⁺ with the [Xe]­4f¹⁴5d¹⁰6s² electronic configuration. The lone pairs on these ions are associated with reduced structural symmetries, including ferroelectric instabilities and other important phenomena. Here we consider the isoelectronic auride Au^– ion with the [Xe]­4f¹⁴5d¹⁰6s² electronic configuration. Ab initio density functional theory methods are employed to probe the effect of the 6s lone pair in alkali-metal aurides (KAu, RbAu, and CsAu) with the CsCl structure. The dielectric constants, Born effective charges, and structural instabilities suggest that the 6s lone pair on the Au^– anion is stereochemically inert to minor mechanical and electrical perturbation. Pressures greater than 14 GPa, however, lead to reorganization of the electronic structure of CsAu and activate lone-pair involvement and Au–Au interactions in bonding, resulting in a transformation from the cubic CsCl structure type to an orthorhombic <i>Cmcm</i> structure featuring zigzag Au–Au chains

Low-Temperature Synthesis and Magnetostructural Transition in Antiferromagnetic, Refractory Nanoparticles: Chromium Nitride, CrN

Nanostructured chromium nitride (CrN), both a hard material and a high-melting compound that is used in the medical industry and for new energy-harvesting applications, was synthesized phase-pure for the first time via low-temperature solution synthesis in liquid ammonia. TEM analysis confirms the nanoscale character of CrN. The antiferromagnetic properties of the agglomerates of nanoparticles are discussed in comparison to literature data on the bulk materials. SQUID and DSC measurements show the transition from paramagnetic to antiferromagnetic at 258.5 K. In situ low-temperature X-ray diffraction patterns confirm the magnetostructural phase transition at this temperature, not seen before for nanoscale CrN. This structural distortion was calculated earlier to be driven by magnetic stress. The bottom-up synthesis of CrN allows for the production of nearly oxygen- and carbon-free and highly dispersed fine particles

Structural Evolution and Atom Clustering in β‑SiAlON: β‑Si_6–<i>z</i>Al_<i>z</i>O_<i>z</i>N_8–<i>z</i>

SiAlON ceramics, solid solutions based on the Si₃N₄ structure, are important, lightweight structural materials with intrinsically high strength, high hardness, and high thermal and chemical stability. Described by the chemical formula β-Si_6–<i>z</i>Al_<i>z</i>O_<i>z</i>N_8–<i>z</i>, from a compositional viewpoint, these materials can be regarded as solid solutions between Si₃N₄ and Al₃O₃N. A key aspect of the structural evolution with increasing Al and O (<i>z</i> in the formula) is to understand how these elements are distributed on the β-Si₃N₄ framework. The average and local structural evolution of highly phase-pure samples of β-Si_6–<i>z</i>Al_<i>z</i>O_<i>z</i>N_8–<i>z</i> with <i>z</i> = 0.050, 0.075, and 0.125 are studied here, using a combination of X-ray diffraction, NMR studies, and density functional theory calculations. Synchrotron X-ray diffraction establishes sample purity and indicates subtle changes in the average structure with increasing Al content in these compounds. Solid-state magic-angle-spinning ²⁷Al NMR experiments, coupled with detailed ab initio calculations of NMR spectra of Al in different AlO_<i>q</i>N_4–<i>q</i> tetrahedra (0 ≤ <i>q</i> ≤ 4), reveal a tendency of Al and O to cluster in these materials. Independently, the calculations suggest an energetic preference for Al–O bond formation, instead of a random distribution, in the β-SiAlON system

Crystal Structure Evolution and Notable Thermal Expansion in Hybrid Perovskites Formamidinium Tin Iodide and Formamidinium Lead Bromide

