Discovery of New Compounds and Scintillators of the A4BX6 Family

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Discovery of New Compounds and Scintillators of the ABX Family: Crystal Structure, Thermal, Optical, and Scintillation Properties Jesse Johnson, Mariya Zhuravleva, Luis Stand, Bryan Charles Chakoumakos, Yuntao Wu, Ian Greeley, Daniel Joseph Rutstrom, Merry Koschan, and Charles L. Melcher Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00661 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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Crystal Growth & Design

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Discovery of New Compounds and Scintillators of the A4BX6 Family: Crystal Structure, Thermal, Optical, and Scintillation Properties

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*Jesse A. Johnson II †‡⸹, Mariya Zhuravleva †‡, Luis Stand †‡, Bryan C. Chakoumakos⸹, Yuntao Wu †‡, Ian Greely †‡, D. Rutstrom†‡, Merry Koschan †‡, Charles L. Melcher †‡

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†

Scintillation Materials Research Center, University of Tennessee, Knoxville, Tennessee 37996, United States ‡

Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States

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⸹

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ABSTRACT

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We report the discovery of four new compounds of Eu2+ doped scintillators [Cs3Rb]SrI6, [Cs3K]SrI6, [Cs3Rb]CaI6, and [Cs3K]CaI6. A compositional engineering strategy dictated by the structural systematics of the base compounds Cs4CaI6 and Cs4SrI6 (space group 𝑅3̅𝑐-hexagonal basis) was conducted to confirm an ideal solid solution for this system. Substitution of smaller monovalent cations Rb1+ and K1+ for Cs1+, maintained the crystal structure, producing an ordered substitution. Crystals were initially grown doped with 0.5% Eu to compare scintillation properties, and a Eu2+ optimization study determined that 7% Eu was the ideal concentration. A 56 mm3 crystal of composition [Cs3.5Rb0.5]SrI6:Eu 7% gave the best measured scintillation performance with an absolute light yield of 75,000 ph/MeV and energy resolution of 3.3% at 662 keV.

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Jesse A. Johnson II

Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831, United States

University of Florida 100 Rhines Hall Gainesville, FL 32611 Phone: 865-771-1078 Fax: 352-392-7219 E-mail: [email protected]

K1 Cs1 Sr1

http://www.engr.utk.edu/smrc/

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ACS Paragon Plus Environment

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Discovery of New Compounds and Scintillators of

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the A4BX6 Family: Crystal Structure, Thermal,

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Optical, and Scintillation Properties

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*Jesse A. Johnson II †‡⸹, Mariya Zhuravleva †‡, Luis Stand †‡, Bryan C. Chakoumakos⸹, Yuntao

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Wu †‡, Ian Greely †‡, D. Rutstrom†‡, Merry Koschan †‡, Charles L. Melcher †‡

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†

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United States

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Page 2 of 29

Scintillation Materials Research Center, University of Tennessee, Knoxville, Tennessee 37996,

‡

Department of Materials Science and Engineering, University of Tennessee, Knoxville,

Tennessee 37996, United States

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⸹

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37831, United States

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*Jesse A Johnson II : [email protected]

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ABSTRACT

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We report the discovery of four new compounds of Eu2+ doped scintillators [Cs3Rb]SrI6,

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[Cs3K]SrI6, [Cs3Rb]CaI6, and [Cs3K]CaI6. A compositional engineering strategy dictated by the

16

structural systematics of the base compounds Cs4CaI6 and Cs4SrI6 (space group 𝑅3̅𝑐-hexagonal

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basis) was conducted to confirm an ideal solid solution for this system. Substitution of smaller

Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee,


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monovalent cations Rb1+ and K1+ for Cs1+, maintained the crystal structure, producing an ordered

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substitution. Crystals were initially grown doped with 0.5% Eu to compare scintillation properties,

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and a Eu2+ optimization study determined that 7% Eu was the ideal concentration. A 56 mm3 crystal

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of composition [Cs3.5Rb0.5]SrI6:Eu 7% gave the best measured scintillation performance with an

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absolute light yield of 75,000 ph/MeV and energy resolution of 3.3% at 662 keV.

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Introduction

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Inorganic metal halide scintillators have continued to be prevalently used as key

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components for advanced radiation detection devices of gamma and X-rays in the fields of high

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energy physics, astronomy, nuclear medical imaging, and homeland security. Tl-doped NaI and

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CsI, as well as Na-doped CsI have predominantly been the scintillators of choice for most

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applications due to their low material cost, high growth yields, mechanical robustness, and

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relatively high light yields between 38,000 and 59,000 ph/MeV.1 They are well suited for single

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photon-emission computed tomography, and CsI(Tl) is particularly useful in X-ray imaging

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applications and dark matter exploration.2-4 However, their lack of radioisotope discrimination

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ability, or energy resolution, makes them less than ideal for homeland security and nuclear

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nonproliferation applications. Within the past decade, the emergence of LaBr3(Ce) and SrI2(Eu),

