Synthesis and characterization of rhombohedral- and tetragonal-lanthanum oxyfluoride powders

Rhombohedral- and tetragonal-lanthanum oxyfluoride (LaOF) powders were prepared by thermal decomposition of lanthanum(III) carbonate fluoride (LaFCO3) powders that had been obtained from boiling an aqueous solution containing lanthanum(III) salt, fluoride ion (F−), and urea. The conversion process of LaFCO3 to LaOF was monitored by thermogravimetry, powder X-ray diffraction, and 19F magic-angle spinning nuclear magnetic resonance spectroscopy. The crystal structure (rhombohedral, tetragonal) of LaOF depended on the mole ratio of F− to La3+ ions in the preparation of LaFCO3, and tetragonal-LaOF was completely transformed into rhombohedral-LaOF at 1000°C. The difference in crystal structures was reflected in the intensity of the 5D0→7F0 transition peak in the luminescence spectra of Eu3+-doped LaOF powders.