Local structure and luminescence in Cs-based metal halide double perovskites

Licentiate thesis, 2026

Metal halide double perovskites are currently attracting large interest, because of their luminescent properties and application potential for various optoelectronic devices. However, fundamental questions surrounding the relationship between local structure and luminescence remain unresolved for these materials, which limits the rational design of better materials and devices. This thesis concerns investigations of the local structure and luminescence in two important metal halide double perovskites, namely Cs₂AgInCl₆ and Cs₂NaEuCl₆, by using a combination of thermoluminescence, photoluminescence, and raman spectroscopy techniques.

The investigations of Cs₂AgInCl₆ focus on how Na⁺ substitution at the Ag⁺ site and Bi³⁺ doping at the In³⁺ site, i.e. Cs₂Na_xAg_1-xIn_1-yBi_yCl₆ (x=0-1; y = 0, 0.025), affect the local structure, luminescence and charge-trapping defects in the materials. Up to six energetically different families of charge-trapping defects are identified. Na⁺ substitution changes the charge-trapping landscape in a non-systematic way, while complete replacement of Ag⁺ by Na⁺ suppresses both charge trapping and the characteristic broadband luminescence as commonly observed in these materials. Bi³⁺ doping promotes deeper trapping states while simultaneously enhancing the intensity of broadband luminescence. The results from Raman spectroscopy indicate that these deeper traps originate from Ag⁺ vacancies associated with nearby BiCl₆ clustering. Increased BiCl₆ clustering may therefore provide a route to materials combining deeper charge trapping with intense broadband luminescence.

The investigations of Cs₂NaEuCl₆ focus on the nature of phonon-induced hypersensitive transitions due to Eu³⁺. The doubly degenerate e_g vibrational mode, corresponding to axial distortions of the EuCl₆ octahedra, progressively relaxes the parity restriction and enhances the hypersensitive ⁵D₀→⁷F₂ transition by a factor of 2-3 relative to the magnetic-dipole ⁵D₀→⁷F₁ transition. Direct excitation of the Eu^{3+ 5}L_6-7 manifold is shown to bypass host-mediated reverse energy transfer through a defect state, resulting in a photoluminescence quantum yield approaching 70%, which is comparable to that of state-of-the-art Eu³⁺-doped phosphors. These findings expand the possibilities for designing hypersensitive-transition-based emitters and provide insight into the competing processes that ultimately limit their efficiency.