Suppression of Sequential Charge Transitions in Ca_0.5Bi_0.5FeO₃ via B‑Site Cobalt Substitution

The perovskite Ca_0.5Bi_0.5FeO₃ containing unusually high-valent Fe^3.5+ undergoes sequentially charge disproportionation (CD) of the Fe centers and intersite charge transfer (CT) between Bi and Fe. From structural, magnetic, and transport property characterization, we found that substitution of Co for Fe occurs isovalently to form Ca_0.5­Bi³⁺_0.5­(Fe_1–<i>x</i>­Co_<i>x</i>)^3.5+­O₃ and destabilizes the CD state. This results in materials exhibiting only intermetallic charge transfer behavior in the region 0.01 < <i>x</i> < 0.67. The CT transitions for these materials only involve Fe^3.5+, whereas Co remains in the 3.5+ oxidation state at all temperatures. The doped Co^3.5+ ions give Pauli-paramagnetic like conducting behavior. The Co-substitution effect is very different from that observed in Ca­Fe_1–<i>x</i>­Co_<i>x</i>O₃

Elucidating the Methylammonium (MA) Conformation in MAPbBr₃ Perovskite with Application in Solar Cells

Hybrid organic–inorganic perovskites, MAPbX₃ (X = halogen), containing methylammonium (MA: CH₃–NH₃⁺) in the large voids conformed by the PbX₆ octahedral network, are the active absorption materials in the new generation of solar cells. CH₃NH₃PbBr₃ is a promising member with a large band gap that gives rise to a high open circuit voltage. A deep knowledge of the crystal structure and, in particular, the MA conformation inside the perovskite cage across the phase transitions undergone below room temperature, seems essential to establish structure–property correlations that may drive to further improvements. The presence of protons requires the use of neutrons, combined with synchrotron XRD data that help to depict subtle symmetry changes undergone upon cooling. We present a consistent picture of the structural features of this fascinating material, in complement with photocurrent measurements from a photodetector device, demonstrating the potential of MAPbBr₃ in optoelectronics