Temperature evolution of the crystal structure of Bi$_{1-x}$Pr$_x$FeO$_3$ solid solutions

The crystal structure of solid solutions in the Bi$_{1-x}$Pr$_x$FeO$_3$ system near the structural transition between the rhombohedral and orthorhombic phases (0.125 $\le x\le$ 0.15) has been studied. The structural phase transitions induced by changes in the concentration of praseodymium ions and in the temperature have been investigated using X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. It has been established that the sequence of phase transformations in the crystal structure of Bi$_{1-x}$Pr$_x$FeO$_3$ solid solutions with variations in the temperature differs significantly from the evolution of the crystal structure of the BiFeO$_3$ compounds with the substitution of other rare-earth elements for bismuth ions. The regions of the existence of the single-phase structural state and regions of the coexistence of the structural phases have been determined in the investigation of the crystal structure of the Bi$_{1-x}$Pr$_x$FeO$_3$ solid solutions. A three-phase structural state has been revealed for the solid solution with $x$ = 0.125 at temperatures near 400$^\circ$C. The specific features of the structural phase transitions of the compounds in the vicinity of the morphotropic phase boundary have been determined by analyzing the obtained results. It has been found that the solid solutions based on bismuth ferrite demonstrate a significant improvement in their physical properties.