Synthesis and Properties of the La1 - x - y Eu y Ca x VO4 (0 ≤ x, y ≤ 0.2) Compounds

The La_1 - x Ca _x VO₄ and La_{1 - x - y} Eu _y Ca _x VO₄ (0 ≤ x, y ≤ 0.2) micro/nanosized powders were prepared by aqueous nitrate-citrate sol-gel synthesis. Phase composition of the sample depends on the x and y values. The La_0.9Ca_0.1VO₄ is crystallized in monoclinic structure up to the x = 0.1. The La_0.9Eu_0.05Ca_0.05VO₄ sample was also attributed to the monoclinic structure. Increasing concentration of europium and calcium ions in La_{1 - x - y}Eu _y Ca _x VO₄ solid solutions leads to the change of the crystal structure, and subsequently, stabilization of the tetragonal phase takes place.The obtained samples were characterized by XRD analysis, SEM microscopy, and IR spectroscopy. Luminescence properties of the synthesized powders were studied. Emission of the La_1 - x Ca _x VO₄ samples is weak and consists of wide bands in the 450-800 nm spectral range. The observed bands at 570 and 630 were ascribed to electron transitions in the distorted VO₄^3- vanadate groups. Emission of the La_{1 - x - y} Eu _y Ca _x VO₄ samples consists of narrow spectral lines in the 550-730 nm spectral range. The lines are caused by the ⁵D₀ → ⁷F_J electron transitions in the Eu³⁺ ions. The Ca²⁺ ions incorporation increases the intensity of the Eu³⁺ ions luminescence. Structure of the spectra depends on Ca²⁺ concentration and excitation wave length. The carried out analysis has revealed that Eu³⁺ ions form at least two different types of emission centers in the La_{1 - x - y} Eu _y Ca _x VO₄ samples. The assumption is made that type I centers are formed by the Eu³⁺ ions in their regular positions in the crystal lattice, while the type II centers have complex structure and consist of Eu³⁺ ions, Ca²⁺ cations, and oxygen vacancies.

Catechin tuned magnetism of Gd-doped orthovanadate through morphology as T1-T2 MRI contrast agents

Tetragonal (t)-LaVO₄ has turned out to be a potential host for luminescent materials. Synthesis of t-LaVO₄ till date has been based on chelating effect of EDTA making it not ideal for bioimaging applications. An alternative was proposed by us through the use of catechin. In recent times there is interest for new MRI contrast agents that can through appropriate doping function both as MRI contrast and optical/upconversion materials. It is generally believed that under appropriate doping, t-LaVO₄ would be a better upconversion material than monoclinic (m)-LaVO₄. Based on these postulations, this work explores the use of gadolinium doped t-LaVO₄ as an MRI contrast agent. From literature, gadolinium oxide is a good T₁ contrast agent. Through this work, using catechin as a template for the synthesis of Gd doped t-LaVO₄, we demonstrate the possible use as a T₁ contrast agent. Interestingly, as the catechin concentration changes, morphology changes from nanorods to square nanoplates and spheres. In this process, a switch from T₁ to T₂ contrast agent was also observed. Under optimal concentration of catechin, with a rod shaped Gd doped t-LaVO₄ an r₂/r₁ value of 21.30 was observed. Similarly, with a spherical shape had an r₂/r₁ value of 1.48 was observed.