Infrared and Raman responses in ferroelectric perovskite crystals: Apparent inconsistencies

Infrared and Raman spectra in the ferroelectric phase of BaTi${\mathrm{O}}_3$ are compared. Inconsistencies about mode coupling are discussed. The mode-coupling formalism has been successfully used by several authors to explain the Raman data but the calculation of an infrared spectrum based on Raman parameters did not systematically agree with the measurement. The factorized form of the dielectric function is shown to fit reflectivity data in BaTi${\mathrm{O}}_3$ and also KNb${\mathrm{O}}_3$.