Green up-conversion of C12A7–Ho3+ prepared by co-precipitation method

• C12A7 polycrystals growing in 3D with bulk nucleation.
• Activation energy calculated by classical and modified Ozawa/Kissinger methods.
• Green up-conversion of C12A7–Ho3+ attributed to dual-photons process.
• Successive energy transfer and cross relaxation for up-conversion of 5F4/5S2 → 5I8.

7CaO⋅12Al2O3 (C12A7) and Ho3+-doped 7CaO⋅12Al2O3 (C12A7–Ho3+) polycrystal powders were synthesized via co-precipitation followed by sintering at 1173 K. Crystal structures, thermodynamics characters and optical properties of crashed samples were examined by XRD, DSC/TG, FT-IR and Raman spectra analysis, respectively. Activation energy of crystallization for C12A7 polycrystals was calculated according to both Ozawa/Kissinger and modified Ozawa/Kissinger methods to be 813 or 791 (for the former, only considering crystallization) and 1084 or 1077 kJ mol−1 (for the latter, taking both crystallization and nucleation into consideration) respectively. Growth morphology parameter (m) and crystallization mechanism index (n) of C12A7 were determined to be m = 3 and n = 4. Fluorescence effects of Raman spectra around 2000–2300 cm−1 and pure visible up-conversion emission centered at 540 nm were observed from C12A7–Ho3+ (1, 2 and 5 mol.%) powders under 488 nm laser diode and 808 nm Xe lamp excitation, respectively. Two mechanisms were proposed to explain up-conversion of pure green light process.