Temperature-dependent Structural and Spectroscopic Studies of (Bi_1–<i>x</i>Fe<i>_x</i>)FeO₃

We report on temperature-dependent structural and spectroscopic features of (Bi_1–<i>x</i>Fe<i>_x</i>)­FeO₃ perovskite for <i>x</i> = 0.15 and 0.25. Samples were synthesized by heating quantum crystalline precursors obtained by the polyol method. Crystal structures of each composition were obtained from in-house X-ray, synchrotron X-ray, and time-of-flight neutron powder diffraction data Rietveld refinements. Partial replacement of the Bi site by the Fe³⁺ cation significantly changes the crystal physicochemical properties, such as thermal expansion, polyhedral distortion, Debye temperature, and vibrational and magnetic properties. Whereas BiFeO₃ is multiferroic, both Bi_0.85Fe_0.15FeO₃ and Bi_0.75Fe_0.25FeO₃ are found to be superparamagnetic, as observed by temperature-dependent Mössbauer and SQUID measurements. Lattice thermal expansion was modeled using the Debye–Einstein-anharmonicity approach. Debye temperatures obtained from the mean-squared atomic displacement parameter and lattice thermal expansion are compared. Temperature dependence of selective Raman modes is also analyzed.