Circumventing Diffusion in Kinetically Controlled Solid-State Metathesis Reactions

Solid-state diffusion is often the primary limitation in the synthesis of crystalline inorganic materials and prevents the potential discovery and isolation of new materials that may not be the most stable with respect to the reaction conditions. Synthetic approaches that circumvent diffusion <i>in solid-state reactions</i> are rare and often allow the formation of metastable products. To this end, we present an <i>in situ</i> study of the solid-state metathesis reactions MCl₂ + Na₂S₂ → MS₂ + 2 NaCl (M = Fe, Co, Ni) using synchrotron powder X-ray diffraction and differential scanning calorimetry. Depending on the preparation method of the reaction, either combining the reactants in an air-free environment or grinding homogeneously in air before annealing, the barrier to product formation, and therefore reaction pathway, can be altered. In the air-free reactions, the product formation appears to be diffusion limited, with a number of intermediate phases observed before formation of the MS₂ product. However, grinding the reactants in air allows NaCl to form directly without annealing and displaces the corresponding metal and sulfide ions into an amorphous matrix, as confirmed by pair distribution function analysis. Heating this mixture yields direct nucleation of the MS₂ phase and avoids all crystalline binary intermediates. Grinding in air also dissipates a large amount of lattice energy via the formation of NaCl, and the crystallization of the metal sulfide is a much less exothermic process. This approach has the potential to allow formation of a range of binary, ternary, or higher-ordered compounds to be synthesized in the bulk, while avoiding the formation of many binary intermediates that may otherwise form in a diffusion-limited reaction

Hybrid Inorganic–Organic Materials with an Optoelectronically Active Aromatic Cation: (C₇H₇)₂SnI₆ and C₇H₇PbI₃

Inorganic materials with organic constituentshybrid materialshave shown incredible promise as chemically tunable functional materials with interesting optical and electronic properties. Here, the preparation and structure are reported of two hybrid materials containing the optoelectronically active tropylium ion within tin- and lead-iodide inorganic frameworks with distinct topologies. The crystal structures of tropylium tin iodide, (C₇H₇)₂SnI₆, and tropylium lead iodide, C₇H₇PbI₃, were solved using high-resolution synchrotron powder X-ray diffraction informed by X-ray pair distribution function data and high-resolution time-of-flight neutron diffraction. Tropylium tin iodide contains isolated tin­(IV)-iodide octahedra and crystallizes as a deep black solid, while tropylium lead iodide presents one-dimensional chains of face-sharing lead­(II)-iodide octahedra and crystallizes as a bright red-orange powder. Experimental diffuse reflectance spectra are in good agreement with density functional calculations of the electronic structure. Calculations of the band decomposed charge densities suggest that the deep black color of tropylium tin iodide is attributed to iodide ligand to tin metal charge transfer, while the bright red-orange color of tropylium lead iodide arises from charge transfer between iodine and tropylium states. Understanding the origins of the observed optoelectronic properties of these two compounds, with respect to their distinct topologies and organic–inorganic interactions, provides insight into the design of tropylium-containing compounds for potential optical and electronic applications

Hybrid Inorganic–Organic Materials with an Optoelectronically Active Aromatic Cation: (C₇H₇)₂SnI₆ and C₇H₇PbI₃

Inorganic materials with organic constituentshybrid materialshave shown incredible promise as chemically tunable functional materials with interesting optical and electronic properties. Here, the preparation and structure are reported of two hybrid materials containing the optoelectronically active tropylium ion within tin- and lead-iodide inorganic frameworks with distinct topologies. The crystal structures of tropylium tin iodide, (C₇H₇)₂SnI₆, and tropylium lead iodide, C₇H₇PbI₃, were solved using high-resolution synchrotron powder X-ray diffraction informed by X-ray pair distribution function data and high-resolution time-of-flight neutron diffraction. Tropylium tin iodide contains isolated tin­(IV)-iodide octahedra and crystallizes as a deep black solid, while tropylium lead iodide presents one-dimensional chains of face-sharing lead­(II)-iodide octahedra and crystallizes as a bright red-orange powder. Experimental diffuse reflectance spectra are in good agreement with density functional calculations of the electronic structure. Calculations of the band decomposed charge densities suggest that the deep black color of tropylium tin iodide is attributed to iodide ligand to tin metal charge transfer, while the bright red-orange color of tropylium lead iodide arises from charge transfer between iodine and tropylium states. Understanding the origins of the observed optoelectronic properties of these two compounds, with respect to their distinct topologies and organic–inorganic interactions, provides insight into the design of tropylium-containing compounds for potential optical and electronic applications

Paracrystalline Disorder from Phosphate Ion Orientation and Substitution in Synthetic Bone Mineral

Hydroxyapatite is an inorganic mineral closely resembling the mineral phase in bone. However, as a biological mineral, it is highly disordered, and its composition and atomistic structure remain poorly understood. Here, synchrotron X-ray total scattering and pair distribution function analysis methods provide insight into the nature of atomistic disorder in a synthetic bone mineral analogue, chemically substituted hydroxyapatite. By varying the effective hydrolysis rate and/or carbonate concentration during growth of the mineral, compounds with varied degrees of paracrystallinity are prepared. From advanced simulations constrained by the experimental pair distribution function and density functional theory, the paracrystalline disorder prevalent in these materials appears to result from accommodation of carbonate in the lattice through random displacement of the phosphate groups. Though many substitution modalities are likely to occur in concert, the most predominant substitution places carbonate into the mirror plane of an ideal phosphate site. Understanding the mineralogical imperfections of a biologically analogous hydroxyapatite is important not only to potential bone grafting applications but also to biological mineralization processes themselves

Paracrystalline Disorder from Phosphate Ion Orientation and Substitution in Synthetic Bone Mineral

Hydroxyapatite is an inorganic mineral closely resembling the mineral phase in bone. However, as a biological mineral, it is highly disordered, and its composition and atomistic structure remain poorly understood. Here, synchrotron X-ray total scattering and pair distribution function analysis methods provide insight into the nature of atomistic disorder in a synthetic bone mineral analogue, chemically substituted hydroxyapatite. By varying the effective hydrolysis rate and/or carbonate concentration during growth of the mineral, compounds with varied degrees of paracrystallinity are prepared. From advanced simulations constrained by the experimental pair distribution function and density functional theory, the paracrystalline disorder prevalent in these materials appears to result from accommodation of carbonate in the lattice through random displacement of the phosphate groups. Though many substitution modalities are likely to occur in concert, the most predominant substitution places carbonate into the mirror plane of an ideal phosphate site. Understanding the mineralogical imperfections of a biologically analogous hydroxyapatite is important not only to potential bone grafting applications but also to biological mineralization processes themselves

Stacking Variants and Superconductivity in the Bi–O–S System

High-temperature superconductivity has a range of applications from sensors to energy distribution. Recent reports of this phenomenon in compounds containing electronically active BiS₂ layers have the potential to open a new chapter in the field of superconductivity. Here we report the identification and basic properties of two new ternary Bi–O–S compounds, Bi₂OS₂ and Bi₃O₂S₃. The former is non-superconducting; the latter likely explains the superconductivity at <i>T</i>_c = 4.5 K previously reported in “Bi₄O₄S₃”. The superconductivity of Bi₃O₂S₃ is found to be sensitive to the number of Bi₂OS₂-like stacking faults; fewer faults correlate with increases in the Meissner shielding fractions and <i>T</i>_c. Elucidation of the electronic consequences of these stacking faults may be key to the understanding of electronic conductivity and superconductivity which occurs in a nominally valence-precise compound