Volkov Sergey, Bubnova Rimma, Krzhizhanovskaya Maria, Galafutnik Lydia
Grebenshchikov Institute of Silicate Chemistry, Makarov Emb, Saint Petersburg, 199053, Russian Federation.
Department of Crystallography, Saint Petersburg State University, University Emb., 7/9, Saint Petersburg, 199034, Russian Federation.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2020 Dec 1;76(Pt 6):992-1000. doi: 10.1107/S2052520620012640. Epub 2020 Nov 10.
The first bismuth borate oxyiodide, BiBOI, 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 OBi tetrahedra; the interlayer space is filled by I and [BO] 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-/[BO] anions are ordered in a complex sequence along [001], i.e. -<-BO-BO-I-I->-I-I-I-<-BO-BO-I-I->-BO-BO-BO-, leading to a structural modulation. The principal feature of the latter is the presence of -I-I-I- and -BO-BO-BO- sequences that cannot be accounted for in the a × b × 4c supercell. The thermal expansion of BiBOI is weakly anisotropic (α = 8, α = 15 and α = 17 × 10 K at 500 K) which is caused by preferential orientation of the borate groups.
