Unusual Spin Exchanges Mediated by the Molecular Anion P2S64−: Theoretical Analyses of the Magnetic Ground States, Magnetic Anisotropy and Spin Exchanges of MPS3 (M = Mn, Fe, Co, Ni)

We examined the magnetic ground states, the preferred spin orientations and the spin exchanges of four layered phases MPS3 (M = Mn, Fe, Co, Ni) by first principles density functional theory plus onsite repulsion (DFT + U) calculations. The magnetic ground states predicted for MPS3 by DFT + U calculations using their optimized crystal structures are in agreement with experiment for M = Mn, Co and Ni, but not for FePS3. DFT + U calculations including spin-orbit coupling correctly predict the observed spin orientations for FePS3, CoPS3 and NiPS3, but not for MnPS3. Further analyses suggest that the ||z spin direction observed for the Mn2+ ions of MnPS3 is caused by the magnetic dipole–dipole interaction in its magnetic ground state. Noting that the spin exchanges are determined by the ligand p-orbital tails of magnetic orbitals, we formulated qualitative rules governing spin exchanges as the guidelines for discussing and estimating the spin exchanges of magnetic solids. Use of these rules allowed us to recognize several unusual exchanges of MPS3, which are mediated by the symmetry-adapted group orbitals of P2S64− and exhibit unusual features unknown from other types of spin exchanges.

Molecular orbitals from group orbitals. IX. The problem of hybrid lone pairs

The quantitative PMO analysis of the ab initio wavefunction of a molecule A—B is based upon a partitioning of the Fock matrix elements of this wavefunction to obtain the fragments A and B, followed by computation of the stabilizing and destabilizing orbital interactions between the orbitals of these fragments that contribute to the HOMO of A—B. However, when one or both of the fragments is NH2 or a congeneric species, neither the 3a1 nor the 1b1 orbital of this fragment is appropriate for overlap with the second fragment, and the PMO analysis cannot be performed. A solution to this problem is proposed, and has been tested by application to various conformational properties of methylamine.