Alkali metal-induced disorder in nanocrystalline fluorite-type RbxBi1-xF3-2x:Ln3+ solid solutions: structural and optical investigation

Bismuth-based compounds are widely acknowledged as a versatile class of materials. Their adaptability stems from their cost-effectiveness, low toxicity and adjustable properties. For instance, the unique features of Bi(III) compounds result from the structure-dependent stereoactivity of their 6s2 electron lone pair, which affects optical and thermoelectric properties of materials. Additionally, due to their intrinsic low phonon energy and high refractive index, bismuth-based compounds have recently spread as a promising category of host materials for luminescent centers such as Ln ions. In this research, nanostructured, crystalline fluorite-type RbxBi1-xF3-2x:Ln3+ solid solutions were successfully yielded and investigated. To disclose how composition-induced structural changes affect the material, structural (XRPD), morphological (SEM, TEM, AFM) and optical (PL-PLE, DRS) analyses were performed. In order to analyze the dopant ion site symmetry within the crystal lattice, Eu3+ was used as a luminescent probe. The UV-VIS-NIR luminescence properties of Ln-doped (Ln=Pr3+, Nd3+) cubic RbxBi1-xF3-2x and hexagonal NaBiF4, used as a benchmark, were assessed via PL and PLE measurements. Finally, preliminary temperature dependent PL analysis was employed to understand the potential of these materials for optical thermometry.