Truncated forms of zero-field splitting (ZFS) Hamiltonians and implications for interpretation of ZFS parameters for Fe2+() ions in

We investigate truncated forms of zero-field splitting (ZFS) Hamiltonians utilized for Fe2+ ions with spin S=2 at tetragonal sites in K2FeF4 and A2MX4 compounds. The ZFS terms determine the easy-plane anisotropy in K2FeF4. This type of truncation consists in omission of some operator parts in the fourth-rank ZFS terms. Relationships between improper (truncated) ZFS parameters and the proper (full) ones are derived. An important implication of this truncation is an additional contribution to the second-rank axial ZFS parameter D, which significantly affects D values for S=2 systems. This hinders comparisons of seemingly compatible ‘D’ values from various sources. The origin of the second-rank rhombic ZFS E-term is also considered and found controversial. Using proper conversions, we analyze discrepancies existing in literature. A survey of available ZFS parameter values provides feedback for microscopic modeling of spin Hamiltonian parameters for Fe2+(S=2) ions in A2MX4 systems.