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

Sergey Volkov, Rimma Bubnova, Maria Krzhizhanovskaya, Lydia Galafutnik

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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.

Original languageEnglish
Pages (from-to)992-1000
Number of pages9
JournalActa crystallographica Section B, Structural science, crystal engineering and materials
StatePublished - 1 Dec 2020

Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Atomic and Molecular Physics, and Optics
  • Metals and Alloys
  • Materials Chemistry


  • bismuth
  • borate
  • incommensurately modulated structure
  • thermal expansion

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