Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite Cs3Bi2Br9, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr2 to the solution-phase synthesis of Cs3Bi2Br9 leads to substitution of up to 7% of the Bi(iii) ions by equal quantities of Sn(ii) and Sn(iv). The nature of the substitutional defects was studied by X-ray diffraction, 133Cs and 119Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(ii) and Sn(iv) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(ii) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(ii) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure-property relationships of lead-free vacancy-ordered perovskite structures.
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The authors thank Dr Christopher N. Savory (University College London) for useful discussions regarding the ground-state properties of Cs3Bi2Br9. SRK acknowledges the EPSRC Centre for Doctoral Training in the Advanced Characterisation of Materials (CDT-ACM)(EP/S023259/1) for funding a PhD studentship. AW acknowledges support from a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (2018R1C1B6008728). KD acknowledges the support of the Cambridge Trust in the form of Cambridge India Ramanujan Scholarship. DOS acknowledges support from the EPSRC (EP/N01572X/1) and from the European Research Council, ERC (Grant No. 758345). We acknowledge the EPSRC National Facility for Photoelectron Spectroscopy (HarwellXPS), operated by Cardiff University and UCL under contract number PR16195. We acknowledge the use of the UCL Kathleen High Performance Computing Facility (Kathleen@UCL), the Imperial College Research Computing Service, and associated support services, in the completion of this work. Via membership of the UK's HEC Materials Chemistry Consortium, which is funded by the EPSRC (EP/L000202, EP/R029431, EP/T022213), this work used the ARCHER UK National Supercomputing Service (www.archer.ac.uk) and the UK Materials and Molecular Modelling (MMM) Hub (Thomas – EP/P020194 & Young – EP/T022213). This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 841136. KG appreciates support from the Polish Ministry of Science and Higher Education within the Mobilnosc Plus program (grant no.1603/MOB/V/2017/0). CPG acknowledges the support from the Royal Society (RP/R1/ 180147) and European Research Council under the European Union's Horizon 2020 research and innovation programme (BATNMR – grant agreement no. 835073). KG and SDS acknowledge the EPSRC (EP/R023980/1) for funding. SDS acknowledges the Royal Society and Tata Group (UF150033). The work has received funding from the European Research Council under the European Union's Horizon 2020 research and innovation programme (HYPERION – grant agreement no. 756962).
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