Effects of distortions in ion-host systems on optical spectra, crystal-field and spin-Hamiltonian parameters of Cr3+ ions doped pyrochlores Y2Ti2O7 and Y2Sn2O7: exchange charge model and superposition model calculations.

Comparative modelling of the crystal-field parameters (CFPs), CF energy levels, and effective spin-Hamiltonian parameters (SHPs), i.e., the g-factors and zero-field splitting parameter (ZFSP), D, of the ground state 4A2g of the Cr3+ dopant ions in Y2Ti2O7 and Y2Sn2O7 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 Cr3+ ions in these crystals is trigonal D3d, a symmetry adapted axis system (SAAS) conforming to Watanabe convention is adopted for CFP modelling. The calculated CF energies and SHPs for Y2Ti2O7:Cr3+ are in good agreement with the experimental results. Variations of the Racah parameter B, as well as ECM and SPM parameters for Y2Sn2-xCrxO7 are correlated with the chromium concentration (x), which affects distortions of CrO6 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-O6/Sn-O6) introduced by Cr3+ ions on the structural and optical properties as well as the SHPs of Cr3+ ions doped in Y2Ti2O7 and Y2Sn2O7.