Orbit-lattice interaction, induced structural distortions and lattice strain affecting energy levels of Ni2+ ions doped pyrochlores: Y2Ti2O7 and Gd2Ti2O7

Effects of orbit-lattice interaction (OLI), structural distortions, and lattice strain on energy levels, crystal-field parameters (CFPs) and zero-field splitting parameter (ZFSPs) for metal ions in ligands polyhedra are largely unexplored. This study examines Ni2+ doped pyrochlores: Y2Ti2O7 and Gd2Ti2O7 involving static and dynamic structural distortions causing stress–strain effects in Ti/Ni-O6 polyhedra. Incorporation of OLI enables correlating lattice strains with changes in CFPs. Utilizing exchange charge model (ECM) and superposition model (SPM) for CFP modelling, OLI and stress–strain effects are investigated. So-obtained CFPs serve as input for CFA/MSH program predicting CF energies, states, and ZFS magnitude (equal ZFSP |D|) of Ni2+ ions. Relation between ZFS magnitude (and CF strength) and nephelauxetic ratio is studied. Results obtained with and without OLI, while including or excluding distortions, reveal profoundly impact of OLI on CF energy levels and ZFSP of Ni2+ ions. The variations of SPM/CFPs due to OLI and/or distortions are analyzed in terms of directional strain components. This computational study offers insights into the structural, spectroscopic, and magnetic properties of Ni2+:Y2Ti2O7 and Ni2+:Gd2Ti2O7. Our combined modelling approach is useful for exploring other hosts doped with transition or rare-earth ions at arbitrary symmetry sites, which are of interest for computational materials science.