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1. 3D Lead‐Organoselenide‐Halide Perovskites and their Mixed‐Chalcogenide and Mixed‐Halide Alloys

   https://doi.org/10.1002/anie.202408443  

   Li, Jiayi; Wang, Yang; Saha, Santanu; Chen, Zhihengyu; Hofmann, Jan; Misleh, Jason; Chapman, Karena W; Reimer, Jeffrey A; Filip, Marina R; Karunadasa, Hemamala I (October 2024, Angewandte Chemie International Edition)

   Abstract We incorporate Se into the 3D halide perovskite framework using the zwitterionic ligand: SeCYS (⁺NH₃(CH₂)₂Se⁻), which occupies both the X⁻and A⁺sites in the prototypical ABX₃perovskite. The new organoselenide‐halide perovskites: (SeCYS)PbX₂(X=Cl, Br) expand upon the recently discovered organosulfide‐halide perovskites. Single‐crystal X‐ray diffraction and pair distribution function analysis reveal the average structures of the organoselenide‐halide perovskites, whereas the local lead coordination environments and their distributions were probed through solid‐state⁷⁷Se and²⁰⁷Pb NMR, complemented by theoretical simulations. Density functional theory calculations illustrate that the band structures of (SeCYS)PbX₂largely resemble those of their S analogs, with similar band dispersion patterns, yet with a considerable band gap decrease. Optical absorbance measurements indeed show band gaps of 2.07 and 1.86 eV for (SeCYS)PbX₂with X=Cl and Br, respectively. We further demonstrate routes to alloying the halides (Cl, Br) and chalcogenides (S, Se) continuously tuning the band gap from 1.86 to 2.31 eV–straddling the ideal range for tandem solar cells or visible‐light photocatalysis. The comprehensive description of the average and local structures, and how they can fine‐tune the band gap and potential trap states, respectively, establishes the foundation for understanding this new perovskite family, which combines solid‐state and organo‐main‐group chemistry. 

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2. Hybrid Organic–Inorganic Halide Derivatives of the 2H Hexagonal Perovskite Structure

   https://doi.org/10.1021/acs.chemmater.2c00688  

   Holzapfel, Noah P.; Milder, Alexander; Woodward, Patrick M. (September 2022, Chemistry of Materials)

   Four quaternary hybrid halide perovskites have been synthesized in hydrohalic acid solutions under hydrothermal conditions. The structures of (CH3NH3)2AgRhX6 and (CH3NH3)2NaRhX6, (X = Cl–, Br–) consist of infinite one-dimensional chains of face-sharing metal-halide octahedra. The structure is closely related to the 2H hexagonal perovskite structure, but the space group symmetry is lowered from hexagonal P63/mmc to trigonal P3 ̅m1 by site ordering of the Rh3+ and Ag+/Na+ cations. All compositions demonstrate broad-spectrum visible light absorption with optical transitions arising from rhodium d-to-d transitions and halide-to-rhodium charge transfer transitions. The bromides show a 0.2 eV red shift in the optical transitions compared to the analogous chlorides. Crystal field splitting energies were found to be 2.6 eV and 2.4 eV for the chloride and bromide compositions, respectively. Band structure calculations for all compositions give rather flat valence and conduction bands, suggesting a zero-dimensional electronic structure. The valence bands are made up of crystal orbitals that are almost exclusively Rh 4d–Cl 3p (Br 4p) π* in character, while the conduction bands have Rh 4d–Cl 3p (Br 4p) σ* character. 

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3. Tin Oxynitride-Based Ferroelectric Semiconductors for Solar Energy Conversion Applications

   https://doi.org/10.1021/acs.chemmater.0c02439  

   Hartman, Steven T.; Thind, Arashdeep S.; Mishra, Rohan (November 2020, Chemistry of Materials)

   null (Ed.)

   Lead halide perovskites have emerged as a promising class of semiconductors; however, they suffer from issues related to lead toxicity and instability. We report results of a first-principles-based design of heavy-metal-based oxynitrides as alternatives to lead halide perovskites. We have used density functional theory calculations to search a vast composition space of ABO2N and ABON2 compounds, where B is a p-block cation and A is an alkaline, alkali-earth, rare-earth, or transition metal cation, and identify 10 new ABO2N oxynitride semiconductors that we expect to be formable. Specifically, we discover a new family of ferroelectric semiconductors with A3+SnO2N stoichiometry (A = Y, Eu, La, In, and Sc) in the LuMnO3-type structure, which combine the strong bonding of metal oxides with the low carrier effective mass and small, tunable band gaps of the lead halide perovskites. These tin oxynitrides have predicted direct band gaps ranging from 1.6 to 3.3 eV and a sizable electric polarization up to 17 μC/cm2, which is predicted to be switchable by an external electric field through a nonpolar phase. With their unique combination of polarization, low carrier effective mass, and band gaps spanning the entire visible spectrum, we expect ASnO2N ferroelectric semiconductors will find useful applications as photovoltaics and photocatalysts as well as for optoelectronics. 

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4. High-pressure characterization of Ag3AuTe2: Implications for strain-induced band tuning

   https://doi.org/10.1063/5.0223472  

   Won, Juyeon; Zhang, Rong; Peng, Cheng; Kumar, Ravhi; Gebre, Mebatsion S; Popov, Dmitry; Hemley, Russell J; Bradlyn, Barry; Devereaux, Thomas P; Shoemaker, Daniel P (November 2024, Applied Physics Letters)

   Recent band structure calculations have suggested the potential for band tuning in the chiral semiconductor Ag3AuTe2 to zero upon application of negative strain. In this study, we report on the synthesis of polycrystalline Ag3AuTe2 and investigate its transport and optical properties and mechanical compressibility. Transport measurements reveal the semiconducting behavior of Ag3AuTe2 with high resistivity and an activation energy Ea of 0.2 eV. The optical bandgap determined by diffuse reflectance measurements is about three times wider than the experimental Ea. Despite the difference, both experimental gaps fall within the range of predicted bandgaps by our first-principles density functional theory (DFT) calculations employing the Perdew–Burke–Ernzerhof and modified Becke–Johnson methods. Furthermore, our DFT simulations predict a progressive narrowing of the bandgap under compressive strain, with a full closure expected at a strain of −4% relative to the lattice parameter. To evaluate the feasibility of gap tunability at such substantial strain, the high-pressure behavior of Ag3AuTe2 was investigated by in situ high-pressure x-ray diffraction up to 47 GPa. Mechanical compression beyond 4% resulted in a pressure-induced structural transformation, indicating the possibility of substantial gap modulation under extreme compression conditions. 

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5. Determination of adsorption-controlled growth windows of chalcogenide perovskites

   https://doi.org/10.1557/mrc.2018.10  

   Filippone, Stephen A.; Sun, Yi-Yang; Jaramillo, R. (March 2018, MRS Communications)

   Ternary sulfides and selenides in the distorted-perovskite structure (“chalcogenide perovskites”) are predicted by theory to be semiconductors with a band gap in the visible-to-infrared and may be useful for optical, electronic, and energy conversion technologies. Here we use computational thermodynamics to predict the pressure–temperature phase diagrams for select chalcogenide perovskites. Our calculations incorporate formation energies calculated by density functional theory, and empirical estimates of heat capacities. We highlight the windows of thermodynamic equilibrium between solid chalcogenide perovskites and the vapor phase at high temperature and very low pressure. These results can guide the adsorption-limited growth of ternary chalcogenides by molecular beam epitaxy. 

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