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

• Tb3+-doped cubic and hexagonal KLaF4 nanocrystals were obtained by solution method.
• Both cubic and hexagonal KLaF4:Tb3+ showed strong green emission under UV light.
• The average lifetime of the (5D4 → 7F5) emission was 1–3 ms.
• Tb3+/Yb3+ co-doped cubic KLaF4 showed intense green upconversion.
• Cubic and hexagonal KLaF4:Tb3+ exhibited paramagnetic character at RT.

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 (5D4 → 7F5) on excitation with 337 nm laser, the intensity increases monotonically with Tb3+ content. The relative intensity of the excited state 5D3 violet emission is weaker than that of 5D4 emissions even at the low Tb3+ amounts. It is proposed that the 5D3 → 5D4 cross-relaxation may be responsible for the decrease in the 5D3 decay rate with increasing Tb3+ concentrations. The average emission decay lifetime of the green emission (545 nm; 5D4 → 7F5) of Tb3+ doped cubic KLaF4 samples are in the order of 1–3 ms. The absence of the characteristic emissions from 5D3 level in hexagonal KLaF4:Tb3+ phosphor suggests the pronounced non-radiative cross relaxation between the 5D3 to 5D4 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 χg value ranging from 1.627 × 10−6 to 1.356 × 10−5 emu/g.