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both with high light yields and excellent energy resolution (~ 3% at volumes > 16 cm3), has

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accelerated the quest for more economically favorable high-performance scintillators with

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comparable scintillation peformance.5-7

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The search for the ideal scintillator for any given application has expanded past the

2

traditional binary compounds to exploring ternary, quaternary, and even quinary systems.8-12

3

Recently, our group has reported the discovery of the known compound Cs4CaI6 as a scintillator,

4

and Cs4SrI6 as a new compound and scintillator when doped with Eu2+. Both compounds

5

crystallize from the melt in the rhombohedral K4CdCl6-structure type (space group 𝑅3̅𝑐-

6

hexagonal basis) and are easy to grow. They also have promising scintillation properties with an

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energy resolution at 662 keV and light yield of 3.6%-50,000 ph/MeV for Cs4CaI6:Eu 4%, and

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3.3%-60,000 ph/MeV for Cs4SrI6:Eu 4% (~6 mm3 crystals). 13 Contiguous with their discovery, it

9

was realized that few alkali-earth alkali halides in the A4BX6 family (A- alkali, B- alkali earth,

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X- halogen) had been reported in literature. An in-depth structural study by Beck et al. focusing

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on ternary Cd halides, A4CdX6, produced a structure field diagram showing the strict dependence

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of the A4BX6 structure-types on the A/X and B/X ionic size ratios.14 Several known compounds

13

including Cs4CaI6 and Rb4CaI6, both congruently melting, were included in the construction of

14

the diagram. They suggested that the K4CdCl6 structure-type, with the structural formula

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[A3B]CX6, could form structurally stable mixed crystals with ordered substitution by introducing

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smaller alkali ions onto the B1+ site.14

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The goal of this work was to follow a composition-engineering strategy dictated by the

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structural systematics of the K4CdCl6 structure-type, and to evaluate the effects on the structural

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stability, thermal properties, ease of growth, and ultimately the scintillation properties. The

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desired properties of a scintillator are specific to the application; however, some of the most

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important properties include: (i) high detection efficiency, (ii) ease of manufacture, (iii) good

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energy resolution, (iv) high light yield, and (v) proportional response to excitation energy.15

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Good detection efficiency is a function of high density and Zeff, while ease of manufacture is


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dependent on low cost raw materials, congruent melting behavior, and growth techniques

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capable of producing high yields, such as the Czochralski and Bridgman methods. Since

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scintillators are insulators, in most cases good spectroscopic performance in terms of energy

4

resolution, light yield, and proportional response is dependent on the doping of activators,

5

namely rare-earth ions, that introduce energy levels within the host bandgap allowing for

6

radiative recombination of excitons. The work presented here focused on the crystal growth of

7

Cs4CaI6 and Cs4SrI6 solid solutions with Rb1+ and K1+ substitutions, characterization of key

8

scintillation properties, structural properties, and evaluating the potential for high volume

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production while maintaining good performance in terms of gamma-ray spectroscopy.

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Experimental

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Crystal growth. We used the vertical Bridgman technique to grow 8 g single crystals

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from the melt in vacuum sealed Ø7 mm quartz ampoules with Ø2 mm capillaries for self-

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seeding. This study was initiated by growing crystals with nominal compositions of [Cs4-xBx]C1-

14

yEuyI6

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characterization of scintillation and structural properties without significantly altering the

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structure from Eu2+ substitution into the host matrix. The latter part of the study involved

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growing crystals with varying Eu2+ concentrations to identify the optimal concentration that

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produced the best scintillation properties. Raw materials of anhydrous CsI, KI, RbI, SrI2, CaI2

19

and EuI2 were acquired from APL Engineered Materials Inc, and were loaded into the quartz

20

ampoules in a dry nitrogen atmosphere (H2O and O2 < 0.5 ppm) due to their hygroscopicity.

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Previous work by our group, Stand et al., showed that Cs4CaI6(Eu) is nearly twice as hygroscopic

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as LaBr3, while Cs4SrI6(Eu) is comparable to NaI(Tl) and slightly more hygroscopic than

23

CsI(Tl).13 All raw materials used were of at least 4N purity apart from RbI, (3N), due to cost.

(B = Cs,Rb,K and C = Ca,Sr) doped with 0.5 mol % Eu2+. This allowed for


5


1

The loaded ampoules were vacuum heat treated at a pressure of 10-6 torr and temperature of

2

220oC for 3 hrs to remove any residual water vapor and oxygen impurities. The ampoules were

3

subsequently sealed at 10-6 torr and transferred to a Bridgman growth furnace. Prior to growth,

4

the compounds were synthesized by melting the raw materials at a temperature 50oC above the

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melting point of the highest melting constituent. This temperature was held for 8 hrs, slowly

6

cooled to room temperature, then the ampoule inverted, and the process repeated to ensure

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proper mixing. Single crystals were then grown through temperature gradients of 25 or 43oC/cm

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at a rate of 1 mm/hr, and cooled to room temperature at a rate of 10oC/hr.

