Structural phase transitions in BaV6O11

BaV₆O₁₁ was synthesized under high pressures and crystallizes in a structure closely related to magnetoplumbite. V(1)O₆]-octahedra share common edges and form a Kagomé lattice normal to the hexagonal 0 0 1] direction. The layers are connected in the direction of c via trigonal V(3)O₅]-bipyramids and V(2)O₆]-octahedra, which share common faces. The Ba-atoms are incorporated into cavities of the vanadium oxide framework and are coordinated by 12 oxygen atoms in the shape of a dodecahedron.Three magnetic anomalies at approximately 250, 115 and 75 K were detected in this compound. All of them are accompanied by anomalies in the specific heat measurement. To characterize possible structural transitions and determine the response of the structure to the magnetic anomalies, single crystal X-ray diffraction studies were carried out in the temperature range from 293 to 80 K. At 250 K the compound undergoes a structural phase transition. The space group above the transition temperature is P6₃/mmc, at lower temperature the symmetry reduces to P6₃mc. For the refinements in P6₃mc an inversion twin model was used, this way accounting for the loss of the center of symmetry. The structural phase transition is characterized by a small displacement of the V(1)-atom (forming the Kagomé lattice) out of its central position in the octahedra. As a consequence part of the octahedral edges/angles are increased, while the opposite ones are decreased. One limiting surface of the octahedral sheet is corrugated, while the other one is smoothened with respect to the high-temperature structure. This deformation of the octahedral sheets leads to the corresponding geometrical changes in the other coordination polyhedra.The structural response to the magnetic anomaly at 115 K is weak and mainly observable in the geometric parameters concerning the V(1)O₆]-octahedra and V(3)O₅]-bipyramids. This may serve as a first indication that the corresponding central atoms play an important role in the mechanism of the magnetic phase transition.