Structural variations of Cr-doped (YREE) AlO3 perovskites

The crystal structure of ten different Y, REE (REE: rare earth elements) perovskite-type aluminates, doped with chromium, has been investigated by means of synchrotron powder diffraction in order to assess the crystallographic modifications related to varying the lanthanide ions. The symmetry change from orthorhombic (Y, Yb-Sm) to rhombohedral (Nd and Pr) has been confirmed. The anomalous behaviour of the unit cell parameters (in the Pnma setting, b and c contract while a expands as a function of the 'lanthanide contraction') has been clarified on the basis of changes in coordination number of the site containing the Y and lanthanide ion: this varies from approximately eight-fold in Yb-, Er-, Ho-, Y-, and Dyorthoaluminates to nine-fold (or larger) in Gd-, Eu-, and Sm-orthoaluminates to twelve-fold (as in the archetypal cubic structure) in rhombohedral perovskites (Nd and Pr). The coordination number appears about two units greater in orthoaluminates with respect to the corresponding orthoferrites and an inverse correlation between the average bond distance of the Y, REE site and that of the octahedral Al (and Cr) hosting site has been observed.