X-ray imaging is fundamental to industrial inspection, medical diagnosis, and scientific research. Desirable characteristics include low-dose irradiation, high stability, and high spatial resolution. Current systems often rely on scintillators—materials converting X-ray photons into visible light detectable by photodiode arrays. Conventional scintillators like CsI:Tl and LuAG:Ce, while effective, require complex and costly synthesis. Lead halide perovskites offer advantages such as facile fabrication, fast response, and good spatial resolution, but their low light yield, toxicity, and instability limit widespread adoption in high-end applications requiring low-dose exposure, safe manufacturing, real-time monitoring, and robustness. Lead-free alternatives, such as double perovskites, copper and bismuth-based metal halides, have emerged as promising candidates. While some, like Rb₂CuBr₃ and Cs₂NaTbCl₃, show high light yield, limitations like long decay times and strong afterglow hinder their application in high-contrast imaging, particularly X-ray computed tomography (CT). This article focuses on developing nontoxic halide scintillators with high light yield, fast decay, and suitable emission wavelengths. The study explores a series of nontoxic double perovskites, Cs₂Ag₀.₆Na₀.₄In₁₋ₓBiₓCl₆ single crystals, with variable Bi³⁺ content to achieve these goals.

The introduction thoroughly reviews existing X-ray imaging technologies and scintillator materials. It highlights the advantages and disadvantages of conventional scintillators (CsI:Tl and LuAG:Ce) and emerging lead halide perovskites. The limitations of lead-based perovskites—low light yield, toxicity, and instability—are emphasized, motivating the search for lead-free alternatives. The literature review also discusses previous research on lead-free emitters, including double perovskites, copper, and bismuth-based metal halides. Specific examples like Rb₂CuBr₃ and Cs₂NaTbCl₃ are mentioned, along with their limitations concerning long decay times and strong afterglow, which hinder real-time applications. This sets the stage for the current research, which aims to overcome these limitations by developing a new class of nontoxic double-perovskite scintillators.

The study synthesized a series of nontoxic double-perovskite single crystals, Cs₂Ag₀.₆Na₀.₄In₁₋ₓBiₓCl₆, with varying Bi³⁺ content. Powder X-ray diffraction (PXRD) confirmed the formation of the pure double-perovskite phase. Scanning electron microscopy-energy-dispersive spectrometry (SEM-EDS) verified the chemical compositions. Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy were used to characterize the optical properties, including PL quantum yield (PLQY), Stokes shift, and absorption band tail width (Urbach energy). Time-resolved photoluminescence (TRPL) spectroscopy, using the time-correlated single-photon counting (TCSPC) method, measured the light decay time. X-ray irradiation was employed to assess scintillation properties, including radioluminescence (RL) spectra, light yield, and afterglow. The light yield was determined by comparing the RL output with a commercial LuAG:Ce scintillator, accounting for differences in X-ray absorption efficiency. A homemade optical system was used for X-ray imaging experiments, employing Cs₂Ag₀.₆Na₀.₄In₀.₈₅Bi₀.₁₅Cl₆ wafers of various thicknesses. The spatial resolution was evaluated using a standard test pattern plate and calculated from modulation transfer function (MTF) curves derived from slanted-edge images. Thermal treatment and X-ray irradiation stability tests were conducted to assess the scintillator's long-term performance.

The introduction of Bi³⁺ into Cs₂Ag₀.₆Na₀.₄InCl₆ successfully improved the X-ray absorption efficiency and luminescence properties. The optimal Bi³⁺ content (15%) yielded a light yield of 39,000 ± 7000 photons/MeV for Cs₂Ag₀.₆Na₀.₄In₀.₈₅Bi₀.₁₅Cl₆, significantly higher than previously reported lead halide perovskites (21,000 photons/MeV) and comparable to commercial CsI:Tl. The large Stokes shift, resulting from self-trapped excitons (STEs), effectively minimized self-absorption. The light decay time was significantly reduced to the nanosecond level, suitable for dynamic real-time imaging. High-quality static and dynamic X-ray images were obtained under extremely low-dose irradiation (~1 µGyₐᵢᵣ for static and 47.2 µGyₐᵢᵣ s⁻¹ for dynamic imaging). The scintillator demonstrated excellent long-term stability under thermal treatment (85 °C for 50 h) and continuous X-ray irradiation (50 h) in ambient air. The spatial resolution of the scintillator was determined to be 4.3 lp mm⁻¹ for a 0.1 mm thick wafer, comparable to selenium direct-type X-ray imagers. The afterglow was exceptionally low, decaying to 0.1% at ~16 µs, superior to CsI:Tl and Rb₂CuBr₃, making it suitable for real-time imaging and CT applications.

The findings address the limitations of existing scintillators by demonstrating a nontoxic, high-performance alternative. The high light yield, fast decay, and negligible self-absorption of Cs₂Ag₀.₆Na₀.₄In₀.₈₅Bi₀.₁₅Cl₆ overcome the key challenges associated with lead-based perovskites and other lead-free options. The excellent stability and low afterglow further enhance its practicality for real-time and low-dose X-ray imaging applications. The achievement of high-quality imaging under extremely low-dose conditions has significant implications for reducing radiation exposure in medical diagnostics and other fields. The tunability of the material's properties through Bi³⁺ content provides flexibility in optimizing performance for specific applications. The results demonstrate the potential of double perovskites as highly competitive scintillators, surpassing lead-based alternatives not only in terms of toxicity but also overall performance.

This study successfully synthesized and characterized a new class of nontoxic double-perovskite scintillators with superior properties. Cs₂Ag₀.₆Na₀.₄In₀.₈₅Bi₀.₁₅Cl₆ exhibited a high light yield, fast decay, and excellent stability, enabling high-quality real-time X-ray imaging under extremely low-dose conditions. The findings open avenues for developing advanced X-ray imaging systems with improved safety and performance. Future work could focus on further optimizing the material's properties, exploring different dopants and crystal structures, and integrating the scintillator into practical imaging devices.

While the study demonstrates excellent performance, several limitations exist. The light yield measurement relies on comparison with a commercial scintillator, introducing some uncertainty. The spatial resolution, although high, may be further improved by reducing scintillator thickness and optimizing the coupling with the detector. The long-term stability assessment was conducted under specific conditions (85°C, 50h) and extended X-ray irradiation, and the behavior under different environmental conditions might be different. Further research is needed to fully understand the impact of various environmental factors on the scintillator's long-term performance. Finally, the scalability and cost-effectiveness of the synthesis process need to be evaluated for commercial viability.