STRUCTURAL STUDIES OF THE T-STAR-PHASES (LA, TB, PB)2CUO4, (LA, TB, SR)2CUO4 - STRUCTURAL TRANSITION IN (LA, TB, PB)2CUO4

The structure of the T* phase compound (La, Tb, Pb)2CuO4 has been investigated by single crystal X-ray diffraction as a function of temperature. It is shown that a structural transition exists around ≈ 470 K below which the structure is orthorhombic with cell parameters a=b=5.451 (2) A ̊ and c=12.462 A ̊ at room temperature. Annealing at temperatures above the transition favors the long range ordering of the superstructure. The tetragonal structure of the high temperature phase (space group P4/ nmm) is similar to that determined by Izumi et al. [3] for the (Nd, Sr, Ce)2CuO4 T*-compound. This is also the case for the structure of (La, Tb, Sr)2CuO4 determined at room temperature by refinement of neutron powder diffraction data. The comparison of the three structures indicates that the disorder of the O(2) apical oxygen atoms is due to a dynamically disordered canting of the CuO5 pyramids rather than to cation ordering on the 9-coordinated site. The structural transition is due to the ordering of this canting and is believed to be a general feature of the T*-phase compounds. © 1990.