Intense red emission on dilute Mn-doped CaYAlO₄-based ceramics obtained by laser floating zone

The laser floating zone (LFZ) technique was used to produce polycrystalline fibers composed by P2₁/c monoclinic Y₄Al₂O₉ (YAM) and \({\text{P}}\bar{4}21{\text{m}}\) tetragonal CaYAl₃O₇ (CYAM) phases embedded into I4/mmm tetragonal CaYAlO₄ matrix. The scanning electron microscopy and X-ray diffraction patterns put in evidence the strong effect of growth rate on the microstructural and phases’ evolution. Besides the microstructural and structural analysis, complementary optical techniques as photoluminescence (PL), PL excitation (PLE), and lifetime measurements were used to characterize the produced fibers. The nonintentionally doped fibers were shown to exhibit an intense deep red emission likely due to Mn⁴⁺ trace impurities. From the PLE measurements, average crystal field strength was estimated with Dq/B ~ 2.94. Temperature-dependent PL measurements revealed that the red luminescence is due to the overlap of transitions from the almost electronic degenerate ²E and ⁴T₂ excited states to the ⁴A₂ ground state. The emission from the two excited states arises due to the breakdown of the adiabatic approximation. The overall luminescence intensity of the red emission was found to decrease from 11 K to RT, and the internal quantum efficiency, estimated from the ratio of the integrated luminescence at high and low temperatures, was found to be 60%. Time-resolved spectroscopy indicates a single decay time of ca. 2.0 ms at room temperature, corresponding to the spin and parity forbidden ²E → ⁴A₂ transition.