🔬 materials science

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

This study reports the successful growth of undoped and Tl/In-doped $Cs_2Li_3I_5$ bulk crystals via the miniaturised vertical Bridgman method, characterizing their structural homogeneity, congruent melting behavior, and enhanced luminescence properties that closely resemble their doped caesium iodide counterparts.

Original authors: Katerina Krehlikova, Vojtech Vanecek, Robert Kral, Romana Kucerkova, Petra Zemenova, Jan Rohlıcek, Petr Prusa, Katerina Rubesova

Published 2026-02-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Katerina Krehlikova, Vojtech Vanecek, Robert Kral, Romana Kucerkova, Petra Zemenova, Jan Rohlıcek, Petr Prusa, Katerina Rubesova

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a super-sensitive "security camera" that can see two very different types of invisible intruders at the same time: neutrons (tiny, ghostly particles) and gamma rays (high-energy light). Usually, you need two different cameras to catch both, but scientists want a single crystal that can do the job for both.

This paper is about growing a new type of "magic crystal" called Cs₂Li₃I₅ (or CLI for short) to see if it can be that single camera. Here is the story of how they made it and what they found, explained simply.

1. The Recipe: Baking a Crystal Cake

Think of the crystal as a cake. To make it, the scientists mixed two main ingredients: Cesium Iodide (like a heavy, dense flour) and Lithium Iodide (a lighter, reactive ingredient). They wanted to bake a "ternary" cake, meaning a perfect blend of three elements (Cesium, Lithium, and Iodine) rather than just a mix of two.

The Oven: They used a special method called the Bridgman method. Imagine a long, thin tube filled with the melted ingredients. They slowly pulled this tube down through a hot zone, like pulling a candy cane through a warm hand. As the tube moved down, the liquid cooled and turned into a solid crystal, growing from the bottom up.
The Flavorings: They made three batches:
1. Plain: Just the basic mix.
2. Thallium-flavored: Added a tiny pinch of Thallium.
3. Indium-flavored: Added a tiny pinch of Indium.

2. The Quality Check: Is the Cake Pure?

After baking, they had to check if the cake was actually what they wanted or if it was a messy mix of burnt flour and raw dough.

The X-Ray Scan: They used X-rays to look at the crystal's internal structure.
- The Plain Cake: It was a bit messy. It had the right "flavor" (the Cs₂Li₃I₅ phase), but it also had chunks of un-mixed ingredients (like leftover Lithium Iodide) scattered inside.
- The Indium Cake: This one was the winner! It was a perfectly uniform, pure crystal from top to bottom.
- The Thallium Cake: It was mostly good, but had some impurities in the middle and bottom.
The Lesson: The scientists realized that for the Indium cake to be so perfect, they had to use a different mixing technique and purify their ingredients better. The Indium batch proved that a pure crystal is possible.

3. The Light Show: How It Glows

When these crystals are hit by radiation (like neutrons or gamma rays), they don't just sit there; they glow. This is called scintillation. Think of it like a firefly blinking when it gets a tap.

The Plain Crystal: It glowed, but not very brightly. It had two main colors of light: a blue-ish one and a green-ish one.
The Thallium Crystal: This was the star of the show. Adding Thallium made it glow 40 times brighter than the plain version! It glowed a deep green color (around 534 nm), which is very similar to how standard Cesium Iodide crystals glow when doped with Thallium.
The Indium Crystal: It also glowed brighter than the plain one, with a greenish-yellow light (around 522 nm), similar to Indium-doped Cesium Iodide.

The Big Discovery: Even though they added Thallium and Indium to make it brighter, the scientists found that the "engine" making the light was actually the crystal itself (the matrix), not just the added flavorings. The added ions just helped the crystal hold onto the energy better, like a better battery, making the light last longer and shine brighter.

4. The Timing: How Fast Does the Blink Happen?

In security, you need to know when something happened. The scientists measured how long the "blink" of light lasted.

All three crystals blinked for about 550 nanoseconds (that's 0.00000055 seconds).
Interestingly, the Thallium-doped crystal blinked in a perfectly smooth, single rhythm (like a metronome), while the others had a tiny, fast "hiccup" at the start. This smooth rhythm is great for sorting out different types of radiation.

5. The Melting Point: How Hot Does It Get?

To grow these crystals, you have to melt them first. The scientists wanted to know exactly when this "magic mix" turns from solid to liquid and back again.

They heated the crystals in a special machine (DSC) and watched the temperature.
They found the crystal starts to melt around 188–190°C (a low temperature, like a very hot oven) and fully melts at 220°C.
The Challenge: When they cooled it back down, the liquid wanted to stay liquid for a long time before turning solid again (a phenomenon called "undercooling"). It's like water in a freezer that refuses to turn into ice until it gets much colder than 0°C. This makes growing perfect crystals tricky because the liquid gets "super-cooled" and might crack or form weird shapes when it finally freezes.

Summary

The scientists successfully grew a new type of crystal (Cs₂Li₃I₅) that is a strong candidate for detecting both neutrons and gamma rays.

