Magnetostructural relationships for Ni(II) ions at octahedral sites in [NixZn1-x(C2O4)(dmiz)2]: Computational study of zero-field splitting and using superposition model

Abstract

Superposition model (SPM) analysis of the zero-field splitting parameters (ZFSPs), for short SPM/ZFSP, is utilized to investigate magnetostructural relationships in Ni(II)-based magnetic systems: the pure Ni(C<inf>2</inf>O<inf>4</inf>)(dmiz)<inf>2</inf> and mixed Ni<inf>x</inf>Zn<inf>1-x</inf>(C<inf>2</inf>O<inf>4</inf>)(dmiz)<inf>2</inf> derivatives, where dmiz is 1,2-dimethylimidazole. The pure system is considered as an example of the S = 1 Heisenberg antiferromagnetic one-dimensional chain. The Haldane gap exhibited by such systems is the major motivation for their studies. SPM/ZFSP is based on knowledge of the model parameters, i.e. the intrinsic parameters - b¯<inf>k</inf>(R0), power law exponents - t<inf>k</inf>, and the reference distance - R<inf>0</inf>. As a first approximation we use the model parameter sets determined earlier for Ni(II) ions in the sixfold coordinated NiO<inf>6</inf> with axial symmetry. Matching of the SPM predicted ZFSPs with the experimental ones is carried out for several model parameter sets and various models of distortions around Ni(II) ions. The axial (D) and rhombic (E) ZFSPs obtained by high magnetic field/high-frequency EMR (HMF-EMR) for the derivatives Ni<inf>x</inf>Zn<inf>1-x</inf>(C<inf>2</inf>O<inf>4</inf>)(dmiz)<inf>2</inf> for x = 0.09 and 0.07 are employed. Three approaches accounting for the local structural distortions with respect to the undistorted host are considered, namely, the angular distortions, the distortions of the ligand-metal bond lengths, and simultaneous distortions of the two types. This enables a thorough study of the magnetostructural relationships for the doped Ni(II) ions in Ni<inf>x</inf>Zn<inf>1-x</inf>(C<inf>2</inf>O<inf>4</inf>)(dmiz)<inf>2</inf> and related crystals. Major finding is that SPM/ZFSP method is capable of yielding satisfactory matching of both the size and the signs of the predicted and experimental ZFSPs. Controversies concerning the very large D-values obtained by ab initio methods as compared with those usually observed in Ni(II) systems are discussed. Several inconsistencies concerning interpretation of experimental and theoretical data for Haldane gap systems are also clarified. © 2023 Elsevier B.V., All rights reserved.