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 NaYF 4 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 approximately octahedral to a convergent square pyramidal coordination. A low-spin configuration was retained at all Co(III) doping concentrations. This work presents a strategy for the design of spin-crossover materials by manipulating the doping concentration of the spin-crossover center.

Spectral Signatures of Positive and Negative Polarons in Lead-Halide Perovskite Nanocrystals

“…For each plot in Figure (a)–(f) the RDF is plotted for the optimized, neutral ground state (black, solid), singlet (green, dash–dash), doublet (red, dot-dot), and triplet (blue, dot-dash) spin configuration for the optimized polaron geometries. Increases (decreases) in peaks of the RDF between the neutral ground state and PGS can be associated with increases (decreases) in coordination number of ions due to polaron formation. Shifts in RDF peaks along the x -axis would correspond to average bond/coordination elongation or contraction.…”

Structural and Magnetic Transitions in CaCo3V4O12 Perovskite at Extreme Conditions

“…CaCo 3 V 4 O 12 features a combination of several dissimilar and potentially competing electronic effects, including strong Coulomb correlations, spin–orbit coupling, and orbital polarization, thus posing a daunting challenge for ab initio calculations and making this compound an interesting system for experimental studies. Other calculations showed that even Co 2+ dopants with the square-planar coordination can essentially modify the electronic band structure of materials …”

“…Other calculations showed that even Co 2+ dopants with the squareplanar coordination can essentially modify the electronic band structure of materials. 46 In this work, we synthesized high-quality single crystals of CaCo 3 V 4 O 12 perovskite and examined the evolution of its structural, vibrational, and transport properties upon strong compression to 60 GPa. We found a crossover near 30 GPa, above which the Co−O bonds in the square-planar oxygen planes demonstrated an abnormally high compressibility.…”