Interconversion between Rare-Earth Metal(III) Chromates(V) and Low-Crystalline Phases by Reduction with Methanol and Oxidation in Air

Abstract

Rare-earth metal(III) chromates(V), LaCrO4, NdCrO4, and Y0.9CrO3.85, were found to show reversible structural changes by reduction with methanol and oxidation in air. This anomalous behavior was investigated by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared, in situ Raman spectroscopy, and other methods. After less than 30 min of reduction at 543 K, zircon-type NdCrO4 in the bulk scale changed to the low-crystalline II-KDP-type phase, which has the same arrangement of metal atoms as zircon. The reduced phase was quickly reverted to the zircon-type phase by air oxidation at 543 K for a few minutes. NdCrO4 was reduced by hydrogen from methanol by forming hydrogen bonds between CrO4 units, which caused the oxygen displacement and the formation of pseudo-octahedral CrO6-m(OH)m units, leading to the distorted II-KDP-type structure. Defect zircon-type Y0.9CrO3.85 changed more quickly to the low-crystalline II-KDP-type phase than NdCrO4, and the structure was not completely restored by the oxidation at 543 K for 1 h, because of the formation of small amounts of YCrO3 and Cr2O3. Monazite-type LaCrO4 changed to an amorphous phase with the formation of the Schottky defects of oxide ions because of the structural differences between the monazite-type and II-KDP-type phases and was restored by air oxidation at 723 K.