Raman scattering from soft modes in barium‐doped lead germanate

Abstract

AbstractThe Raman spectrum of the soft mode and adjacent low‐frequency A TO modes in pure and barium‐doped lead germanate was measured as a function of temperature, T, through the ferroelectric‐paraelectric phase transition. The barium concentrations studied varied from 2.3 to 6.6%. The Raman spectra were analysed for all the temperatures and dopings by least‐squares fitting of a model response function. In general, the phonon line widths increase with the amount of barium doping whereas the soft frequency decreases. Both the soft optic frequency and the mode strength display Curie‐Weiss behaviour close to the Curie temperature, Tc. When they are plotted as a function of T c‐T, the various fitted soft‐mode parameters generally behave in a qualitatively similar fashion for the different specimens. However, on a quantitative basis, the agreement is less good, showing that the behaviour of the soft mode does not scale simply as Tc‐T. This may indicate that the influence of the different polarizabilities of the barium defects is important in lead germanate.