Spectroscopic, Electronic, and Magnetic Signatures of Cation Disorder in Ca3R3Ta2Ti7O26.5 (r=dy3+, Nd3+) Pyrochlores Elucidated Using Ab Initio Computations and Crystal-Field Calculations

Herein, we report spectroscopic, electronic, and magnetic investigation of the mixed-cation pyrochlores Ca3R3Ta2Ti7O26.5 (R = Dy3+, Nd3+), elucidating key signatures of A-and B-site cation disorder and oxygen nonstoichiometry. Structural characterization, Raman and infrared spectroscopy, complemented by lattice dynamical analysis, reveal disorder-induced distortions of the pyrochlore lattice and relaxation of the vibrational selection rules. The ab initio density-functional theory computations using LDA+U and LDA-1/2 approaches reproduce experimentally observed wide optical band gaps (approximate to 3 eV), highlighting the sensitivity of the electronic states near band edges to oxygen rearrangements and cation mixing. Crystal-field (CF) calculations employing the exchange charge model reproduce UV-visible and photoluminescence spectra, yielding reliable CF energy levels and wavefunctions for R3+ ions. Dy3+ ions in Ca3Dy3Ta2Ti7O26.5 retain a well-isolated (111) Ising-like Kramers doublet with large magnetic moment. The dc and ac magnetic susceptibility measurements reveal absence of long-range spin-ice freezing, with only slow spin dynamics and short-range correlations persisting arising from disorder-induced disruption of exchange interactions. Nd3+ ions in Ca3Nd3Ta2Ti7O26.5 exhibit pronounced CF-induced J mixing, reduced Ising anisotropy, and dominant antiferromagnetic interactions, leading to short-range "all-in, all-out" correlations that are enhanced under applied magnetic fields. Mean-field modeling incorporating CF effects, anisotropic exchange, and long-range dipolar interactions quantitatively accounts for the magnetic response of both compounds. Electrical resistivity measurements on Nd-based compound reveal thermally activated hopping transport, consistent with a wide-gap insulating state and disorder-induced electronic localization. Our results establish chemical disorder as an effective tuning factor controlling the spectroscopic, electronic, magnetic, and transport properties of rare-earth pyrochlores and provide a robust platform for exploring emergent phenomena in frustrated materials.