Prediction and Verification of the Structural Chemistry of New One-Dimensional Barium/Copper/Iridium Oxides

We propose a classification scheme for the commensurate phases in the family of pseudo one-dimensional structures formed from the mixed hcp stacking of A 3 O 9 and oxygen-deficient A 3 A‘O 6 layers. We envisage these compounds as composites of two substructures having common a and b unit cell parameters but different parameters c 1 and c 2 . Use of the ratio c 1 / c 2 facilitates the prediction of new commensurate structures while allowing for the commonly incommensurate nature of materials in this family. The structures of the new commensurate phases Ba 5 CuIr 3 O 12, Ba 14 Cu 3 Ir 8 O 33, Ba 16 Cu 3 Ir 10 O 39, and Ba 9 Cu 2 Ir 5 O 21 are predicted and subsequently verified by powder X-ray diffraction and HRTEM. Ba 5 CuIr 3 O 12 has a 10 layer structure with space group P 321, a = 10.143 82(8) Å, c = 21.6553(2) Å; Ba 14 Cu 3 Ir 8 O 33 has a 14 layer structure with space group P 321, a = 10.145 85(8) Å, c = 29.9574(3) Å; Ba 16 Cu 3 Ir 10 O 39 has a 16 layer structure with space group P 321, a = 10.136 43(7) Å; c = 35.0616(3) Å; Ba 9 Cu 2 Ir 5 O 21 has an 18 layer structure with space group R 32, a = 10.144 64(11) Å, c = 38.2455(6) Å. Sequences of trigonal prismatic sites and octahedral sites run in chains parallel to z, with Ba cations located between the chains; the distribution of iridium and copper cations in the octahedral and trigonal prismatic sites is disordered in each case. Electron diffraction patterns and lattice images show evidence for modulation in the structures of Ba 5 CuIr 3 O 12, Ba 14 Cu 3 Ir 8 O 33, and Ba 16 Cu 3 Ir 10 O 39 but not in that of Ba 9 Cu 2 Ir 5 O 21 . The magnetic susceptibilities of all four phases obey a modified Curie−Weiss law above 150 K, with no long-range magnetic order observed between 5 and 300 K. They are all electrical insulators.