The effects of isovalent and non-isovalent impurities on the ferroelectric phase transition in barium titanate

X-ray scattering expts. on lightly-doped powders of BaTiO3 were extended to study transitions in materials doped with cations of lower valency than the ions they replace. These materials are insulators with a resistivity of ∼1010 Ω cm at room temp.; 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 102 Ω cm at room temp., and so the impurities are screened. Surprisingly the behavior of the 2 types is very similar: both types of impurity lower the transition temp., even though they favor the formation of the low-temp. phase. The presence of small clusters of polarized material appears to inhibit the phonon softening, as 1st proposed by Halperin and Varma. The authors have studied the phase transition in BaTiO3-rich solid soln.; the crit. behavior in these materials is very different from that of the impure systems. [on SciFinder(R)]