Revealing the peculiar local behavior of manganese and cobalt in diffusion-doped calcium orthovanadate Ca3(VO4)2

Abstract

Laser Ca₃(VO₄)₂ single crystals (СVO; space group R3c, Z = 21) with crystallochemical formula Ca1₁₈Ca2₁₈Ca3₁₈Ca4₆(Ca5 + Са5А)₃(V1₁₈V2₁₈V3₆)O₁₆₈ grown by the Czochralski method were doped with Мn₂O₃ (CVO:Мn₂O₃) and Сo₃O₄ (CVO:Co₃O₄) by high-temperature diffusion annealing. Statistical and local structures were comprehensively studied by single-crystal and powder synchrotron X-ray diffraction and conventional single-crystal X-ray diffraction as well as by X-ray absorption spectroscopy for the first time. The presence of Mn^(2+δ)+ ions with a mixed formal charge and Co²⁺ ions in the Ca2 (monocapped trigonal prism), Ca3 (two-capped trigonal prism), and Ca4 (octahedron) sites was revealed by diffraction methods. The tetrahedral coordination and the coordination intermediate between tetrahedral and octahedral were established for Mn^(2+δ)+ and Co²⁺ ions, respectively. A fundamentally different structural behavior of dopant ions (concentration, formal charge, and coordination environment) in the CVO crystals doped with manganese in the form of Mn₂O₃ during the crystal growth or by high-temperature diffusion annealing was observed. The relationship between characteristic bands in the absorption, fluorescence, and excitation spectra of CVO:Мn₂O₃ and CVO:Co₃O₄ samples and transition metals with specific formal charges is discussed.