Metastable phase relations and europium(II) luminescence of barium hexa-aluminates prepared by gel to crystallite conversion

Wet chemical reaction of hydrated alumina gels, Al2O3 · yH2O(80 < y < 120) with Ba(OH)2 gives rise to crystallites of the precursor, Ba1+xAl12O19+x−z2(OH)z · nH2O where −0.2 < x < 0.32, 15 < z < 20 and 5 < n < 9. Thermal decomposition of the precursor above 1150 °C produces monophasic hexa-aluminate, Ba1+xAl12O19+x. Metastability of this nonstoichiometric phase is evident from the splitting of (00ℓ) reflections, on heat treatment above 1600 °C, indicating the subsolidus decomposition to 0.82BaO 6Al2O3 and 1.32BaO 6Al2O3. High resolution electron microscopy reveals extensive distribution of the blocking defects and structural intergrowths. The metastable Ba-hexaaluminates doped with Eu(II) exhibit predominantly the 436nm emission band for all the ‘x’ values. No deterioration in emission intensity is noticed on long term preservation of the phosphors. Differences prevailing in literature on the phase relations in Ba-hexa-aluminates can be attributed to preparative route dependent metastability.