Crystal structure and thermal expansion of a CsCe₂Cl₇ scintillator

Highlights

• Crystal structure of CsCe₂Cl₇ was solved through X-ray diffraction.

• Linear coefficients of thermal expansion were determined from in-situ XRD in 25–650 °C.

• Anisotropy of the a axis with respect to b and c axes (21.3 vs 27.0×10^–6/°C) was found.

• No solid–solid phase transitions were observed via XRD and thermal analysis.

Abstract

We used single-crystal X-ray diffraction data to determine crystal structure of CsCe₂Cl₇. It crystallizes in a P112₁/b space group with a=19.352(1) Å, b=19.352(1) Å, c=14.838(1) Å, γ=119.87(2)°, and V=4818.6(5) Å³. Differential scanning calorimetry measurements combined with the structural evolution of CsCe₂Cl₇ via X-ray diffractometry over a temperature range from room temperature to the melting point indicates no obvious intermediate solid–solid phase transitions. The anisotropy in the average linear coefficient of thermal expansion of the a axis (21.3×10^–6/°C) with respect to the b and c axes (27.0×10^–6/°C) was determined through lattice parameter refinement of the temperature dependent diffraction patterns. These findings suggest that the reported cracking behavior during melt growth of CsCe₂Cl₇ bulk crystals using conventional Bridgman and Czochralski techniques may be largely attributed to the anisotropy in thermal expansion.

Introduction

CsCe₂Cl₇ has been recently reported as an intrinsic scintillator for X-ray and gamma ray detection [1], [2]. It has a density of 3.6 g/cm³, scintillation light yield of ~27,000 ph/MeV and a primary scintillation decay time of ~50 ns. While possessing attractive scintillation performance for applications in medical imaging, border security, and high-energy physics, it is still early in its development and little is understood about its bulk growth behavior. A more complete understanding of relevant structural properties such as thermal expansion and existence of any solid–solid phase transitions is crucial for developing melt crystal growth of CsCe₂Cl₇ and for evaluation of its potential for large-scale manufacturing. Small size single crystals have been previously grown using the Bridgman and Czochralski technique, however attempts to produce large size crystals were not successful due to severe cracking of the boules during solidification and cooling. Among possible factors that may contribute to cracking in halide crystals is anisotropy in thermal expansion behavior. For many materials, which exhibit large or anomalous anisotropy in their thermal expansion, measures to accommodate this behavior may be incorporated into the growth process to improve the yield of large uncracked crystals and ultimately reduce their production cost. These measures may include seeding growth along an axis of minimal anisotropy and a tapered ampoule geometry [3].

Of the two congruently melting compounds existing in the CsCl–CeCl₃ binary system, CsCe₂Cl₇ is considered to be more promising for bulk growth from the melt compared with Cs₃CeCl₆, which has a transition from a high temperature phase to its room temperature stable monoclinic structure around 400°C [4]. Pyrosilicate chlorides with a general formula ARE₂Cl₇ (where A represents an alkali metal and RE represents a rare earth element) exist in a number of phases. Powder diffraction structure studies reported that they belong to either the P21/c monoclinic space group or the Pnma orthorhombic space group, yet CsCe₂Cl₇ was not investigated directly [5], [6], [7]. Several similar compounds such as RbGd₂Br₇:Ce, CsGd₂Cl₇:Ce and KGd₂Cl₇:Ce scintillators exhibit a plate-like structure and, therefore, are not promising for large size crystal growth [8]. Additional thermodynamic reports indicate that CsCe₂Cl₇ crystallizes in the hexagonal P6₃/m space group or the orthorhombic Pna2₁ space group [2], [9].

Due to the inconsistencies in literature, this study aims to produce a definitive study of the structural properties of CsCe₂Cl₇ scintillator crystals by determining its thermal expansion coefficients and searching for evidence of solid to solid phase transitions potentially responsible for the reported polycrystallinity and cracking. The hygroscopic nature of CsCe₂Cl₇ and the difficulty in producing an uncracked single crystal test specimen with known orientation makes conventional dilatometry measurements inaccessible for determining the linear coefficients of thermal expansion. As a result of this, in situ observations of the lattice expansion through temperature dependent X-ray diffraction measurements on powder samples sealed in quartz capillaries heated to the melting point were determined to be a more direct approach. This measurement, in combination with differential scanning calorimetry data, is expected to provide a more complete view of the cracking phenomenon in CsCe₂Cl₇.