Optical properties of Tb3+ doped KLaF4 in cubic and hexagonal symmetries

Tb3+ doped cubic and hexagonally ordered KLaF4 nano-sized crystals have been synthesized by solution based method with the simultaneous evaluation of their optical and magnetic properties. While higher amounts of Tb3+ (10%) can be doped in cubic KLaF4 lattice, only 3% of Tb3+ can be doped in hexagonal KLaF4 by this method. Cubic KLaF4:Tb3+ samples show a very strong green emission centered at 545 nm (D-5(4) -> F-7(5)) on excitation with 337 nm laser, the intensity increases monotonically with Tb3+ content. The relative intensity of the excited state D-5(3) violet emission is weaker than that of D-5(4) emissions even at the low Tb3+ amounts. It is proposed that the D-5(3) -> D-5(4) cross-relaxation may be responsible for the decrease in the D-5(3) decay rate with increasing Tb3+ concentrations. The average emission decay lifetime of the green emission (545 nm; D-5(4) -> F-7(5)) of Tb3+ doped cubic KLaF4 samples are in the order of 1-3 ms. The absence of the characteristic emissions from D-5(3) level in hexagonal KLaF4:Tb3+ phosphor suggests the pronounced non-radiative cross relaxation between the D-5(3) to D-5(4) energy levels. Further, these cubic KLaF4:Tb3+ samples show intense green upconversion emission when co-doped with moderate concentrations of sensitizer Yb3+ ions. Both cubic and hexagonal KLaF4:Tb3+ samples show paramagnetic behavior at room temperature with chi(g) value ranging from 1.627 x 10(-6) to 1.356 x 10(-5) emu/g. (C) 2013 Elsevier B.V. All rights reserved.