The Good News: It glows very brightly (especially when doped with Thallium), it has a high density (good for stopping radiation), and it contains a lot of Lithium (essential for catching neutrons).
The Catch: Growing a perfect, pure crystal is hard because the ingredients are tricky to mix, and the liquid likes to stay liquid too long before freezing.
The Verdict: With the right recipe (using the Indium method) and better ingredient purification, this crystal could become a powerful tool for seeing invisible radiation in a single, compact device.

Technical Summary: Bridgman Method Grown Cs2Li3I5 Scintillators

Problem Statement
Neutron detection is critical for applications ranging from nuclear power and security to medicine. While the $^6$ Li(n, $\alpha$ ) $^3$ H reaction offers advantages for pulse shape discrimination and gamma-ray rejection, there is a need for alternative materials that can detect mixed neutron/gamma fields in a single detector. Ternary alkali halides, specifically caesium lithium iodide (Cs $_2$ Li $_3$ I $_5$ , or CLI), have been proposed as such materials due to their high lithium content. However, despite previous reports of thin films and basic crystallographic data, the growth of large bulk CLI single crystals and the characterization of their thermal and optical properties had not been reported. Furthermore, the phase stability and congruent melting behavior of the ternary CLI compound remained unverified.

Methodology
The study employed the miniaturised vertical Bridgman method (mVB) to grow bulk crystals of undoped Cs $_2$ Li $_3$ I $_5$ (CLI), as well as crystals doped with thallium (CLI:Tl) and indium (CLI:In) at 0.1 at%. Two synthesis approaches were tested:

Synthesis I: Direct synthesis of purified CsI and raw LiI, followed by zone refining and powder grinding.
Synthesis II: Purification of CsI, followed by the addition of stoichiometric LiI and dopants (TlI or InI) within a glovebox, followed by melting to ensure homogenization.

The crystals were grown in a single-zone, inductively heated furnace with a pulling rate of 0.6 mm h $^{-1}$ . To prevent degradation, all handling was performed in an inert nitrogen atmosphere due to the hygroscopic nature of the materials.

Characterization techniques included:

X-ray Powder Diffraction (XRPD): To confirm phase purity and identify secondary phases across the front, middle, and tail sections of the ingots.
Optical Spectroscopy: Radioluminescence (RL), photoluminescence (PL), and photoluminescence excitation (PLE) spectra were measured at room temperature, alongside PL decay kinetics.
Differential Scanning Calorimetry (DSC): To determine thermal properties, including melting points and eutectic behavior.

Key Results

Crystal Growth and Phase Purity: XRPD confirmed the presence of the ternary Cs $_2$ Li $_3$ I $_5$ phase in all samples. The CLI:In crystal exhibited the highest homogeneity, appearing as a pure monoclinic phase throughout the ingot (except for trace unknown impurities in the tail). In contrast, undoped CLI and CLI:Tl crystals showed significant phase segregation, with secondary phases (Li $_2$ O, CsI, Li $_2$ O $_2$ , LiI) present in the middle and tail sections. The study concluded that Synthesis II combined with the purification of raw LiI is necessary for high-quality growth.
Thermal Properties: DSC measurements revealed two endothermic peaks for both CLI and CLI:In, corresponding to a eutectic point (188–190 °C) and a liquidus temperature. By correlating the melting temperatures with the phase purity determined by XRPD, the authors estimated the congruent melting point of pure CLI to be approximately 220 °C. The cooling curves indicated a high degree of undercooling (50 °C), which poses challenges for crystal growth but suggests potential energy efficiency advantages for commercialization.
Luminescence Properties:
- Undoped CLI: Exhibited two broad emission peaks at 313 nm and 468 nm. The primary luminescence was attributed to trapped excitons (TE) within the matrix.
- Doped Crystals: Doping with Tl $^+$ and In $^+$ suppressed the 313 nm peak and shifted the main emission to the red (534 nm for Tl, 522 nm for In), resembling the emission of CsI:Tl and CsI:In, respectively.
- Efficiency: The CLI:Tl crystal demonstrated a radioluminescence efficiency approximately 900% of the BGO reference (over 40 times higher than undoped CLI).
- Decay Kinetics: All crystals showed a dominant decay component around 550 ns. Notably, only the CLI:Tl crystal exhibited a single exponential decay (557 ns), whereas undoped CLI and CLI:In contained minor fast components. This suggests that ns $^2$ ion doping enhances carrier trapping and stabilizes impurity-trapped excitons.

Significance and Claims
The paper establishes Cs $_2$ Li $_3$ I $_5$ as a promising candidate for combined neutron/gamma detection due to its high lithium content (30.0 at%), density (3.93 g cm $^{-3}$ ), and effective atomic number ( $Z_{eff}$ = 53). The study successfully demonstrates the feasibility of growing bulk CLI crystals and confirms their congruent melting behavior.

The authors claim that the primary luminescence centers originate from the CLI matrix itself, with doping (specifically Tl $^+$ ) significantly enhancing luminescence efficiency through the formation of impurity-trapped excitons. The high radioluminescence efficiency of CLI:Tl, comparable to established CsI:Tl scintillators, combined with the material's ability to facilitate pulse shape discrimination (inferred from the distinct decay times and high lithium content), supports its potential for use in mixed radiation fields. The study concludes that while high undercooling presents a growth challenge, the low melting point of ~220 °C offers a distinct advantage for reduced energy consumption in future commercial applications.