Defect Structure and Anion Conduction in Lanthanum Oxychloride Solid Solutions Revealed by X-ray Excited Optical Luminescence and Auger Emission

Ion transport in crystalline solids is governed by the interplay between mobile ions and the host framework; ion mobility is facilitated by local structural distortions, dynamical evolution in the local coordination environment of the mobile ion, and coupling with lattice phonons. Ion dynamics are strongly influenced by the size and hardness of the mobile ion and the rigidity and polarizability of the host framework, as well as by the nature and concentration of intrinsic or engineered defects. While the design of superionic cation conductors has attracted much recent attention, mechanisms of bulk anion diffusion and their coupling with defect dynamics remain incompletely understood. In this work, we examine the role of anion vacancies introduced by site-selective aliovalent modification of LaOCl in modifying local structure and modulating halide-ion diffusion. Distinctively, we use Dy- and Tb-ion chromophores situated on the La sublattice of LaOCl as local structure probes of the modulation of local symmetry, defect-mediated trap states, and phonon relaxation pathways using X-ray excited optical luminescence (XEOL) and Auger emission. Upon excitation of La core-levels, XEOL measurements reveal two distinct dissipative channels: (a) excitation at the giant resonance engenders nonradiative emission of Auger electrons, which is intensified with increasing concentration of chloride vacancies; (b) excitation above or below the giant resonance sensitizes La → Dy/Tb energy transfer, followed by activation of radiative recombination channels at the luminescent chromophores. Stronger blue emission from defect-mediated midgap states and thermally populated states of Dy and Tb dopants is observed. As such, the ratio of luminescence between thermally populated and thermalized states and the excitation range of Auger emission provides a sensitive measure of the concentration of chloride vacancies and maps with high fidelity to anion conductivity. Ionic conductivity in the range of 2.76 × 10^-5-4.3 × 10^-5 S/cm can be achieved at 300 °C at Cl vacancy concentrations of ca. 20 at. %, which holds promise for utilization as thermally robust and low electronic conductivity ceramic solid electrolytes of halide-ion batteries proposed as sustainable and safe alternatives to current electrochemical energy storage technologies. In addition to establishing an XEOL analytical probe of defect dynamics, the results illuminate key mechanistic understanding and provide design principles for site-selective modification and tuning of defect concentrations to modulate phonon band structure and promote anion diffusion.