Novel Fluorite Structured Superparamagnetic RbGdF₄ Nanocrystals as Versatile Upconversion Host

Fluorite structured nanocrystals of RbGdF₄ in cubic symmetry have successfully been synthesized by employing a simple, one-step, and template-free wet chemical method at room temperature. Considering the structural model of cubic KLaF₄ in the <i>Fm̅</i>3<i>m</i> space group, the observed powder X-ray diffraction (PXRD) pattern was fitted by the Le Bail procedure with the cubic lattice constant of <i>a</i> = 5.8244 (1) Å. Both high-resolution transmission electron microscopic (HR-TEM) and dynamic light scattering (DLS) measurements revealed the monodispersity of the nanocrystals with their size in the range of 2–18 nm. Upon excitation at 980 nm, Yb³⁺, (Er³⁺/Ho³⁺/Tm³⁺) codoped RbGdF₄ nanocrystals showed multicolor upconversion including red, yellow, blue, and the combination of basic color (near-white) emissions. Also, near-white upconversion emission from Yb³⁺, Ho³⁺, Tm³⁺ triply doped cubic RbGdF₄ nanocrystals was observed at varying laser power densities. RbGdF₄ nanocrystals exhibited superparamagnetic behavior with a molar magnetic susceptibility of 2.61 × 10^–2 emu·Oe^–1·mol^–1 at room temperature, while at low temperature (5 K) a saturation magnetization value of 90.41 emu·g^–1 at an applied field of at 10 kOe was observed. Non-interaction of the localized magnetic moment of Gd³⁺ ions in the host matrix has been reasoned out for the observed superparamagnetic behavior. From the Langevin fit of the magnetic data, the average particle diameter obtained was approximately 2.2 nm, matching well with the values from other measurements. RbGdF₄ nanocrystals exhibited a large ionic longitudinal relaxivity (<i>r</i>₁ = 2.30 s^–1·mM^–1), suggesting their potential applicability as a promising agent for <i>T</i>₁ contrast magnetic resonance imaging (MRI) in addition to the applications arising from the coupling of optical and magnetic functions such as multiplexing biodetection, bioimaging (optical and MRI), and other optical technologies