Jf. Meng et al., SOFT OPTICAL PHONONS AND THE STRUCTURE TRANSITION OF BISMUTH TITANATESYSTEM DOPED WITH ND AND SB, Physica. B, Condensed matter, 203(1-2), 1994, pp. 186-192
Raman scattering results for Bi2-xNdxTi4O11 and Bi2-xSbxTi4O11 systems
show that the square of the frequency of the 'soft' phonon changes li
nearly with the Nd and Sb concentration for x less than or equal to 0.
3 and x less than or equal to 0.4 respectively, and that the structure
phase transition occurring at x = 0.3 and 0.4 may be associated with
an instability of the 'soft' phonon. It is shown that changes in the s
hort-range harmonic forces play an important role in the x dependence
of the 'soft' phonon frequency. The phase transition pressure occurrin
g in Bi1.8Nd0.2Ti4O(11) is 3.32 GPa.