Relationship between Site Symmetry, Spin State, and Doping Concentration for Co(II) or Co(III) in β-NaYF4

Abstract

Magnetic dopants provide versatile tunability of properties for transparent host matrix materials, semiconductors, and nanostructured materials. The relationship between doping concentration, site symmetry, crystal-field splitting, and spin state for Co(II) and Co(III) doped in hexagonal-phase NaYF4 crystals and nanocrystals was investigated in a spin-polarized density functional theory approach. With decreasing dopant concentrations, the geometry of the Co(II) fluoride nearest-neighbor coordination changes from approximately square pyramidal to square planar to cubic before converging to pentagonal bipyramidal coordination. A transition from low-spin to high-spin was observed, accompanying the geometry transformation from square planar to cubic, which is consistent with expectations from basic crystal-field theory. Magnetic ordering of adjacent ions was found to have little influence on this spin-transition process. For Co(III), decreasing concentration resulted in a geometry transformation from approxim...