High-Frequency and -Field Electron Paramagnetic Resonance Studies of Yellow/Orange/Red Pigments Based on Fe3+ in Trigonal Bipyramidal Coordination.

The material YInO3 is amenable to doping of its trigonal bipyramidal (TBP) In3+ site by a wide range of metal ions. Among these are Mn3+ (S = 2) and Fe3+ (S = 5/2). The former, YIn1-xMnxO3, was previously the subject of an earlier investigation by high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy. HFEPR is here applied to YIn1-xFexO3 (0 ≤ x ≤ 0.3). This material exhibits a strong HFEPR response and spectra that can be readily analyzed using an S = 5/2 spin Hamiltonian with only second-order zero-field splitting (zfs) terms: D = -1.095 cm-1, E = 0. The spectral appearance is strongly dependent on both temperature and iron doping level, even for these In-rich samples. The zfs is semiquantitatively analyzed using a ligand-field theory (LFT) model, which suggests that the small-magnitude, negative D value arises from spin-orbit coupling (SOC) of the 6A1' (in D3h idealized point group symmetry) ground state primarily with the lowest-lying 4E″ excited state.