Tetragonal-Orthorhombic Phase Transition in Barium Titanate via Monoclinic MCType Symmetry

The transition of barium titanate crystals from the tetragonal to the orthorhombic phase has been investigated by XRD measurements during slow cooling. Additional diffuse scattering intensity between the 002 and 200 peaks develops with decreasing temperature and with time. This can be understood by assuming that this order-disorder phase transition is initiated by increasing monoclinic short-range order (SRO) of the only partly ordered Ti ions within the tetragonal domains. This leads to rearranging the Ba and O ions towards the orthorhombic symmetry analogous to the processes initiated by tetragonal SRO below the Burns temperature above the cubic-tetragonal transition. This results, finally, in monoclinic nucleation and transition to the orthorhombic phase. The last step can obviously be impeded by compositional disorder as in Ba(Ti,Sn)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 et al. with higher concentration of “inactive” B ions (Sn, Mg et al.). The reason of the high electromechanical response of monoclinic “phases” of perovskite type ferroelectrics is probably field-induced transformation of nuclei towards long-range order.