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Raman spectra of Bi 4- x La x Ti 3 O 12 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 Bi 2 O 2 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 behavior 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- x La x Ti 3 O 12 . 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.
Osada et al. (Sat,) studied this question.