SOFT OPTICAL PHONONS AND THE STRUCTURE TRANSITION OF BISMUTH TITANATESYSTEM DOPED WITH ND AND SB

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.