Cation distribution and structural instability in Bi_4-xLa_xTi₃O₁₂

Raman spectra of Bi_4-xLa_xTi₃O₁₂ have been investigated. For an increasing degree of La doping, we find a substantial hardening of the vibration involving Bi atoms at the perovskite A site, whereas the Bi mode at the Bi₂O₂ layer is negligibly changed. From a comparison with a simple mass consideration, we identify a precise cation distribution, which indicates a pronounced site selectivity of La ions for the A site for x < ∼ 1. At room temperature, the lowest mode at 28 cm^-1 shows a significant softening with La doping. This behaviour resembles the temperature dependence in the nondoped material (at constant composition). A remarkable correlation is found between the soft-mode frequency and T_C for Bi_4-xLa_xTi₃O₁₂. We suggest that this correlation, as well as the soft-mode frequency for substituted materials, offers useful guidelines for designing materials in a controlled manner.