Bridgman method grown $Cs_2Li_3I_5$: an inter-alkali metal scintillator with high lithium content

In this study, we report on the growth of ternary caesium lithium iodide ($Cs_2Li_3I_5$, CLI) bulk crystals, both undoped and doped with thallium (Tl) and indium (In), using the miniaturised vertical Bridgman method (mVB). X-ray Powder Diffraction (XRPD) confirmed the presence of the ternary CLI phase in all three crystals, with CLI : In appearing homogeneous structure-wise throughout the entire ingot. Measurements of radioluminescence (RL), photoluminescence emission (PL), photoluminescence excitation (PLE) spectra, and photoluminescence decay kinetics (PL decay) demonstrated that the primary luminescence centers originate from the matrix itself. When doped with thallium, the efficiency of the luminescence was significantly increased. Furthermore, CLI : Tl and CLI : In crystals exhibited emission spectra similar to those of their doped caesium iodide counterparts, CsI : Tl and CsI : In, respectively. The main component of the PL decay was 523 ns, 557 ns, and 554 ns for the undoped, Tl+-doped, and In+-doped crystals, respectively. It is worthy of note that only CLI : Tl exhibited a single exponential decay. Differential Scanning Calorimetry (DSC) measurements revealed two endothermic peaks corresponding to the eutectic and liquidus temperature for CLI and CLI : In, indicating that this ternary compound has a congruent melting behaviour. Finally, the melting point of CLI was estimated to be approximately 220 °C.

💡 Research Summary

This paper reports the successful growth of bulk single crystals of the ternary alkali halide Cs₂Li₃I₅ (CLI) using a miniaturised vertical Bridgman (mVB) technique, and investigates the influence of monovalent ns² dopants thallium (Tl⁺) and indium (In⁺) on its structural, optical, and thermal properties. Two synthesis routes were explored. “Synthesis I” involved direct mixing of purified CsI with commercial LiI, while “Synthesis II” added pre‑purified TlI or InI dopants before melting. X‑ray powder diffraction confirmed the monoclinic C2/m phase (PDF 01‑076‑1482) in all samples, but impurity phases (Li₂O, CsI, Li₂O₂, LiI) were detected in the undoped and Tl‑doped crystals, especially when LiI was not pre‑purified. The In‑doped crystal, prepared via Synthesis II, showed essentially single‑phase material throughout the ingot, highlighting the importance of raw‑material purification.

Radioluminescence (RL) measurements revealed that undoped CLI exhibits two broad bands at ~313 nm and ~468 nm. Doping with Tl⁺ or In⁺ suppresses the short‑wavelength band and enhances a broad red‑shifted emission centered between 520 nm and 540 nm. The Tl‑doped crystal reaches an RL intensity of 915 % relative to a BGO reference, corresponding to more than a 40‑fold increase over undoped CLI, and its emission maximum (≈534 nm) matches that of the well‑known scintillator CsI:Tl. The In‑doped crystal shows a similar red‑shifted band (≈522 nm) but with a lower relative intensity (≈53 %). Photoluminescence excitation (PLE) spectra for all samples contain two overlapping absorption bands near 270 nm and 295 nm, and photoluminescence (PL) emission recorded at the respective RL maxima displays broad bands with large Stokes shifts, indicating that the luminescence originates from the host matrix, likely trapped excitons, rather than from the dopant ions themselves.

Time‑resolved PL decay measurements show that undoped CLI and CLI:In exhibit multi‑exponential decays with dominant components of 523 ns and 554 ns, respectively. In contrast, CLI:Tl displays a single exponential decay of 557 ns, suggesting that Tl⁺ introduces a more uniform recombination pathway. These decay times are sufficiently distinct to enable pulse‑shape discrimination (PSD) between neutron‑induced and gamma‑induced events, a critical requirement for mixed‑field radiation detectors.

Differential scanning calorimetry (DSC) identified two endothermic peaks for CLI and CLI:In around 220 °C, corresponding to eutectic and liquidus transitions. The presence of a single, well‑defined melting point indicates congruent melting behavior, which simplifies crystal growth and improves reproducibility.

The authors conclude that Cs₂Li₃I₅ combines a high lithium content (three Li atoms per formula unit) with a relatively low melting point (~220 °C), making it attractive for neutron detection applications that benefit from the high (n,α) cross‑section of ⁶Li. Tl⁺ doping dramatically boosts scintillation efficiency while preserving a decay time compatible with PSD, positioning CLI:Tl as a promising candidate for compact, multifunctional detectors capable of simultaneous neutron, gamma, and X‑ray detection. However, the material’s strong hygroscopicity demands strict handling under inert atmosphere, and the presence of impurity phases when using unpurified LiI underscores the need for improved raw‑material processing for scalable production. Future work should focus on detailed deconvolution of the red‑shifted emission band, optimization of dopant concentrations, and long‑term stability studies under realistic operating conditions.