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Thermodynamic properties. Thermodynamic profiles were measured via Differential

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Scanning Calorimetry (DSC) using a Labsys EVO instrument, which revealed melting/freezing

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points, congruency, and solid-solid phase transitions. A heat-cool-heat cycle was used in a high

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purity Ar atmosphere at a rate of 5oC/min to a temperature ~50oC above the expected melting

13

point of the new compounds. Covered alumina crucibles were used, and baseline measurements

14

were conducted to subtract any thermal contributions from the crucible. Approximately 50 mg

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crystals were loaded into the covered alumina crucibles in a dry nitrogen atmosphere. The

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crucibles were then loaded into a lidded glass jar to minimize exposure of the crystals to ambient

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air during transfer to the instrument.

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Crystal structure refinement. High resolution single crystal X-ray diffraction was used

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to derive the unknown standard structural parameters of new compounds such as: lattice

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constants, Wyckoff atomic positions, and atomic displacements. Approximately 0.001 mm3

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crystals were suspended in paratone oil, and mounted from a plastic loop attached to a copper

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pin/goniometer. Measurements were conducted at 260K in a dry nitrogen flow using a Rigaku

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XtaLAB PRO diffractometer with Mo Kα radiation (λ = 0.71073 Å), a Dectris Pilatus 200K


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detector, and Oxford N-Helix cryocooler. Peak indexing and integration were done using the

2

CrysAlisPro and AutoChem3.0 structure analysis software.16 Structure solution, and refinement

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were done using the SIR-2011 in WinGX and SHELX-2013 software packages.17-19

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Phase purity analysis. Phase purity was evaluated by measuring powder X-ray

5

diffraction profiles of ~2 g samples using a PANalytical Empyrean diffractometer with θ-θ

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goniometer (45 kV and 40mA), Cu Kα radiation (λ = 1.5406 Å), and a PIXel 3D solid-state

7

detector. Initially, an instrument parameter file was created to determine the instrumental

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contribution to peak broadening by measuring a powdered form of NIST SRM 660c, LaB6,

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which exhibits no strain. A diffraction pattern was collected in the range of 15-145o 2θ, and an

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initial parameter file was created by peak fitting, followed by conducting a Rietveld refinement.

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The resulting instrument parameters were saved for evaluating the materials of interest. Samples

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were prepared from single crystals by grinding to a fine powder and loaded into a zero-

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background Si sample holder. The samples were then covered with a Kapton foil in an airtight

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sample holder. Data were collected at range from 15-90o 2θ at a step-size of 0.013o 2θ with 80

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s/step. The measurement optics for the incident beam included 0.02 rad soller slits, 1/8o

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programmable divergence slit, and a 1/4o anti-scatter slit. For the diffracted beam, the

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measurement optics included a 1/4o programmable anti-scatter slit, 0.02 rad Soller slits, and a Ni

18

filter. Phase analysis and structural parameters were evaluated via Rietveld Refinements with

19

the General Structure Analysis System II software (GSASII). A χ2 value < 1.90% was achieved

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for each refinement.

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Optical properties measurements. Photoluminescent (PL) emission and excitation

22

spectra were measured using a HORIBA Jobin Yvon Fluorolog-3 spectro-fluorometer. A 450 W

23

continuous Xe-broad spectrum lamp was used as the excitation source, passing through an


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excitation monochromator set at 1 nm bandpass for wavelength selection. The emission

2

monochromator was also set to 1 nm bandpass. PL lifetime was also measured using a HORIBA

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Jobin Yvon Fluorolog-3 spectro-fluorometer, adapted for time correlated single photon counting.

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A 370-nm wavelength HORIBA Jobin Yvon NanoLED excitation source with < 2 ns pulse width

5

was used as an excitation source.

6

Scintillation properties measurements. The absolute light yield was measured by using

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a Hamamatsu R2059 photomultiplier tube (PMT) operated at -1500 Vbias, and estimating the

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photoelectron yields via the single photoelectron peak method. The crystals were housed in a

9

quartz holder wrapped in Teflon and filled with mineral oil, and the quartz holder was coupled to

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the PMT using mineral oil. The coupled PMT and quartz holder were also covered with Teflon

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to maximize light collection. The crystals were irradiated using a 15 μCi Cs137 source and the

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light was collected using a pulse processing chain consisting of the PMT, a Canberra model 2005

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pre-Amp, an Ortec 672 Amp set at a 10μs shaping time, and a Tukan 8k multichannel analyzer.

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The light yield was determined by accounting for the amplifier gain, gamma source, and was

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also corrected by the emission weighted quantum efficiency of the PMT. This was determined by

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the wavelength response of the PMT and measuring the X-ray excited luminescence spectra

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using an X-ray tube with Tungsten anode operated at 35 kV and 0.1 mA.

