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1. Small‐Band‐Gap Halide Double Perovskites

   https://doi.org/10.1002/anie.201807421  

   Slavney, Adam H.; Leppert, Linn; Saldivar Valdes, Abraham; Bartesaghi, Davide; Savenije, Tom J.; Neaton, Jeffrey B.; Karunadasa, Hemamala I. (August 2018, Angewandte Chemie International Edition)

   Abstract Despite their compositional versatility, most halide double perovskites feature large band gaps. Herein, we describe a strategy for achieving small band gaps in this family of materials. The new double perovskites Cs₂AgTlX₆(X=Cl (1) and Br (2)) have direct band gaps of 2.0 and 0.95 eV, respectively, which are approximately 1 eV lower than those of analogous perovskites. To our knowledge, compound2displays the lowest band gap for any known halide perovskite. Unlike in A^IB^IIX₃perovskites, the band‐gap transition in A^I₂BB′X₆double perovskites can show substantial metal‐to‐metal charge‐transfer character. This band‐edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B‐ and B′‐site metal frontier orbitals. Calculations reveal a shallow, symmetry‐forbidden region at the band edges for1, which results in long (μs) microwave conductivity lifetimes. We further describe a facile self‐doping reaction in2through Br₂loss at ambient conditions. 

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2. 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 (September 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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3. 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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4. Mosaic Cu ^I −Cu ^II −In ^III 2D Perovskites: Pressure‐Dependence of the Intervalence Charge Transfer and a Mechanochemical Alloying Method

   https://doi.org/10.1002/anie.202300957  

   Li, Jiayi; Matheu, Roc; Ke, Feng; Liu, Zhenxian; Lin, Yu; Karunadasa, Hemamala I. (April 2023, Angewandte Chemie International Edition)

   Abstract The perovskite (BA)₄[Cu^II(Cu^IIn^III)_0.5]Cl₈(1_BA; BA⁺=butylammonium) allows us to study the high‐pressure structural, optical, and transport properties of a mixed‐valence 2D perovskite. Compressing1_BAreduces the onset energy of Cu^I/IIintervalence charge transfer from 1.2 eV at ambient pressure to 0.2 eV at 21 GPa. The electronic conductivity of1_BAincreases by 4 orders of magnitude upon compression to 20 GPa, when the activation energy for conduction decreases to 0.16 eV. In contrast, Cu^IIperovskites achieve similar conductivity at ≈50 GPa. The solution‐state synthesis of these perovskites is complicated, with more undesirable side products likely from the precursor mixtures containing three different metal ions. To circumvent this problem, we demonstrate an efficient mechanochemical synthesis to expand this family of halide perovskites with complex composition by simply pulverizing together powders of 2D Cu^IIsingle perovskites and Cu^IIn^IIIdouble perovskites. 

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5. 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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