High-Pressure Polymorphism in Silver Ferrite Delafossite, AgFeO2.

The delafossites are a class of layered metal oxides that are notable for being able to exhibit optical transparency alongside an in-plane electrical conductivity, making them promising platforms for the development of transparent conductive oxides. Pressure-induced polymorphism offers a direct method for altering the electrical and optical properties in this class, and although the copper delafossites have been studied extensively under pressure, the silver delafossites remain only partially studied. We report two new high-pressure polymorphs of silver ferrite delafossite, AgFeO2, that are stabilized above ∼6 and ∼14 GPa. In situ X-ray diffraction and vibrational spectroscopy measurements are used to examine the structural changes across the two phase transitions. The high-pressure structure between 6 and 14 GPa is assigned as a monoclinic C2/c structure that is analogous to the high-pressure phase reported for AgGaO2. Nuclear resonant forward scattering reveals no change in the spin state or valence state at the Fe3+ site up to 15.3(5) GPa.

Pressure-Modulated Anomalous Organic-Inorganic Interactions Enhance Structural Distortion and Second-Harmonic Generation in MHyPbBr3 Perovskite.

Organic-inorganic halide perovskites possess unique electronic configurations and high structural tunability, rendering them promising for photovoltaic and optoelectronic applications. Despite significant progress in optimizing the structural characteristics of the organic cations and inorganic framework, the role of organic-inorganic interactions in determining the structural and optical properties has long been underappreciated and remains unclear. Here, by employing pressure tuning, we realize continuous regulation of organic-inorganic interactions in a lead halide perovskite, MHyPbBr3 (MHy+ = methylhydrazinium, CH3NH2NH2+). Compression enhances the organic-inorganic interactions by strengthening the Pb-N coordinate bonding and N-H···Br hydrogen bonding, which results in a higher structural distortion in the inorganic framework. Consequently, the second-harmonic-generation (SHG) intensity experiences an 18-fold increase at 1.5 GPa, and the order-disorder phase transition temperature of MHyPbBr3 increases from 408 K under ambient pressure to 454 K at the industrially achievable level of 0.5 GPa. Further compression triggers a sudden non-centrosymmetric to centrosymmetric phase transition, accompanied by an anomalous bandgap increase by 0.44 eV, which stands as the largest boost in all known halide perovskites. Our findings shed light on the intricate correlations among organic-inorganic interactions, octahedral distortion, and SHG properties and, more broadly, provide valuable insights into structural design and property optimization through cation engineering of halide perovskites.

Dehydration kinetics of nanoconfined water in beryl probed by high temperature single crystal synchrotron X-ray diffraction.

Understanding changes in material properties through external stimuli plays a key role in validating the expected performance of materials and engineering material properties in a controlled manner. Here, we introduce a fundamental protocol to deduce dehydration reactions kinetics of water confined in nanopore channels, with the cyclosilicate beryl as the scaffold of interest, using time-resolved synchrotron X-ray diffraction (SXRD), in the temperature interval of 298-1038 K. The temperature-dependent intensity ( I ) of the strongest reflection (112) was used as the crystallite variable. An estimation of an isobaric thermal crystallite coefficient, k , analogous with the isobaric thermal expansion coefficient, established the rate of relative crystallization as a function of temperature, ∂ I ∂ T . A plot of lnk and 1 T gives rise to two kinetic steps, indicating a slow dehydration stage up to ~ 700 K and a fast dehydration stage up to the investigated temperature 1038 K. The crystal structure of beryl determined up to 1038 K, in temperature increment as small as 10 K, indicates the presence of channel ions Na and Fe and a gradual decrease of water upon heating.

Room temperature facile synthesis of olivine-Co2SiO4 nanoparticles utilizing a mechanochemical method.

Co2SiO4 is a ceramic pigment and promising battery material of significant technological interest, as well as a model end-member of one of the most important mineral families in the Earth's crust and upper mantle. All previously developed methods for synthesis of Co2SiO4 require high-temperature processing, which promotes grain growth, while the nanocrystalline form is required for some important technological applications. Here, we report a successful method for synthesizing nanocrystalline Co2SiO4 via a simple and inexpensive high-energy ball milling mechanochemical process. Products of the synthesis were characterized by a combination of XRD and TEM, and their crystal structures and elemental compositions are reported.

Fe4(OAc)10[EMIM]2: Novel Iron-Based Acetate EMIM Ionic Compound.

We synthesized and characterized a novel iron(II) aceto EMIM coordination compound, which has a simplified empirical formula Fe4(OAc)10[EMIM]2, in two different hydration forms: as anhydrous monoclinic compound and triclinic dihydrate Fe4(OAc)10[EMIM]2·2H2O. The dihydrate compound is isostructural with recently reported Mn4(OAc)10[EMIM]2·2H2O, while the anhydrate is a superstructure of the Mn counterpart, suggesting the existence of solid solutions. Both new Fe compounds contain chains of Fe2+ octahedrally coordinated exclusively by acetate groups. The EMIM moieties do not interact directly with the Fe2+ and contribute to the structural framework of the compound through van der Waals forces and C-H···O hydrogen bonds with the acetate anions. The compounds have a melting temperature of ∼94 °C; therefore, they can be considered metal-containing ionic liquids. Differential thermal analysis indicates three endothermic transitions associated with melting, structural rearrangement in the molten state at about 157 °C, and finally, thermal decomposition of the Fe4(OAc)10[EMIM]2. Thermogravimetric analyses indicate an ∼72 wt % mass loss during the decomposition at 280-325 °C. The Fe4(OAc)10[EMIM]2 compounds have higher thermal stability than their Mn counterparts and [EMIM][OAc] but lower compared to iron(II) acetate. Temperature-programmed desorption coupled with mass spectrometry shows that the decomposition pathway of the Fe4(OAc)10[EMIM]2 involves four distinct regimes with peak temperatures at 88, 200, 267, and 345 °C. The main species observed in the decomposition of the compound are CH3, H2O, N2, CO, OC-CH3, OH-CO, H3C-CO-CH3, and H3C-O-CO-CH3. Variable-temperature infrared vibrational spectroscopy indicates that the phase transition at 160-180 °C is associated with a reorientation of the acetate ions, which may lead to a lower interaction with the [EMIM]+ before the decomposition of the Fe4(OAc)10[EMIM]2 upon further heating. The Fe4(OAc)10[EMIM]2 compounds are porous, plausibly capable of accommodating other types of molecules.