Study of cobalt ions diffusion in calcium orthovanadate crystal

High-temperature diffusion doping has been used for the introduction of active cobalt ions in calcium orthovanadate Ca₃(VO₄)₂ single crystals (CVO). The test samples have been produced from nominally pure Cz CVO single crystals. High-temperature diffusion conditions have been optimized for the obtaining of doped optical quality single crystals in open or closed zone annealing. The diffusion coefficients of cobalt ions have been calculated for different conditions. Temperature regime, process time, diffusant type were defined. The samples have been annealed at 1150–1300 °C for 24–48 h with Co₃O₄, Ca₁₀Co_0.5(VO₄)₇ and Ca₃(VO₄)₂ : 2 wt.% Co₃O₄ as diffusants. The diffusion direction has been parallel or perpendicular to the optical axis of the CVO crystal. The diffusion coefficients have been calculated to be in the range of 2.09 · 10^-8 – 1.58 · 10^-7 cm²/s. The diffusion activation energies have been determined to be 2.58 ± 0.5 and 2.63 ± 0.5 eV for the [001] and [100] directions, respectively. The maximum cobalt concentration in the doped CVO crystals has been found to be 2 · 10²⁰ cm^-3. The absorption spectrum of the diffusion doped Ca₃(VO₄)₂ : Co samples has exhibited absorption bands typical of the Co²⁺ and Co³⁺ ions. It has been shown that the intensity ratio between the characteristic absorption bands varies depending on crystal obtaining technique. The optical anisotropy of the crystal increases with dopant concentration.