Optical and dielectric properties of Sillén Bi₂Y_1-xM_xO₄Cl (M = Cu, Cd)

Mixed-anion solids with reduced dimensionality and layered (2D) structures exhibit a range of advantages arising from their tunable bonding environments and structural flexibility. Studies involving cationic substitution in the Sillen family of bismuth oxychloride-containing triple fluorite blocks, Bi₂YO₄Cl, to impart new functions are scarce and further investigations are required. In this study, we examined the optical and dielectric property changes in Bi₂YO₄Cl by introducing Cu²⁺ and Cd²⁺ at the Y³⁺ site. Up to 20 mol% Y³⁺ in Bi₂YO₄Cl could be substituted with Cu²⁺ and Cd²⁺ while retaining the layered tetragonal structure (S.G. P4/mmm). The inclusion of these two divalent ions increased the unit cell volume. Doping with Cu²⁺ and Cd²⁺ ions produced nearly equivalent amounts of Bi⁵⁺ ions, as confirmed by XPS analysis and iodometric redox titration. The EPR spectra of Bi₂Y_0.80Cu_0.20O₄Cl indicated a pseudo-octahedral environment around Cu²⁺. The absorption edge in the UV-visible spectra of the Cu²⁺ and Cd²⁺ doped Bi₂YO₄Cl samples was redshifted, leading to a reduction in the optical bandgap. For the Cu²⁺ doped sample, the CIE 1976 a* coordinate became more negative, indicated by the deepening of the green shade. The a* value became positive with the addition of Cd²⁺, imparting a reddish tinge to the sample. Temperature-dependent dielectric measurements of Cu²⁺ and Cd²⁺-doped samples in the frequency range of 1-10⁴ Hz revealed Maxwell-Wagner-type polarization and pseudo-Debye-type behavior, respectively.