Limits of Solid Solution and Evolution of Crystal Morphology in (LaRE)FeO Perovskites by Low Temperature Hydrothermal Crystallization.

The crystallization of a new series of A-site substituted lanthanum ferrite materials (LaRE)FeO was explored by the hydrothermal method at 240 °C, for rare earth (RE) = Nd, Sm, Gd, Ho, Er, Yb, and Y, with 0 ≤ ≤ 1. The effect of elemental substitution on the morphological, structural, and magnetic properties of the materials was studied using high-resolution powder X-ray diffraction, energy dispersive spectroscopy (EDS) on the scanning electron microscope, Raman spectroscopy, and SQUID magnetometry. If the radius of the La and the substituent ions is similar, such as for Nd, Sm, and Gd, homogeneous solid solutions are formed, with the orthorhombic GdFeO-type structure, and a continuous evolution of Raman spectra with composition and distinct magnetic behavior from the end members. When the radius difference between substituents and La is large, such as for Ho, Er, Yb, and Y, then instead of forming solid solutions, crystallization in separate phases is found. However, low levels of element mixing are found and intergrowths of segregated regions give composite particles. In this case, the Raman spectra and magnetic behavior are characteristic of mixtures of phases, while EDS shows distinctive elemental segregation. A-site replacement induces an evolution in the crystallite shape with an increasing amount of substituent ions and this is most evident for RE = Y from cube-shaped crystals seen for LaFeO to multipodal crystals for (LaY)FeO, providing evidence for a phase-separation-driven evolution of morphology.