Single-Crystal X-ray Study of Ba2Cu2Te4O11Br2 and Its Incommensurately Modulated Superstructure Companion

• Compounds containing lone-pair elements such as TeIV are very interesting from the structural point of view, as the lone-pair nonbonding regions create low-dimensional geometrical arrangements. We have synthesized two new compounds with these features—Ba2Cu2Te4O11Br2 (I) and Ba2Cu2Te4O11−δ(OH)2δBr2 (II, δ≈0.57)—as members of the AE-M-Te-O-X (AE=alkaline-earth metal, M=transition metal, X=halide) family of compounds by solid-state reactions. Preliminary single-crystal X-ray analysis indicated that compound I crystallizes in the orthorhombic system, but attempts at refinement proved unsatisfactory. Closer inspection of the reciprocal lattice revealed systematic, non-crystallographic absences that indicate twinning. The structure is in fact triclinic, space group C (equivalent to P), with unit cell parameters (at 120 K) of a=10.9027(9), b=15.0864(7), c=9.379(2) Å, β=106.8947°. It is layered and built from [TeO3E] tetrahedra, [TeO3+1E] trigonal bipyramids (where E is the lone pair of TeIV), [CuO4] squares and irregular [BaO10Br] polyhedra. The crystal structure of II shows the same basic structure as I but contains additional oxygen, probably in the form of OH groups. The presence of satellites reveals that ordering on this O site creates an incommensurate modulation, primarily affecting Br and Te. The modulated structure of II was solved in the triclinic superspace group X(αβγ)0 with the vector q≈1/16 c*.

Ämnesord

MEDICIN OCH HÄLSOVETENSKAP  -- Medicinska och farmaceutiska grundvetenskaper -- Andra medicinska och farmaceutiska grundvetenskaper (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Basic Medicine -- Other Basic Medicine (hsv//eng)

Nyckelord

alkaline earth metals

layered compounds

solid-state reactions

structure elucidation

tellurium

Chemistry

Kemi

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