THE EFFECTS OF ISOVALENT AND NON-ISOVALENT IMPURITIES ON THE FERROELECTRIC PHASE-TRANSITION IN BARIUM-TITANATE

X-ray scattering experiments on lightly-doped powders of BaTiO3 using the diffractometer on station 9.1 of the SRS at Daresbury have been extended to investigate transitions in materials doped with cations of lower valency than the ions they replace. These materials are insulators with a resistivity of about 10(10) omega cm at room temperature; the defect sites are therefore unscreened. Materials studied earlier had been doped with impurities of higher valency than the cations they replace: these are extrinsic n-type semiconductors with resistivity typically 10(2) OMEGA cm at room temperature, and so the impurities are screened. Surprisingly the behaviour of the two types is very similar: both types of impurity lower the transition temperature, even though they favour the formation of the low-temperature phase. The presence of small clusters of polarized material appears to inhibit the phonon softening, as first proposed by Halperin and Varma. We have also studied the phase transition in BaTiO3-rich solid solutions; the critical behaviour in these materials is very different from that of the impure systems.