Uniaxial birefringence in “cubic” Pb(Fe0.5Ta0.5)O₃: Is there a hidden symmetry breaking?

The room temperature phase of the B-site disordered perovskite Pb(Fe0.5Ta0.5)O3 (PFT) has been studied by single crystal X-ray diffraction and by polarized light microscopy. While pyramidal (100) growth sectors display a weak birefringence, indicating a tetragonal symmetry with an optically uniaxial positive indicatrix, the structure of PFT crystals can be well refined in cubic space group Pm3m, however, the introduction of disorder on the A site is necessary. In the light of the optical symmetry, the (x100) A-site disordered cubic structure was considered as a mean structure averaged over tetragonal “domains,” each one containing Pb displaced just towards one of the three [100]cuh equivalent directions. Attempts at refinements in tetragonal symmetry and at mechanical (ferroelastic) switching of domains will be discussed.