Effect of chromium substitution on structural and optical properties of Ca3(VO4)2 for laser and SRS applications

Abstract

Crystal structures of Czochralski-grown Ca3(VO4)2 single crystals doped with over-stoichiometric 0.01, 0.05, and 0.1 wt% Cr2O3 (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 (Cr3+) and tetrahedral (Cr4+) coordination, which suggests structural superpositions of CaO8, CaO7, CaO6 and Cr3+O6 or Cr4+O4 polyhedra. X-ray diffraction, EPR and spectroscopic studies showed that green or yellow color of CVO:Cr is related to chromium valence state, Cr3+ or Cr4+, respectively. An increase in the Cr4+ 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 Cr3+ ions, are revealed in the UV-VIS spectra of green samples. An additional broad band with a maximum at 21254 cm−1 refers to Cr4+ ions, the appearance of which after annealing in air was established by X-ray diffraction analysis.