Czochralski growth of the mixed halides BaBrCl and BaBrCl:Eu

doi:10.1016/j.jcrysgro.2015.11.032

Journal of Crystal Growth

Volume 435, 1 February 2016, Pages 42-45

https://doi.org/10.1016/j.jcrysgro.2015.11.032 Get rights and content

Highlights

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Halide crystals grown by pressurized Czochralski method (CZ).
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Cubic inch crack-free crystals of undoped and doped BaBrCl are grown.
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The growth pressure is up to 5 atm at any step of the growth.
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The method effectively prevents volatilization from melting.
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The technique makes it possible to grow stoichiometric halide crystals.

Abstract

We present results from the growth of BaBrCl and BaBrCl:Eu single crystals, using the Czochralski method. Cubic inch crack-free crystals of both undoped and 5% Eu doped BaBrCl were obtained. The BaBr₂–BaCl₂ phase diagram was acquired by differential thermal analysis revealing that the system forms a solid solution at all concentrations with no significant separation between the solidus and liquidus curves. Details of the Czochralski process used to prevent cracking are presented. The scintillation performance of the Czochralski grown crystals is presented.

Introduction

Large, crack-free scintillator crystals with excellent energy resolution and good light output are necessary for national security applications. These crystals should be produced at a rather low cost to enable large area deployment. The relatively inexpensive NaI:Tl and CsI:Tl crystals have been widely used and a higher performance crystal LaBr₃ [1] has also become a commercial product in the early 2000s but is still too costly for applications requiring large volume. Glodo et al. introduced the concept of mixed La halides in 2008 [2], [3] however the performance of the mixed La halides was not significantly better than that of the LaBr₃. A hardening effect was reported earlier [4] for CsI using small amount of Br. The full solid solution range was not reported. However the mixed Ba halides activated with Eu²⁺ and first reported in 2009 defined a new class of mixed halides with performance far superior to that of the simple binary Ba halides constituting them [5], [6], [7], [8]. Reviews of that topic have been published recently [9], [10].

BaBrI:Eu has the best scintillation performance within the family of Ba based mixed halides BaX_1−xY_x (X and Y=F, Cl, Br or I), followed by BaBrCl:Eu [8]. This paper presents the growth of BaBrCl activated by 5% Eu. Small size crystals have been grown via the vertical Bridgman–Stockbarger technique in a glassy carbon crucible sealed in an evacuated quartz tube to prevent oxidation and hydration; however the technique is limited by the lack of well-defined nucleation sites, which impedes reproducibility, the contact with the crucible and the one time use of the ampoule assembly [8], [11]. Among the alternate crystal growth techniques that can overcome these limitations, the Czochralski (CZ) method [12] may be the best option for reproducible and controlled growth of large crystals in a production environment. It is one of the oldest and most widely used crystal growth techniques for commercial production of large-size single crystalline boules including the scintillator NaI. However, there are only few reports to use the CZ method to grow novel halides [13], [14]. Growth of halides is complicated by their reactivity and corrosive nature [15], [16]. In addition Eu has a very strong affinity for oxygen, a contaminant. BaBrCl is a solid solution. Its phase diagram is of critical importance to the development of a growth technique to achieve uniform composition. Therefore we first present the phase diagram of the BaCl₂–BaBr₂ system. We then explore the possibility to use a pressurized CZ method to grow crack-free BaBrCl:Eu crystals. We present details of the growth process and the scintillation properties of the produced crystals.

Section snippets

Experimental methods

For all experiments high-purity 5N (99.999%) starting materials in the form of beads were purchased from Sigma-Aldrich. The oxygen and carbon content of the starting materials was not available. All compounds and products were handled in an Ar-filled glove box maintained below 0.1 ppm of O₂ and H₂O.

BaBr₂–BaCl₂ phase equilibrium

BaBrCl crystallizes in an orthorhombic, three-dimensional PbCl₂ structure-type with a Pnma space group. Details of its structure were reported in [11]. BaCl₂ exhibits a phase transition with a cubic high temperature phase from 925 °C to its melting point (963 °C) and a low temperature orthorhombic phase. BaBr₂ and BaCl₂ form a solid solution in the whole range of composition 0<x≤2 BaBr_xCl_2−x. While we didn׳t prepare samples with composition between x=1.8 and x=2, it appears that small amount of

Conclusion

The phase diagram of the BaCl₂–BaBr₂ (BaBr_xCl_2–x) system was determined. The result shows that mixtures of BaCl₂ and BaBr₂ forms solid solutions at all concentrations of the two binary compounds of BaCl₂ and BaBr₂ and there is no significant gap between the melting and solidification temperatures of the ternary phases. Both undoped and 5% Eu²⁺ doped BaBrCl crystals of about 62 mm in length and 1 in. in diameter were successfully grown by the modified pressurized CZ method. The cracking issues

Acknowledgments

The authors would like to thank late Chris Ramsey for his engineering expertise growth instruments, Steve Hanrahan for his professional photography, Eric C. Samulon, Gautam Gundiah, Martin Gascon and Stephen Derenzo for their valuable scientific discussions and support for this work. This work was supported by the U.S. Department of Homeland Security/DNDO and was carried out at the Lawrence Berkeley National Laboratory under Contract no. DE-AC02-05CH11231.

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Czochralski growth of the mixed halides BaBrCl and BaBrCl:Eu

Highlights

Abstract

Introduction

Section snippets

Experimental methods

BaBr2–BaCl2 phase equilibrium

Conclusion

Acknowledgments

Radiat. Meas.

Nucl. Instrum. Methods Phys. Res. A

Nucl. Instrum. Methods Phys. Res. A

J. Cryst. Growth

J. Lumin.

J. Cryst. Growth

Nucl. Instrum. Methods Phys. Res. A

Nucl. Instrum. Methods Phys. Res. A

BaBr₂–BaCl₂ phase equilibrium