Effect of cobalt content on point defects and local structure in activated Ca3(VO4)2 single-crystal solid solutions

Doping-induced structural changes of Co-activated calcium orthovanadate Ca3(VO4)2 (CVO) single crystals and specific local behavior of cobalt ions depending on the dopant content are presented in this paper. CVO is activated with over-stoichiometric Co3O4 of different concentration (CVO:xCo; x ​= ​0.1, 0.5, 1.0, 1.5, 2.0 ​wt %) during the Czochralski growth. Single-crystal synchrotron X-ray diffraction study shows gradual increase in the number of Ca crystallographic sites occupied by cobalt ions with increasing dopant concentration: from Ca3 site for CVO:0.1Co to additionally Ca2 and Ca4 sites for CVO:0.5–1.5Co, and also to Ca1 site for CVO:2.0Co. Vacancies in the tetrahedral vanadium V2 site are detected for all samples, and those in the tetrahedral V1 site are additionally found for CVO:2.0Co. Disordering of O1, O4, O7 oxygen sites is observed in all structures. Combined results of X-ray diffraction analysis and X-ray absorption spectroscopy showed that the formal charge of Co ions is mixed between 2+ and 3+ with the prevalence of 2+. A trend towards a decrease in formal charge with an increase in Co concentration is observed. The coordination number of Co(2+δ)+ ions is found to be 4.6 and polyhedron is intermediate between tetrahedral and octahedral.