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 (α_a = 8, α_b = 15 and α_c = 17 × 10^-6 K^-1 at 500 K) which is caused by preferential orientation of the borate groups.

Keywords: bismuth; borate; incommensurately modulated structure; thermal expansion.