Structural variations of Cr-doped (Y, REE)Al2O3 perovskites

The crystal structure of ten different Y, REE (REE: rare earth elements) perovskite-type aluminates, doped with Cr, has been investigated by means of synchrotron powder diffraction in order to assess the crystallographic modifications related to varying the lanthanide ions. The simmetry change from orthorhombic (Y, Yb-Sm) to rhomboedral (Nd and Pr) has been confirmed. The anomalous behaviour of the unit cell parameters has been clarified on the basis of changes in coordination number of the site containing Y and lanthanide ion: this varies from aproximately eight-fold in Yb-, er-, Ho-, Y and Dy- orthoaluminates to nine fold (or larger) in Gd-, Eu-, and Sm-orthoaluminates to twelve fold in rhomboedral 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 tha average bond distance of the Y, REE site and that of the octaedral Al (and Cr) hostin site has been observed.