Single‐Crystal X‐ray Study of Ba 2 Cu 2 Te 4 O 11 Br 2 and Its Incommensurately Modulated Superstructure Companion

Compounds containing lone‐pair elements such as Te IV 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—Ba 2 Cu 2 Te 4 O 11 Br 2 ( I ) and Ba 2 Cu 2 Te 4 O 11− δ (OH) 2 δ Br 2 ( 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 $\bar 1$ (equivalent to P $\bar 1$ ), 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 [TeO 3 E] tetrahedra, [TeO 3+1 E] trigonal bipyramids (where E is the lone pair of Te IV ), [CuO 4 ] squares and irregular [BaO 10 Br] 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 $\bar 1$ ( αβγ )0 with the vector q ≈1/16 c *.