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Energy resolution was measured using a high quantum efficiency Hamamatsu R6231-100

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PMT operated at -1000 Vbias. Crystals were housed in a quartz holder wrapped in Teflon and

20

filled with mineral oil, and the holder was coupled to the PMT using mineral oil. The coupled

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PMT and quartz holder were covered in Teflon to maximize light collection. The energy

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resolution was calculated as the full width at half maximum divided by the centroid of the Cs137

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662 keV photopeak. Non-proportional light yield measurements of γ-photon energies were also


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conducted using this setup. We used Cd109, Cs137, Am241, Co57, and Ba133 sources using a range

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of γ-photon energies from 22.1 to 356 keV.

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Scintillation decay time was measured using the Bollinger-Thomas time-correlated

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single-photon counting technique under Ba133 source excitation.

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Results and Discussion

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Crystal growth. All crystals doped with 0.5 mol % Eu2+ were transparent and mostly

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crack free with the exception of the Rb1+ containing compounds, which were heavily cracked.

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Figure 1a-f shows the grown crystal boules prior to removal from the quartz ampoules. Ø7 mm

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x 4 mm slabs were cut from the crack-free boules and polished down to ~3 mm thick using 600,

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800, and 1200 grit SiC polishing pads with mineral oil and are shown in Figure 1g. Large

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transparent crystals were collected from the Rb containing compounds, and a flat surface was

12

polished for comparable spectroscopic measurements.

(a)

(d)

(b)

(e)

(c)

(f)

(g)

Figure 1. Crystal growth results for 0.5 molar % Eu2+ doped (a) Cs4CaI6:Eu, (b) [Cs3Rb]CaI6:Eu, (c) [Cs3K]CaI6:Eu, (d) Cs4SrI6:Eu, (e) [Cs3Rb]SrI6:Eu, (f) [Cs3K]SrI6:Eu, and (g) the polished slabs used for spectroscopic measurements. 13

Differential scanning calorimetry. Previous studies of AX-BX2 binary systems have

14

shown that many A4BX6 compounds are formed at dystectic or peritectic points in phase

15

diagrams.14 For the Sr based compounds, neither the CsI-SrI2, RbI-SrI2, or KI-SrI2 phase


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diagrams have been reported. The thermal profile of Cs4SrI6 suggest that the compound is

2

incongruently melting, and is most likely formed at a peritectic point that lies close to the

3

liquidus, Figure 2a. Upon melting, two endothermic minima, partially convolved, are clearly

4

discerned. Upon cooling, a small exothermic maxima (liquidus) can be seen relative to the main

5

exothermic maxima with a ΔTonset of 30oC. Developing a phase diagram of the CsI-SrI2 system is

6

beyond the scope of this study, however the argument that the compound is formed by a

7

peritectic point can be corroborated by confirming high phase purity from powder X-ray

8

diffraction which is addressed later. DSC also confirms that the ternary system of CsI-RbI-SrI2

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produces the compound [Cs3Rb]SrI6, Figure 2b. Although two exothermic maxima can be seen,

10

the latent heat of the smaller maxima lies at the same crystallization temperature as Cs4SrI6,

11

suggesting that [Cs3Rb]SrI6 is congruently melting and Cs4SrI6 may be present due to bulk

12

polycrystalline nucleation and growth versus the seeded single crystal growth process. The DSC

13

profile for [Cs3K]SrI6 resembles that of Cs4SrI6 although only one endothermic minima can be

14

seen, Figure 2c. Given the two observed exothermic maxima (ΔTonset of 8oC) and the broad

15

endothermic base, it can be assumed that two endothermic minima are completely convoluted,

16

and that the peritectic point is closer to the liquidus than in Cs4SrI6. The phase diagrams for the

17

CsI-CaI2 and RbI-CaI2 systems have been reported, both showing that the compounds Cs4CaI6

18

and Rb4CaI6 are congruently melting. In most cases, specifically if two compounds share the

19

same crystal structure, these conditions should allow for the entire range of solid solutions

20

between the two end compounds to also be congruently melting. This was confirmed in the case

21

of [Cs3Rb]CaI6, seen in Figure 2d-e, where a single endothermic melting peak and exothermic

22

crystallization peak was measured via DSC. Contrarily, the KI-CaI2 system does not form a


10

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1

K4CaI6 phase, however DSC measurements show that [Cs3K]CaI6 is congruently melting, Figure

2

2f.

