Effects of charge transfer band position and intensity on the photoluminescence properties of Ca1.9M2O7:0.1Eu3+ (M = Nb, Sb and Ta)

Abstract

The effects of the position and intensity of the charge transfer band (CTB) on photoluminescence properties are elucidated in the Eu3+ doped Ca2M2O7 (M = Nb, Sb and Ta) system. The position of the CTB is dependent on the electronegativity and coordination of counter cation at the B-site and is shifted to higher wavelengths from Nb to Sb to Ta. The CT band of Nb system is shallow and located at a lower wavelength (295 nm) due to more polarizability of anion (O2−). While, the CTB of Sb system is broad and intense that lies at 298 nm wavelength due to highly polaraizable environment of Eu–O bond whereas charge transfer energy of Ta system is maximum shifted to the lower energy (318 nm) which is mainly attributed to the higher covalency of Eu3+ surroundings due to lower electronegativity of Ta ion. It is further observed that the intensity of the CTB is inversely related to the f-f excitation levels of Eu3+ which indicates poor energy transfer for the intense CTB system. Further, the Eu3+ emissions due to 5D0-7FJ (J = 0, 1, 2) transitions illustrate the site symmetry of the Eu3+ ions in the lattice to distinct among these ambiguity structures. Among these phosphors, the Ca1.9Nb2O7:0.1Eu3+ exhibit intense red emission due to efficient energy transfer from the host to the Eu3+ excitation levels. The lifetime of 5D0-7F2 increases in the order of Nb > Ta > Sb in accordance with the emission intensities. These results facilitate to design the phosphor using the factors of position and intensity of charge transfer band in rare earth activated phosphors.