High-frequency EMR data for Fe2+ (S = 2) ions in natural and synthetic forsterite revisited – Fictitious spin S′ = 1 versus effective spinS˜ = 2 approach

Recent wide-band (65–850 GHz) electron magnetic resonance (EMR) study determined the zero field splitting (ZFS) parameters (ZFSPs) for Fe2+ in natural and synthetic forsterite (Mg2SiO4) based on the fictitious spin S′ = 1 approach. So-obtained ZFSPs (D′, E′) are incompatible with those available in literature for 3d6 (Fe2+) and 3d4 (Fe4+, Mn3+, Cr2+) ions with the electronic spin S = 2, which predominantly pertain to the effective spin S ˜ = 2. Background for the effective spin S ˜ versus the fictitious ‘spin’ S′ (J′) approaches is provided with focus on application to Fe2+ ions. To enable comparison of the S′ = 1 ZFSPs with the S ˜ = 2 ones methodology for conversions has been worked out and appropriate conversion relations derived for various combinations of the possible energy level schemes for the spin S ˜ = 2 and S′ = 1. The second-rank S′ = 1 ZFSPs (D′, E′) measured by high-frequency EMR for Fe2+ in Mg2SiO4 are converted to the S ˜ = 2 ZFSPs (D, E) and compared with literature data. Suitability of Fe2+:Mg2SiO4 systems for application as high-pressure probes for high-magnetic field and high-frequency EMR (HMF-EMR) is then considered. The results of this study may be applied to other cases of 3d6 and 3d4 (S = 2) ions in various hosts.