Crystal structure and thermal expansion of CsCaI3:Eu and CsSrBr3:Eu scintillators
Introduction
In recent years, much research has been focused on the development of novel scintillator materials suitable for gamma ray spectroscopy for national security applications. A large part of this effort has been focused on metal halide compositions that can be grown from the melt in relatively large sizes. Compositions such as LaBr3:Ce and SrI2:Eu are currently the top performers, with high light yields and excellent energy resolutions [1], [2]. Many of these compositions are grown via the vertical Bridgman method, yielding single crystals up to 2 inches in diameter [3], [4]. However, there are challenges in the development of metal halide crystals; for example, the majority of the halide scintillators are hygroscopic. As a result, sealed quartz ampoules are used during growth to prevent exposure to the atmosphere. One unintended side effect is the constriction on the crystal during heating and cooling, which can lead to stresses and cracking due to thermal expansion and contraction. Rapid changes in temperature, which detectors deployed in the field will undergo, can also induce cracking in grown crystals due to thermal stress. In order to better understand these properties, the thermal expansion coefficients have been calculated for crystals such as SrI2, CsCe2Cl7, and Lu2SiO5 [5], [6], [7] via high temperature powder X-ray diffraction (XRD).
National security applications require large crystals with uniform optical quality. Two potential compositions that have been studied at the University of Tennessee are CsCaI3:Eu and CsSrBr3:Eu [8], [9]. These materials show promise for gamma ray spectroscopy, with light yields of 40,000 ph/MeV and energy resolution of 4% and 5% at 662 keV, respectively. Both compositions are distorted perovskites with cloudy appearances due to solid-to-solid phase transitions [10], [11], [12]. Previous work has shown that eliminating phase transitions yields superior crystals; for instance, substituting some of the iodine in CsCaI3:Eu suppresses the phase transition to below room temperature, resulting in extremely transparent crystals. Both compounds crystalize into a previously unknown phase, and transform into room temperature orthorhombic Pnma phases at ∼250 °C [13], [14]. The goal of this work is to utilize powder diffraction at high temperatures to resolve these phases, as well as to quantify the thermal expansion in both CsCaI3:Eu and CsSrBr3:Eu.
Section snippets
Experimental
Due to the hygroscopic nature of these compositions, special procedures were taken during preparation, handling, and measurement. All raw materials were purchased from APL or Sigma Aldrich in the form of anhydrous beads with purity levels of 99.99% or greater. The crystals for powder diffraction were prepared inside of a nitrogen maintained glovebox with less than 1 ppm oxygen and moisture. Starting materials of CsI, CaI2, CsBr, SrBr2, and EuI2 were loaded into a quartz ampoule with a ratio of
Results and discussion
Scans on both compounds were completed up to 400 °C, well below the melting temperatures of both compositions (686 °C CsCaI3:Eu and 760 °C for CsSrBr3:Eu). Fig. 1 contains the patterns acquired for CsCaI3:Eu, while Fig. 2 contains those for CsSrBr3:Eu. The low temperature patterns for both materials had similar peaks in accordance with their published structures. As seen in the region of 20–30° 2θ, both patterns lose peaks as they transform to high symmetry phases at their transition
Conclusion
High temperature powder X-ray diffraction patterns were collected for scintillator compounds CsCaI3:Eu and CsSrBr3:Eu. Both compositions undergo a reversible phase transition when heated from their orthorhombic room temperature phases to higher symmetry phases that leaves the crystals cloudy. The high-temperature phases of each composition were resolved via Rietveld refinement for the first time in this work. The lattice parameters as a function of temperature were measured using sequential
Acknowledgements
This work has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded grant #2012-DN-077-ARI067-05. This support does not constitute an express or implied endorsement on the part of the Government. The X-ray diffraction experiments were performed using the instruments procured through the general infrastructure grant of DOE-Nuclear Energy University Program #DE-NE0000693.
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