A Mixed-Anion Design Approach for High-Efficiency X-ray Scintillators: A Study of KLn(SO₄)F₂ (Ln = Eu, Gd, Tb, Dy, Ho, and Tm)

High-quality single crystals of mixed anion rare earth sulfate fluorides KLn(SO₄)F₂ (Ln = Eu, Gd, Tb, Dy, Ho, and Tm), monoclinic space group <i>P</i>2₁/<i>m</i>, were obtained using an HF-free mild hydrothermal synthesis. Thermal, magnetic, and optical properties of these materials were investigated after determining their crystal structures and their chemical compositions using a combination of single-crystal and powder X-ray diffraction. Notably, the photoluminescence and radioluminescence behavior of KEuSO₄F₂ and KTbSO₄F₂ were studied for the first time, featuring the intense green radioluminescence of KTbSO₄F₂, which is comparable to the integrated intensity of the commercial bismuth germanium oxide (Bi₄Ge₃O₁₂) scintillator, while KEuSO₄F₂ scintillates in red with an integrated intensity of 49% of Bi₄Ge₃O₁₂. The experimental results indicate that doping with appropriate amounts of Tb³⁺ significantly improves the luminescence performance of KEuSO₄F₂.