The Subbromide Bi5Br4 – On the Existence of a Hidden Phase

Article abstract of DOI:10.1002/zaac.201800149

Black and irregularly shaped crystals of the bismuth-rich bromide Bi₅Br₄ were obtained as a by-product of the reaction of CsBr, Bi, and BiBr₃. X-ray diffraction on a single-crystal revealed its orthorhombic structure with the space group Pmmn (no. 59) and lattice parameters a = 1800.0(2) pm, b = 1476.1(1) pm, and c = 924.5(2) pm at 296 K. The structure is composed of Bi₈ ²⁺ and Bi₉ ⁵⁺ polycations and bromidobismuthate(III) anions according to the structured formula Bi₅Br₄ = Bi₂₀Br₁₆ = Bi₈ ²⁺Bi₉ ⁵⁺[BiBr₅]^2–[Bi₂Br₁₁]^5–. Bi₅Br₄ is the bismuth-richest among the bismuth subhalides containing isolated polycations. Extensive differential scanning calorimetry studies indicate that Bi₅Br₄ decomposes at 262 °C, i.e. one degree below the bismuth-rich eutectic at 263 °C. All attempts towards a rational synthesis yielded predominantly the neighboring phases BiBr and Bi₆Br₇.