Direct Hydrothermal Synthesis and Physical Properties of Rare-Earth and Yttrium Orthochromite Perovskites

We describe a systematic study of the hydrothermal synthesis of rare-earth orthochromites, RCrO3 (R = La, Pr, Sm, Gd, Dy, Ho, Yb, and Lu) and YCrO3. All nine of these materials can be prepared in a single step by hydrothermal treatment of an amorphous mixed-metal hydroxide at temperatures above 300 degrees C upon heating around 24 h, with no post-synthesis annealing needed. The as-made materials are highly crystalline powders with submicrometer particle size. In the case of LaCrO3 the addition of solution additives to the hydrothermal synthesis allows some modification of crystallite form of the material, and in the presence of sodium dodecylsulfate nanocrystalline powders are produced. Profile refinement of powder X-ray diffraction (XRD) data show that each of the RCrO3 materials adopts an orthorhombic distorted (Pbmn) perovskite structure. Detailed, magnetization studies as a function of temperature reveal the high quality of the specimens, with low temperature antiferromagnetic behavior seen by direct current (DC) magnetization, and T-N values that correlate with the structural distortion. A high temperature structural phase transition (similar to 540 K to rhombohedral R (3) over barc) seen by variable temperature XRD in the hydrothermally prepared LaCrO3 is consistent with the reported behavior of the same composition prepared using conventional high temperature synthesis.