Cation distribution and structural instability in Bi4-xLaxTi3O12

Raman spectra of Bi4-xLaxTi3O12 have been investigated. For an increasing d egree of La doping, we, find a substantial hardening of the vibration invol ving Bi atoms at the perovskite A site, whereas the Bi mode at the Bi2O2 la yer is negligibly changed. From a comparison with a simple mass considerati on, we identify a precise cation distribution, which indicates a pronounced site selectivity of La ions for the A site for x < similar to 1. At room t emperature, the lowest mode at 28 cm(-1) shows a significant softening with La doping. This behavior resembles the temperature dependence in the nondo ped material (at constant composition). A remarkable correlation is found b etween the soft-mode frequency and Tc for Bi4-xLaxTi3O12. We suggest that t his, correlation, as well as the soft-mode frequency for substituted materi als, offers useful guidelines for designing materials in a controlled manne r.