The complete Raman spectrum of paraelectric and ferroelectric lead germanate

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Abstract
New measurements have been made of the complete Raman spectrum of Pb5Ge3O11 at temperatures above and below the ferroelectric transition at Tc approximately=450K. The data set is of much higher quality than that previously reported. Numerous first-order Raman bands are observed at low temperature consistent with group theoretical predictions. TO-LO splittings of polar modes were investigated and, contrary to other work, were found to be small for most cases. A molecular crystal approximation gives a reasonable description of the spectrum, where modes at higher frequencies can be classified as internal vibrations of GeO44- tetrahedra. Several A-symmetry modes, including the soft mode, exhibit a strong temperature dependence in their intensities in the ferroelectric phase. Their behaviour is consistent with expectations, but there are some E-symmetry modes whose intensities change unusually between 110 and 293K.