Herein, we report spectroscopic, electronic, and magnetic investigation of the mixed-cation pyrochlores $\mathrm{C}{\mathrm{a}}_3{R}_3\mathrm{T}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_7{\mathrm{O}}_{26.5}$ ($R={\mathrm{Dy}}^{3+}, {\mathrm{Nd}}^{3+}$), 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 (≈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 $R^{3+}$ ions. $\mathrm{D}{\mathrm{y}}^{3+}$ ions in $\mathrm{C}{\mathrm{a}}_3\mathrm{D}{\mathrm{y}}_3\mathrm{T}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_7{\mathrm{O}}_{26.5}$ 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. $\mathrm{N}{\mathrm{d}}^{3+}$ ions in $\mathrm{C}{\mathrm{a}}_3\mathrm{N}{\mathrm{d}}_3\mathrm{T}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_7{\mathrm{O}}_{26.5}$ 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.

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• Crystal-field excitations
• Electronic structure
• Luminescence
• Magnetic anisotropy
• Magnetic susceptibility
• Phonons
• Structural properties
• Topological materials