X-ray excited photoluminescence near the giant resonance in solid-solution Gd1−xTbxOCl nanocrystals and their retention upon solvothermal topotactic transformation to Gd1−xTbxF3

Abstract

Design rules for X-ray phosphors are much less established as compared to their optically stimulated counterparts owing to the absence of a detailed understanding of sensitization mechanisms, activation pathways and recombination channels upon high-energy excitation. Here, we demonstrate a pronounced modulation of the X-ray excited photoluminescence of Tb³⁺ centers upon excitation in proximity to the giant resonance of the host Gd³⁺ ions in solid-solution Gd_1−xTb_xOCl nanocrystals prepared by a non-hydrolytic cross-coupling method. The strong suppression of X-ray excited optical luminescence at the giant resonance suggests a change in mechanism from multiple exciton generation to single thermal exciton formation and Auger decay processes. The solid-solution Gd_1−xTb_xOCl nanocrystals are further topotactically transformed with retention of a nine-coordinated cation environment to solid-solution Gd_1−xTb_xF₃ nanocrystals upon solvothermal treatment with XeF₂. The metastable hexagonal phase of GdF₃ can be stabilized at room temperature through this topotactic approach and is transformed subsequently to the orthorhombic phase. The fluoride nanocrystals indicate an analogous but blue-shifted modulation of the X-ray excited optical luminescence of the Tb³⁺ centers upon X-ray excitation near the giant resonance of the host Gd³⁺ ions.