(a)

(d)

(b)

(e)

(c)

(f)

Figure 2. (a-f) Differential scanning calorimetry heat flow curves for the new and base compounds. The Ca-based compounds all melt congruently while the Sr-based compounds are incongruent, with the exception of [Cs3Rb]SrI6. 3

X-Ray diffraction. Single crystal diffraction measurements show that the nominal

4

compositions of [Cs3Rb]SrI6, [Cs3K]SrI6, [Cs3Rb]CaI6, and [Cs3K]CaI6 are all new compounds

5

that retained the rhombohedral K4CdCl6 structure-type, space group 𝑅3̅𝑐-hexagonal basis. Their

6

structures are composed of three unique polyhedra that share edges and faces. The (Ca1,Sr1)I6

7

polyhedron is an octahedron, the Cs1I8 a distorted square antiprism, and the (K1,Rb1,Cs2)I6 is a

8

trigonal prism. Confirming the hypothesis of Beck et al., the introduction of smaller alkali ions

9

preferentially occupied the monovalent site with lower coordination number and volume,

10

producing an ordered substitution. With the exception of [Cs3K]CaI6, a full stoichiometric

11

substitution was not obtained, potentially due to segregation during the growth process and the

12

particular part of the boule where the measured crystal was selected. During refinement, the


11


Page 12 of 29

1

smaller alkali ions were placed on the larger monovalent site, however the agreement factor

2

worsened and produced a negative occupation. The structures of the new compounds are

3

illustrated in Figure 3 a-e where the structures are projected along the (110) plane. The most

4

notable feature are the columns of (Ca1,Sr1)I6 and (Rb1,K1,Cs2)I6 polyhedra along the [001]

5

direction. These columns are linked together by the Cs1I8 polyhedra. Beck et al. pointed out that

6

the K4CdCl6 structure is a stuffed substitution variant of the hexagonal close packed

7

arrangement, where the K ions substitute for Cl ions on two unique sites and the Cd ions are

8

positioned in 1/6 of the octahedral voids 14. This gives a densely packed arrangement for

9

compounds of this structure-type. The results of the Rietveld refinements are also shown


12

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1


(a) (f)

(b)

(g)

(c) (h)

(d)

(i) (e)

Figure 3. (a,b,d,e) The unit cells projected along the (110) plane and individual polyhedra that compose the rhombohedral crystal structure, space group 𝑅3̅𝑐-hexagonal basis, as determined via single crystal diffraction. The substitution of Rb1+ and K1+ into the Cs4CaI6 and Cs4SrI6 matrices formed ordered substitutions by preferentially occupying the lower coordinated monovalent site. These distorted trigonal prisms are shown to highlight the fractional occupation of the substitutions. (c) The common polyhedra among the Sr2+ and Ca2+ based compounds. Both the Sr2+ and Ca2+polyhedra are nearly ideal octahedrons. (f-i) Results of the Rietveld refinements showing high phase purity for all new compounds.


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in Figure 3 f-i. The CIF’s produced from the single crystal diffraction refinements served as the

2

starting parameters for the Rietveld refinements, and the most important result is that all new

3

compounds had high phase purity. Without having a phase diagram to reference, this gives merit

4

to the argument that [Cs3K]SrI6 forms at a peritectic point. A summary of the structural

5

parameters is included in the references, and tables of the atomic positions, occupation, and

6

displacement parameters are included in the supplemental information.i,ii


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1


Eu2+ centers and radioluminescence. The X-ray excited luminescence spectra of the 0.5

2

mol % Eu2+ doped crystals are shown in Figure 4. The emission maxima are a result of the

3

recombination of e-h pairs/excitons via Eu2+ 5d-4f transition. The emissions of the Sr-based

4

compounds were in the range of 462-460 nm, while the Ca-based were in the range of 460-468

Figure 4. X-ray excited luminescence spectra of the 0.5 molar % Eu2+ doped crystals. The inset with normalized intensity in log scale highlights the weak low energy emission relative to the Eu2+ emission.


15


Page 16 of 29

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5. (a-f) The photoluminescence excitation and emission spectra of the 0.5 molar % Eu2+ doped compounds. 1

nm. The weak low energy emissions observed from ~510-650 nm in the emission spectra have

2

been attributed to radiative recombination from trace impurities and/or surface hydrate defects

3

caused by the hygroscopic nature of metal halides.20-21

4

The photoluminescence excitation and emission spectra of the 0.5 mol % Eu2+ doped

5

crystals are shown in Figure 5 a-f. All crystals had a single emission band centered at 460 nm

6

which signifies that the Eu2+ ions only occupied the single divalent octahedral site in the

7

matrices. Additionally, no weak low energy emissions were observed above 500 nm, confirming

8

that the weak emissions seen in the radioluminescence spectra were not due to Eu2+

9

luminescence centers. The excitation spectra are a product of broad absorption bands from the

10

Eu2+ 4f7-4f65d1 transitions. From the low Eu2+ concentrations a near fully resolved broad band is

11

observed at ~280 nm, followed by less resolved broad bands at 350 nm and 370 nm, and

12

unresolved bands from 390-420 nm. Another result of low Eu2+ concentration is the diminished


16

Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

excitation spectra intensity in the range of 250-340 nm. This effect has been observed in similar

2

metal-halide scintillators doped with < 5% Eu2+.22 Self-absorption is typical in Eu2+ doped

3

compounds, and is observed in the overlapping of the emission and excitation spectra.

