Heavily doped nanocrystals of host KLaF₄ with rare earth (RE³⁺ = Er³⁺, Tm³⁺, and Yb³⁺) ions prepared by a simple one-step template-free wet-chemical route have been reported. Prepared KLaF₄ nanocrystals reveal phase-pure cubic structures (lattice constant a = 5.931Å) with space group Fm3m. Precisely defined molar ratios of heavily dopant RE³⁺ ions allow us to achieve wide color upconversion (UC) emission tunability (blue, green to yellow–orange–red) and white light, without any morphology and structure changes. The enhanced red emission by a factor of ∼120 has been achieved in 20% Yb³⁺ and 5% Tm³⁺ ions in KLaF₄:1% Er³⁺ nanocrystals, which is due to an efficient sensitizer–acceptor (Yb³⁺ to Er³⁺ and Tm³⁺ ions) energy transfer and interexchange energy process between acceptors. For the first time, the key role of sensitizer (Yb³⁺) for UC emission energy transfer to Er³⁺ and/or Tm³⁺ is experimentally demonstrated. The evidence of upconversion photoluminescence excitation spectra reveals a broad safe biological excitation window (690–1040 nm), which can be well demonstrated by low-cost NIR diode lasers/LEDs. The applicability of these cubic nanophosphors is demonstrated as light-emitting polymer composite coatings and blocks for LEDs and solar cell panels. These well-dispersed UC nanocrystals can also be found to have greater use in bioimaging and spectral studies.

Systematic and strong rare-earth photoluminescence (PL) color flips that are highly sensitive to structural phase transformation in KLaF₄:RE³⁺ (RE³⁺ = Eu³⁺, Er³⁺/Yb³⁺) nanocrystals are demonstrated.

Strong and site selective red-emitting photoluminescent/MRI multi-functional KLa _(0.95−x) Gd _x F ₄ :Eu ³⁺ ( x = 0–0.4) bio-compatible nanomaterials for targeted in-vitro liver cancer cell imaging.