Structurally Modulating the Upconversion Emission and Superparamagnetism in KLa(Gd)F ₄ :Er ³⁺ /Yb ³⁺ Nanoparticles

Compositional tuning in KLa(Gd)F4:RE3+ nanoparticles reveals a strong correlation among crystal phase, upconversion emission, and magnetic response. In the systematic compositions of KLa(0.79–x)GdxF4:1% Er3+/20% Yb3+ (x = 0–0.5), the progressive Gd3+ substitution induces a transformation from a cubic to a hexagonal crystal phase. This phase evolution governs the upconversion photoluminescence under 980 nm excitation, producing pronounced color switching from yellow to red emissions. The nanoparticles simultaneously exhibit composition-dependent magnetic behavior, transitioning from paramagnetism at room-temperature to superparamagnetism at 5 K, with the fully hexagonal composition (x = 0.50) reaching a saturation magnetization of 105.74 emu g–1. Electron paramagnetic resonance confirms that higher Gd3+ content strengthens magnetic interactions between neighboring Gd3+ ions. The present comprehensive understanding of structural, upconversion emission and magnetic features enable the rational design of multifunctional lanthanide nanomaterials with implications for bioimaging, theranostics, and optoelectronic technologies.