The temperature-dependent structure evolution of the hybrid halide perovskite compounds, formamidinium tin iodide (FASnI₃, FA⁺ = CH­[NH₂]₂⁺) and formamidinium lead bromide (FAPbBr₃), has been monitored using high-resolution synchrotron X-ray powder diffraction between 300 and 100 K. The data are consistent with a transition from cubic <i>Pm</i>3<i>m</i> (No. 221) to tetragonal <i>P</i>4/<i>mbm</i> (No. 127) for both materials upon cooling; this occurs for FAPbBr₃ between 275 and 250 K, and for FASnI₃ between 250 and 225 K. Upon further cooling, between 150 and 125 K, both materials undergo a transition to an orthorhombic <i>Pnma</i> (No. 62) structure. The transitions are confirmed by calorimetry and dielectric measurements. In the tetragonal regime, the coefficients of volumetric thermal expansion of FASnI₃ and FAPbBr₃ are among the highest recorded for any extended inorganic crystalline solid, reaching 219 ppm K^–1 for FASnI₃ at 225 K. Atomic displacement parameters of all atoms for both materials suggest dynamic motion is occurring in the inorganic sublattice due to the flexibility of the inorganic network and dynamic lone pair stereochemical activity on the <i>B</i>-site. Unusual pseudocubic behavior is displayed in the tetragonal phase of the FAPbBr₃, similar to that previously observed in FAPbI₃

An Efficient, Thermally Stable Cerium-Based Silicate Phosphor for Solid State White Lighting

A novel cerium-substituted, barium yttrium silicate has been identified as an efficient blue-green phosphor for application in solid state lighting. Ba₉Y₂Si₆O₂₄:Ce³⁺ was prepared and structurally characterized using synchrotron X-ray powder diffraction. The photoluminescent characterization identified a major peak at 394 nm in the excitation spectrum, making this material viable for near-UV LED excitation. An efficient emission, with a quantum yield of ≈60%, covers a broad portion (430–675 nm) of the visible spectrum, leading to the blue-green color. Concentration quenching occurs when the Ce³⁺ content exceeds ≈3 mol %, whereas high temperature photoluminescent measurements show a 25% drop from the room temperature efficiency at 500 K. The emission of this compound can be red-shifted via the solid solution Ba₉(Y_1–<i>y</i>Sc_<i>y</i>)_1.94Ce_0.06Si₆O₂₄ (<i>y</i> = 0.1, 0.2), allowing for tunable color properties when device integration is considered

Metastable Ni₇B₃: A New Paramagnetic Boride from Solution Chemistry, Its Crystal Structure and Magnetic Properties

We trapped an unknown metastable boride by applying low-temperature solution synthesis. Single-phase nickel boride, Ni₇B₃, was obtained as bulk samples of microcrystalline powders when annealing the amorphous, nanoscale precipitate that is formed in aqueous solution of nickel chloride upon reaction with sodium tetrahydridoborate. Its crystal structure was solved based on a disordered Th₇Fe₃-type model (hexagonal crystal system, space group <i>P</i>6₃<i>mc</i>, no. 186, <i>a</i> = 696.836(4) pm, <i>c</i> = 439.402(4) pm), using synchrotron X-ray powder data. Magnetic measurements reveal paramagnetism, which is in accordance with quantum chemical calculations. According to high-temperature X-ray diffraction and differential scanning calorimetry this nickel boride phase has a narrow stability window between 300 and 424 °C. It crystallizes at ca. 350 °C, then starts decomposing to form Ni₃B and Ni₂B above 375 °C, and shows an exothermic effect at 424 °C

Dielectric and Thermodynamic Signatures of Low-Temperature Glassy Dynamics in the Hybrid Perovskites CH₃NH₃PbI₃ and HC(NH₂)₂PbI₃

Hybrid main group halide perovskites hold great technological promise in optoelectronic applications and present rich and complex evolution of structure and dynamics. Here we present low-temperature dielectric measurements and calorimetry of <i>A</i>PbI₃ [<i>A</i> = CH₃NH₃⁺, HC­(NH₂)₂⁺] that suggest glassy behavior on cooling. In both compounds, the dielectric loss displays frequency-dependent peaks below 100 K characteristic of a glassy slowing of relaxation dynamics, with HC­(NH₂)₂PbI₃ exhibiting greater glass fragility. Consistent with quenched disorder, the low-temperature heat capacity of both perovskites deviates substantially from the ∼<i>T</i>³ acoustic phonon contribution predicted by the Debye model. We suggest that static disorder of the <i>A</i>-site molecular cation, potentially coupled to local distortions of the Pb–I sublattice, is responsible for these phenomena. The distinct low-temperature dynamics observed in these two perovskites suggest qualitative differences in the interaction between the molecular cation and the surrounding inorganic framework, with potential implications for defect screening and device performance at ambient temperatures