Cation Substitution Dependent Bimodal Photoluminescence in Whitlockite Structural Ca3–xSrx(PO4)2:Eu2+ (0 ≤ x ≤ 2) Solid Solution Phosphors

Cation substitution dependent tunable bimodal photoluminescence behavior was observed in the Ca3–xSrx(PO4)2:Eu2+ (0 ≤ x ≤ 2) solid solution phosphors. The Rietveld refinements verified the phase purity and whitlockite type crystal structure of the solid solutions. The tunable photoluminescence evolution was studied as a function of strontium content, over the composition range 0.1 ≤ x ≤ 2. In addition to the emission band peak at 416 nm in Ca3(PO4)2:Eu2+, the substitution of Ca2+ by Sr2+ induced the emerging broad-band peak at 493–532 nm. A dramatic red shift of the emission peak located in the green-yellow region was observed on an increase of x in the samples with 0.75 ≤ x ≤ 2.00. The two emission bands could be related to the EuOn–Ca9 and EuOn–Ca9–xSrx emitting blocks, respectively. The values for the two kinds of emitting blocks in the solid solutions can be fitted well with the observed intensity evolution of the two emission peaks.