Effect of chromium substitution on structural and optical properties of Ca 3 (VO 4 ) 2 for laser and SRS applications

Crystal structures of Czochralski-grown Ca 3 (VO 4 ) 2 single crystals doped with over-stoichiometric 0.01, 0.05, and 0.1 wt% Cr 2 O 3 (CVO:Cr) are solved and refined. A whitlockite-like structure with five Ca sites, two-capped (Ca1 and Ca3) and mono-capped (Ca2) trigonal prisms, distorted octahedron (Ca4) and tetrahedron (Ca5), is determined for CVO:Cr. Single-crystal synchrotron X-ray diffraction analysis revealed the presence of chromium in Ca2, Ca3, Ca4 sites. In CVO:Cr, chromium has octahedral (Cr 3+ ) and tetrahedral (Cr 4+ ) coordination, which suggests structural superpositions of CaO 8, CaO 7, CaO 6 and Cr 3+ O 6 or Cr 4+ O 4 polyhedra. X-ray diffraction, EPR and spectroscopic studies showed that green or yellow color of CVO:Cr is related to chromium valence state, Cr 3+ or Cr 4+, respectively. An increase in the Cr 4+ concentration is accompanied by increased vacancy content in two of three vanadium sites. Structural behavior of transition metal ions with a variable formal charge (Co, Mn, Cr) in CVO structure is summarized, analyzed and discussed. Three absorption bands around 23900, 16300 and 14000 cm −1, typical for Cr 3+ ions, are revealed in the UV-VIS spectra of green samples. An additional broad band with a maximum at 21254 cm −1 refers to Cr 4+ ions, the appearance of which after annealing in air was established by X-ray diffraction analysis.