Well-defined crystallites autoclaved from the nitrate/NH4OH reaction system as the precursor for (Y,Eu)2O3 red phosphor: Crystallization mechanism, phase and morphology control, and luminescent property

Autoclaving the rare-earth nitrate/NH4OH reaction system under the mild conditions of 120–200 °C and pH 6–13 have yielded four types of well-crystallized compounds with their distinctive crystal shapes, including Ln2(OH)5NO3·nH2O (Ln=Y and Eu) layered rare-earth hydroxide (hexagonal platelets), Ln4O(OH)9NO3 oxy-hydroxyl nitrate (hexagonal prisms and microwires), Ln(OH)2.94(NO3)0.06·nH2O hydroxyl nitrate (square nanoplates), and Ln(OH)3 hydroxide (spindle-shaped microrods). The occurrence domains of the compounds are defined. Ammonium nitrate (NH4NO3) as a mineralizer effectively widens the formation domains of the NO3− containing compounds while leads to larger crystals at the same time (up to 0.3 mm). Crystallization mechanisms of the compounds and the effects of NH4NO3 were discussed. Optical properties (PLE/PL) of the four phases were characterized in detail and were interpreted from the different site symmetries of Eu3+. The compounds convert to cubic-structured (Y0.95Eu0.05)2O3 by annealing at 600 °C while retaining their original crystal morphologies. The resultant phosphor oxides of diverse particle shapes exhibit differing optical properties, in terms of luminescent intensity, asymmetry factor of luminescence and fluorescence lifetime, and the underlying mechanism was discussed.