Figure 1
(a) Representation of the HTT structure of the Ruddlesden-Popper n = 1 cuprates, packed according to the low-temperature supercell and showing its relationship to the I4/mmm aristotype unit-cell (for LTO and LTT, a = aHTT − bHTT, b = aHTT + bHTT and c = cHTT); A-site cation (e.g., La, Ba,…) in green, copper (B-site) in blue and oxygen in red. (b) and (c) show representations of the LTO structure (A-site cations omitted for clarity) viewed just off (a) and (b), respectively. (d) Presents a similar depiction of the LTT phase viewed just off (a). Note how, in LTO (b, c), the sense of the distortion is such that they propagate solely along (b) and all Cu–O–Cu bonds are buckled while, in the LTT phase (d), distortions propagate along alternating diagonals in adjacent layers, [1 1 0] at z = 0 and [1 −1 0] at z = 0.5 with respect to the super-cell, and so buckle only half the Cu–O–Cu bonds, along orthogonally alternating vectors in each layer. (e) and (f) show the CuO6 octahedra in the ab planes of the LTO and LTT phase, respectively, viewed parallel to (c); black arrows depict the direction in which the tilts occur, i.e., their ‘sense’, and the blue dashed lines the associated ‘buckles’ in the CuO2 planes.
