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1. Chemical Origin of the Stability Difference between Copper(I)‐ and Silver(I)‐Based Halide Double Perovskites

   https://doi.org/10.1002/ange.201705113  

   Xiao, Zewen ; Du, Ke‐Zhao ; Meng, Weiwei ; Mitzi, David B. ; Yan, Yanfa ( August 2017 , Angewandte Chemie)

   Recently, Cu^I‐ and Ag^I‐based halide double perovskites have been proposed as promising candidates for overcoming the toxicity and instability issues inherent within the emerging Pb‐based halide perovskite absorbers. However, up to date, only Ag^I‐based halide double perovskites have been experimentally synthesized; there are no reports on successful synthesis of Cu^I‐based analogues. Here we show that, owing to the much higher energy level for the Cu 3d¹⁰orbitals than for the Ag 4d¹⁰orbitals, Cu^Iatoms energetically favor 4‐fold coordination, forming [CuX₄] tetrahedra (X=halogen), but not 6‐fold coordination as required for [CuX₆] octahedra. In contrast, Ag^Iatoms can have both 6‐ and 4‐fold coordinations. Our density functional theory calculations reveal that the synthesis of Cu^Ihalide double perovskites may instead lead to non‐perovskites containing [CuX₄] tetrahedra, as confirmed by our material synthesis efforts.

    

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2. 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)

   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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3. 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/ange.202300957  

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

   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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4. 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)

   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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5. Small‐Band‐Gap Halide Double Perovskites

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

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

   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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