In this work, a Raman study of powder samples of multiferroic Bi₅Fe₁₊_xTi₃₋_xO₁₅ solid solutions and Bi₆Fe₂Ti₃O₁₈ as a function of temperature from 27 °C (room temperature) to 850 °C is presented. The values of x (i.e., the Fe composition) for the solid solutions were 1.0, 1.1, 1.3, and 1.4. The temperature coefficients of eight phonon frequencies were determined for all the samples. The large observed phonon broadenings with increasing temperature precluded the observation of several of the phonon bands above defined temperatures in the range of 200–700 °C depending on the sample. These phonon broadenings were explained on the basis of the Klemens model, which considers that the broadenings are due to the thermal expansion of the lattice with a major contribution in terms of magnitude from anharmonic phonon–phonon interactions. However, some evidence for the presence of several of the phonons persisted up to 800–850 °C. These solid solutions are expected to exhibit a ferroelectric–paraelectric phase transition at 742 to 750 °C and a ferromagnetic–antiferromagnetic transition at 426 °C. We also observed changes in the slopes of the temperature dependence of the phonon frequencies for the lines at 228 cm⁻¹ for Bi₅FeTi₃O₁₅ and 330 cm⁻¹ for Bi₆Fe₂Ti₃O₁₈ at temperatures of 247 °C and 347 °C, respectively. No similar temperature–frequency slope changes indicative of possible phase transitions were observed for any of the phonon lines of the other three Bi₅Fe₁₊_xTi₃₋_xO₁₅ solid solutions examined.

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