Synthesis and Properties of the La_{1 - 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.