Temperature-dependent Structural and Spectroscopic Studies of (Bi<sub>1–<i>x</i></sub>Fe<i><sub>x</sub></i>)FeO<sub>3</sub>

We report on temperature-dependent structural and spectroscopic features of (Bi<sub>1–<i>x</i></sub>Fe<i><sub>x</sub></i>)­FeO<sub>3</sub> 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 Fe<sup>3+</sup> cation significantly changes the crystal physicochemical properties, such as thermal expansion, polyhedral distortion, Debye temperature, and vibrational and magnetic properties. Whereas BiFeO<sub>3</sub> is multiferroic, both Bi<sub>0.85</sub>Fe<sub>0.15</sub>FeO<sub>3</sub> and Bi<sub>0.75</sub>Fe<sub>0.25</sub>FeO<sub>3</sub> 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.