TrivalentAl Insertion into Layered-Oxide-DerivedPerovskite Oxynitrides for Defect-Controlled Dielectric Response

Al-containing perovskite oxynitrides, SrAl0.2M0.8O2.4N0.6 (M = Nb, Ta), were synthesized by ammonolysis of layered Sr5M4O15 precursors with α-Al2O3. This reaction demonstrates that the cation-insertion chemistry of layered-perovskite-derived oxynitrides can be extended to trivalent Al3+, beyond the mono- and divalent cations explored in previous systems. Neutron Rietveld refinements revealed tetragonal perovskite frameworks with strongly anisotropic O/N distributions, where nitrogen preferentially occupies the axial anion site. Solid-state 27Al MAS NMR confirmed octahedral, largely O-rich Al coordination, whereas 93Nb MAS NMR revealed highly disordered and low-symmetry Nb-centered local environments in SrAl0.2Nb0.8O2.4N0.6 despite its well-defined average structure. Diffuse-reflectance spectroscopy showed discernible band gap widening and marked suppression of subgap absorption relative to SrNbO2N and SrTaO2N, indicating that Al incorporation modifies both the anion composition and defect-related electronic structure. Impedance analysis indicated an appreciable dielectric response for SrAl0.2Nb0.8O2.4N0.6, with the high-frequency bulk-related component giving effective dielectric constants of approximately 350–360 over the temperature range of 290–350 K. This result suggests that Nb-centered local disorder can contribute substantially to dielectric polarizability when defect-related electronic conduction is sufficiently suppressed. Trivalent-cation insertion therefore offers a useful route to tune anion chemistry, defect states, local structure, and dielectric behavior in mixed-anion perovskites.