The first bismuth borate oxyiodide, Bi4BO7I: commensurate or incommensurate?

Abstract

The first bismuth borate oxyiodide, Bi4BO7I, has been prepared by solid-state reaction in evacuated silica ampoules. Its crystal structure [space group Immm(00γ)000] comprises litharge-related layers of edge-sharing OBi4 tetrahedra; the interlayer space is filled by I− and [BO3]3− anions. The wavevector, q = 0.242 (3)c*, is very close to the rational value of c*/4, yet refinement based on commensurate modulation faces serious problems indicating the incommensurate nature of the modulation. The I-/[BO3]3− anions are ordered in a complex sequence along [001], i.e. –<–BO3–BO3–I–I–> n = 28–I–I–I–<–BO3–BO3–I–I–> n = 28–BO3–BO3–BO3–, leading to a structural modulation. The principal feature of the latter is the presence of –I–I–I– and –BO3–BO3–BO3– sequences that cannot be accounted for in the a × b × 4c supercell. The thermal expansion of Bi4BO7I is weakly anisotropic (αa  = 8, αb  = 15 and αc  = 17 × 10−6 K−1 at 500 K) which is caused by preferential orientation of the borate groups.