Synthesis, crystal chemistry, and optical properties of two methylammonium silver halides: CH3NH3AgBr2 and CH3NH3Ag2I3

Abstract

Two novel ternary compounds from the pseudobinary CH₃NH₃X–AgX (X = Br, I) phase diagrams are reported. CH₃NH₃AgBr₂ and CH₃NH₃Ag₂I₃ were synthesized via solid state sealed tube reactions and the crystal structures were determined through a combination of single crystal and synchrotron X-ray powder diffraction. Structurally, both compounds consist of one-dimensional ribbons built from silver-centered tetrahedra. The structure of CH₃NH₃AgBr₂ possesses orthorhombic Pnma symmetry and is made up of zig-zag chains where each silver bromide tetrahedron shares two edges with neighboring tetrahedra. The tetrahedral coordination of silver is retained in CH₃NH₃Ag₂I₃, which has monoclinic P2₁/m symmetry, but the change in stoichiometry leads to a greater degree of edge-sharing connectivity within the silver iodide chains. With band gaps of 3.3 eV (CH₃NH₃Ag₂I₃) and 4.0 eV (CH₃NH₃AgBr₂) the absorption onsets of the ternary phases are significantly blue shifted from the binary silver halides, AgBr and AgI, due in part to the decrease in electronic dimensionality. The compounds are stable for at least one month under ambient conditions and are thermally stable up to approximately 200 °C. Density functional theory calculations reveal very narrow valence bands and moderately disperse conduction bands with Ag 5s character. Bond valence calculations are used to analyze the hydrogen bonding between methylammonium cations and coordinatively unsaturated halide ions. The crystal chemistry of these compounds helps to explain the dearth of iodide double perovskites in the literature.