Synthesis and luminescent properties of AGd1−xLnxP2O7 (A = Rb, Cs; Ln = Ce, Eu, Tb) under VUV and X-ray excitations

AGdP2O7 (A = Rb, Cs) diphosphates were obtained through solid state reactions. Besides the monoclinic β-RbGdP2O7 and CsGdP2O7, the high temperature phase of RbGdP2O7, α-RbGdP2O7, was reported for the first time, which is isostructural with α-KYP2O7 (S.G. Cmcm, Z = 4), and the unit cell volume of α-phase is 5.4% smaller than that of β-phase. However, no reverse α ↔ β polymorphic transition was observed. The VUV-UV and X-ray excited spectroscopic properties of undoped and Ln3+ (Ln = Ce, Eu, Tb) doped AGdP2O7 (A = Rb, Cs) were presented. Under X-ray excitation, strong cascade emission of Gd3+ was observed in each undoped AGdP2O7 samples. The Gd–Ln energy transfer is relatively weak, which is due to the large Gd–Gd distance. The energy transfer probability among Gd3+ ions follows a sequence of CsGdP2O7 < β-RbGdP2O7 < α-RbGdP2O7, which is inversely proportional with the Gd–Gd distance.