4

Luminescence kinetics. The Eu2+ lifetimes and scintillation decay times of the 0.5 molar

5

% Eu2+ doped crystals are shown in Figure 6 a-e. Comparison of the two decay mechanisms

6

allows for an indirect evaluation of the presence of charge carrier traps and complex processes

7

that delay the rate of energy transfer and recombination of excitons at Eu2+ luminescent centers.

8

Decay times for Eu2+ doped compounds generally range between 500 and 1500 ns depending on

9

the hosts crystal matrix.23 The measured Eu2+ lifetimes of the crystals in this study fell within

10

this range with decay times between 1 and 1.2 μs. Significant differences were observed in the

11

scintillation kinetics however. Crystals of 3 compounds, Cs4SrI6:Eu, Cs4CaI6:Eu, and

(a)

(b)

(c)

(d)

(e)

(f)

Figure 6. (a-f) The scintillation decay times compared with the Eu2+ lifetimes of the 0.5 molar % Eu2+ doped compounds. The reported error is based on the quality of fit from the calculated decay time to the measured data.


17


Page 18 of 29

1

[Cs3K]SrI6:Eu all had significantly longer primary decay components than their measured Eu2+

2

lifetimes, coupled with exceedingly slow components. On the other hand, the Rb containing

3

compounds and [Cs3K]CaI6:Eu had primary decay components comparable to their Eu2+

4

lifetimes with secondary slow components < 4 μs. In alkali halides, the strong electron-hole

5

interactions quickly form excitons which in turn quickly create self-trapped excitons (STE) due

6

to the highly polarizable nature of host matrix. It was proposed by Alexandrov et al. that

7

scintillation decay has two competing mechanisms for excitons to reach luminescence centers.

8

The fast component, a phonon induced transport of an STE to a luminescence center, and the

9

slow component ascribed to the thermal decomposition of an STE to electron and holes followed

10

by capture at luminescence centers via diffusion processes.24 Another model based on theoretical

11

and experimental work by Payne et al. to derive the origins of non-proportional light yield

12

explains that the processes that contribute to luminescence include STE’s, free excitons, and free

13

electrons and holes.25 Future studies of these compounds would benefit from

14

thermoluminescence, afterglow, and proportional electron response measurements to better

15

understand the specific processes that contribute to the scintillation kinetics in these crystals. It

16

can be deduced that because all of the crystals have the same crystal structure and were

17

synthesized from the same raw materials, the observed differences in the luminescence kinetics

18

must be due to 1) the band gap and relative position of Eu2+ centers within the bandgap, and 2)

19

charge carrier traps, perhaps created by defects associated with strain and residual stress. Future

20

development of this family of scintillators could be advanced by using DFT calculations to

21

determine the effect of the ordered substitution of Rb1+ and K1+ on the electronic band structure,

22

complimented by experimentally following strain and band gap engineering strategies to

23

determine the effect on scintillation efficiency.


18

Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1


Gamma Ray Spectroscopy. The energy resolution and light yields of ~ 100 mm3 single

2

crystals doped with 0.5 mol % Eu2+ are shown in Figure 7 a-f. The top performer was

3

[Cs3Rb]CaI6:Eu with an energy resolution of 4.5% and light yield of 38,000 ph/MeV. At these

4

low Eu2+ concentrations the Ca-based compounds all outperformed the Sr-based compounds.

5

Interestingly, Cs4SrI6:Eu had a worse energy resolution than Cs4CaI6:Eu by nearly 2%, despite

6

having equivalent light yields. This is contrary to their previously reported performance when

7

doped with 4 molar % Eu2+, where Cs4SrI6:Eu had better light yield and energy resolution.13 The

8

proportional response to γ-photon irradiation is shown in Figure 8, where the ideal response is at

9

unity. It was observed that the trend in performance of energy resolution is perfectly correlated

(a)

(b)

(c)

(d)

(e)

(f)

Figure 7. (a-f) The absolute light output and energy resolution of the 662 keV photopeak of Cs137spectra for the 0.5 mol% Eu2+ doped compounds. The energy resolution is calculated as the full width at half maximum divided by the photopeak centroid. The fits include the unresolved Cs and Iodine K-edge X-ray escape peaks with energies of 33.9846 and 33.1694 keV, respectively.


19


1

with the trend in proportional response, from Cs4SrI6:Eu 0.5% having the worst energy

2

resolution and least proportional response to [Cs3Rb]CaI6:Eu 0.5% having the best energy

3

resolution and most proportional response.

Page 20 of 29

Figure 8. The non-proportional light yield response to γphoton energy for the 0.5 molar % Eu2+ doped compounds. 4

Eu2+ optimization. To obtain the best scintillation performance in terms of energy

5

resolution and light yield at these volumes, single crystals were grown with varying Eu2+

6

concentrations from 1 to 9 mol %. [Cs3K]CaI6 was chosen from the new compounds as the host

7

matrix because of its relatively good energy resolution and because it was the easiest to grow.

