Lattice mismatch and energy transfer of Eu- and Dy-codoped MO-Al2O3-SrO (M=Mg, Ca, Ba) ternary compounds affecting luminescence behavior

A systematic investigation of energy transfers and luminescence behaviors for MxSr0.94−xAl2O4:Eu0.02, Dy0.04 (M=Mg, Ca, Ba; x=0, 0.235, 0.47, 0.705, 0.94) ternary compounds was accomplished. The results demonstrated that six phenomena must be fitted into the energy-transfer mechanisms of the ternary compounds: (1) the optical band-gap energy of Mg0.94Al2O4:Eu0.02Dy0.04 is extremely low and does not allow photoemission; (2) Ca2+ and Ba2+ ions are the main hosts when x≥0.47 in CaxSr1−xAl2O4:Eu0.02Dy0.04 and BaxSr1−xAl2O4:Eu0.02Dy0.04, respectively; (3) Eu3+ ions are the main activator ions in CaxSr1−xAl2O4:Eu0.02Dy0.04 with x=0.47 and in BaxSr1−xAl2O4:Eu0.02Dy0.04 with x=0.353−0.705; (4) Sr2+ and Eu2+ ions are the main host and activator ions, respectively, when x<0.353 in each ternary compound; (5) energy transfers from the MO phases to the SrO phase because the conduction band energy of SrO is the lowest; and (6) mutual substitution between alkaline-earth ions does not alter the resultant structures' crystal field and nephelauxetic effects, as determined by measuring their luminescence. Two energy transfer paths were discovered to be possible in CaO-Al2O3-SrO and BaO-Al2O3-SrO ternary compounds, and the boundaries determining which path was chosen were the atomic ratios Ca:Sr and Ba:Sr, both approximately 1.6:1 (x=0.353). Because second path increased the energy transferred from the MO band gap to the SrO band gap, the corresponding structure's spectrum emission intensity was approximately 4.3 times higher than that of the SrO−Al2O3 binary compound, and their photoluminescence was thus substantially higher.