Synthesis and Properties of the La1 − x − yEuyCaxVO4 (0 ≤ x, y ≤ 0.2) Compounds

The La1 - x Ca x VO4 and La1 - 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 La0.9Ca0.1VO4 is crystallized in monoclinic structure up to the x = 0.1. The La0.9Eu0.05Ca0.05VO4 sample was also attributed to the monoclinic structure. Increasing concentration of europium and calcium ions in La1 - x - yEu 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 La1 - 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 VO43- vanadate groups. Emission of the La1 - 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 → 7FJ 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 La1 - 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.