8

High growth yields of quality transparent single crystals were obtained, and Ø7 mm x 3 mm

9

slabs were harvested from the same position in the boules. The polished slabs are shown in

10

Figure 9a and their measured energy resolutions and light yields are shown in Figure 9b. The

11

crystal from Eu 5% (not pictured) came out heavily cracked, however a large single crystal was

12

able to be harvested and its light yield and energy resolution are included in the plot. Despite the

13

absence of a clear trend in energy resolution at low Eu2+ concentrations, it was concluded that 7

14

mol% Eu2+ was the ideal concentration for this system of compounds. The erratic change in


20

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

Figure 9. (a) The polished Ø7 mm x 3 mm slabs of [Cs3K]CaI6:Eu of various Eu2+ concentrations, and (b) their absolute light yield (blue curve) and energy resolution (black curve). 1

energy resolution in contrast to the monotonic change in light yield at low Eu2+ concentrations is

2

most likely due to non-statistical factors such as spatial inhomogeneity of Eu2+ and potential

3

impurities. Additional crystal growth development will be necessary to unambiguously identify

4

the cause. Due to Cs4SrI6:Eu out performing both Cs4CaI6:Eu and Cs3KCaI6:Eu at higher Eu2+

5

concentrations, two other Sr-based crystals, Cs4SrI6:Eu 7% and [Cs3.5Rb0.5]SrI6:Eu 7% were

6

chosen to study. A partial substitution of Rb1+ for Cs1+ was chosen instead of a full substitution

7

for a higher Zeff, and to determine if the solid solution was still congruently melting. Due to the

8

larger degree of supercooling in the solid solution, [Cs3.5Rb0.5]SrI6:Eu was grown through a

9

thermal gradient of 43oC/cm and in an ampoule with a longer capillary than previous growths.

10

The crystal, shown in Figure 10 a, was still cracked but had good optical quality and a large

11

single crystal was harvested for measurements. The crystals used for scintillation measurements

12

are shown in Figure 10 b. The DSC measurement of [Cs3.5Rb0.5]SrI6:Eu is shown in Figure 10 c

13

and was observed to have the same characteristics of [Cs3Rb]SrI6:Eu. A single sharp

14

endothermic peak was observed with an onset temperature of 525oC, 10oC lower than the onset

15

temperature of Cs4SrI6. Two exothermic peaks are also observed, with the minor peak at the


21


(a)

Page 22 of 29

(d)

(b)

(c)

60 mm

3

Figure 10. (a) Crystal growth result of [Cs3.5Rb0.5]SrI6:Eu 7%, (b) the 7 molar % Eu2+ crystals used for scintillation measurements, (c) the differential scanning calorimetry heat flow curve for [Cs3.5Rb0.5]SrI6:Eu 7%, and (d) the absolute light yield and energy resolution of the 662 keV photopeak of Cs137spectra for the 7 molar % Eu2+ doped compounds . 1

crystallization temperature of Cs4SrI6. Cs137 spectra highlighting the 662 keV photopeak are

2

shown in Figure 10 d. The performance of [Cs3.5Rb0.5]SrI6:Eu 7% and Cs4SrI6:Eu 7% are

3

comparable with an energy resolution of 3.3% and 3.5%, and light yields of 75,000 and 74,000

4

ph/MeV respectively. [Cs3K]CaI6:Eu 7% had an energy resolution of 3.9% and light yield of

5

62,000 ph/MeV. The scintillation decay curves and proportional response measurements are

6

shown in Figure 11a and 11b respectively. The scintillation decay for [Cs3.5Rb0.5]SrI6:Eu 7%


22

Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

and Cs4SrI6:Eu 7% are approximately equal and are defined by two components of ~ 1.35 and

2

2.3 μs of nearly equal magnitude. [Cs3K]CaI6:Eu 7% had a decay time of 1.9 μs for the fast

3

component (76%), and 3.3 μs (24%) for the slow component. [Cs3.5Rb0.5]SrI6:Eu 7% and

4

Cs4SrI6:Eu 7% also had nearly identical good proportional light yields to γ-excitation. The non-

5

proportional response of [Cs3K]CaI6:Eu 7% was intermediate between the measured non-

6

proportional light yield of a packaged CsI:Tl crystal and unity. Proportional response is directly (a)

(b)

7

Figure 11. (a) scintillation decay curves of the 7 molar % Eu2+ doped compounds, and (b) their non-proportional light yield response to γ-photon energy compared with a packaged CsI:Tl crystal (~12 cm3). proportional to the efficiency of activators to capture charge carriers, as well as the efficiency of

8

activators to emit photons.25 For Eu2+doped crystals, an increase in concentration leads to both

9

improvement in proportional response and light yield up to a point, until self-absorption and

10

quenching dominates. A summary of physical, thermal, and scintillation properties of the crystals

11

presented in this work are listed along with benchmark scintillators in Table 1.27-28

12 13


23


Page 24 of 29

Table 1. summary of physical, thermal, and scintillation properties.

