Reaction−Diffusion Programming of Morphology and Excited-State Relaxation in Sm3+-Doped La2O3

Abstract

Nonequilibrium precipitation offers a route to inorganic materials in which composition, supersaturation, and growth history are encoded spatially during synthesis. Here we show that hydrogel reaction−diffusion can couple these macroscopic gradients to dopant-controlled morphology and excited-state dynamics in Sm3+-doped La2O3. Diffusion of NH4OH into La3+/Sm3+-loaded agar gels produces La(OH)3:Sm3+ architectures that are thermally converted to A-type La2O3:Sm3+. Varying the Sm3+ fraction and total lanthanide concentration reveals two related kinetic crossovers: damping of macroscopic Liesegang banding at moderate Sm3+ loading and a microscopic sphere-to-platelet/rod crossover centered near 5−6% Sm, whose exact expression depends on total inner-lanthanide concentration and reaction−diffusion position. Counterion-control experiments show that nitrate precursors preserve microsphere formation under conditions where chloride precursors yield rods, demonstrating that reaction−diffusion patterning and microscopic growth-mode selection can be chemically decoupled. PXRD, ATR-FTIR, Rietveld refinement, EDX, and ICP−MS show that these morphological changes occur without detectable Sm-rich crystalline secondary phases. Photoluminescence measurements identify 5% Sm as the optimal emission composition for the 50 mM R2 series, whereas higher dopant loading leads to concentration quenching and strongly dispersive decay kinetics. Stretched-exponential analysis identifies a concentration-dependent crossover near 5−6% Sm3+, above which shorter characteristic decay times and lower βK values indicate faster and more heterogeneous relaxation, consistent with increased Sm−Sm interactions and possible cross-relaxation. At fixed 5% Sm3+, progression across the morphology map primarily modifies the breadth and spatial organization of the emitting environments without imposing a universal shortening of the characteristic decay time. Direct comparison with the Inokuti−Hirayama model supports the Kohlrausch function as a consistent comparative description but does not support assigning a unique energy-transfer mechanism from the decay profiles alone. These results establish reaction−diffusion synthesis as a chemically programmable route for linking precipitation kinetics, morphology selection, and the spatial organization of lanthanide excited-state relaxation.