Effects of distortions in ion-host systems on optical spectra, crystal-field and spin-Hamiltonian parameters of Cr ions doped pyrochlores YTiO and YSnO: exchange charge model and superposition model calculations.

Comparative modelling of the crystal-field parameters (CFPs), CF energy levels, and effective spin-Hamiltonian parameters (SHPs), , the -factors and zero-field splitting parameter (ZFSP), , of the ground state A of the Cr dopant ions in YTiO and YSnO is carried out. The CFPs are calculated using XRD structural data by employing two semi-empirical models: the exchange charge model (ECM) and superposition model (SPM). This two-fold approach ensures increased reliability of CFP modelling and thus of the final results. The modelled CFPs serve as the input to the crystal-field analysis/microscopic spin-Hamiltonian (CFA/MSH) program to predict CF energy levels and wave functions, and to extract SHPs. Since the site symmetry of Cr ions in these crystals is trigonal , a symmetry adapted axis system (SAAS) conforming to Watanabe convention is adopted for CFP modelling. The calculated CF energies and SHPs for YTiO:Cr are in good agreement with the experimental results. Variations of the Racah parameter , as well as ECM and SPM parameters for YSnCrO are correlated with the chromium concentration (), which affects distortions of CrO structures. We find that the SHPs originate predominantly from the spin-orbit coupling, though contributions from spin-spin and spin-other-orbit couplings are also appreciable and thus important for analysis of lattice distortions and structural disorder. The uniqueness of the SAAS used for CFP modelling is also discussed. The present study enables exploring the influence of the radial and angular distortions of host clusters (Ti-O/Sn-O) introduced by Cr ions on the structural and optical properties as well as the SHPs of Cr ions doped in YTiO and YSnO.