Study of Structure of Defect Centers in Europium Vanadate Nanoparticles with Heterovalent Dopants

The EuVO$_4$ nanoparticles doped with Ca$^{2+}$ impurities were synthesized by citrate-nitrate sol-gel method and investigated. Diffuse reflection spectra of the EuVO$_4$:Ca samples contain two additional bands around 400 and 470 nm those were not observed for the undoped EuVO$_4$ . These bands were ascribed to Ca-induced defects in the vanadate crystal lattice. Origin of these defects is studied taking into account results of high resolution XRD measurements. The additive XRD peaks were found for the Eu$_{0.8}$Ca$_{0.2}$VO$_4$ , sample. These peaks were identified as traces of the Ca$_2$ V$_2$ O$_7$ second crystal phase in the synthesized nanoparticles. The maximal concentrations of the Ca$^{2+}$ heterovalent impurity permits to keep single phase EuVO$_4$:Ca sol-gel nanoparticles were estimated.