27 27 28

1

Conclusion. In this article we present the structural, thermal, optical, and scintillation

2

properties of the newly discovered compounds and scintillators [Cs3Rb]SrI6:Eu, [Cs3K]SrI6:Eu,

3

[Cs3Rb]CaI6:Eu, and [Cs3K]CaI6:Eu along with the base compounds Cs4SrI6:Eu and Cs4CaI6:Eu.

4

It was shown via DSC that [Cs3Rb]SrI6:Eu, [Cs3Rb]CaI6:Eu, and [Cs3K]CaI6:Eu are all

5

congruently melting. Single crystal X-ray diffraction confirmed that all four compositions are in

6

fact new compounds and maintained the same crystal structure as the base compounds.

7

Additionally, Rietveld refinements based on powder X-ray diffraction data showed all

8

compounds had high phase purity, including [Cs3K]SrI6:Eu despite incongruent melting. PL

9

emission spectra of all crystals doped with 0.5 mol% Eu2+ show a single emission peak,

10

signifying that Eu2+ only occupies the single divalent octahedral site. Despite having the same

11

crystal structure, it was shown that the different matrices affect the scintillation kinetics to

12

varying degrees when compared with Eu2+ emission lifetimes. The 0.5 mol% Eu2+ doped crystals

13

had light yields ranging from 27,000-38,000 ph/MeV, and the energy resolution had a perfect


24

Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

correlation with the proportional light yield response to γ irradiation. The Ca-based compounds

2

outperformed the Sr-based at low Eu2+ concentrations, however, at the optimal concentration of

3

7 mol% the Sr-based compounds had the best performance. An energy resolution of 3.3% and

4

light yield of 75,000 ph/MeV was measured for a 60 mm3 crystal of [Cs3.5Rb0.5]SrI6:Eu 7%.

5

The initial assessment of the Eu2+-doped alkali-earth alkali halides from the A4BX6

6

family has shown good potential as scintillators with energy resolution and light yields that

7

surpasses that of benchmark scintillators CsI:Tl and NaI:Tl. While the scintillation performance

8

did not reach that of SrI2:Eu, the measured absolute light yields surpassed that of LaBr3:Ce. A

9

competitive advantage of these Cs rich compounds is the inexpensive cost of raw materials

10

compared to pure SrI2 and LaBr3. Further development of these crystals will continue to address

11

the optimization of growth parameters, particularly for the Rb1+ containing compounds.

12

Additionally, exploration into the electronic structure, optical properties of un-doped crystals,

13

and proportional light yield response to electron energy could provide valuable insights into

14

developing compositional engineering strategies to achieve energy resolution < 3%.

15

Acknowledgments

16

This project was funded in part by Oak Ridge Associated Universities (ORAU) via the

17

Ralph E. Powe Junior Faculty Enhancement Award, and the Center for Materials Processing at

18

the University of Tennessee - Knoxville. This work has also been supported by the US

19

Department of Homeland Security, Domestic Nuclear Detection Office, under competitively

20

awarded grants # 2014-DN-077-ARI088 and #2012-DN-077-ARI067. This support does not

21

constitute an express or implied endorsement on the part of the Government. A portion of this

22

research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility

23

operated by the Oak Ridge National Laboratory.


25


1 2

Supporting Information Available Tables of the atomic positions, occupation, and displacement parameters are included as

3

supplemental information. This information is available free of charge via the Internet at

4

http://pubs.acs.org/ .

5

References

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

Page 26 of 29

1. Derenzo, S. E.; Moses, W. W., EXPERIMENTAL EFFORTS AND RESULTS IN FINDING NEW HEAVY SCINTILLATORS. Editions Frontieres: Dreux, 1993; p 125-135. 2. Girard, T. A.; Giuliani, F., On the direct search for spin-dependent WIMP interactions. Phys. Rev. D 2007, 75 (4), 7. 3. Lee, H. S.; Bhang, H. C.; Choi, J. H.; Dao, H.; Hahn, I. S.; Hwang, M. J.; Jung, S. W.; Kang, W. G.; Kim, D. W.; Kim, H. J.; Kim, S. C.; Kim, S. K.; Kim, Y. D.; Kwak, J. W.; Kwon, Y. J.; Lee, J.; Lee, J. H.; Lee, J. I.; Lee, M. J.; Lee, S. J.; Li, J.; Li, X.; Li, Y. J.; Myung, S. S.; Ryu, S.; So, J. H.; Yue, Q.; Zhu, J. J., Limits on interactions between weakly interacting massive particles and nucleons obtained with CsI(Tl) crystal detectors. Phys. Rev. Lett. 2007, 99 (9), 4. 4. Nagarkar, V. V.; Gupta, T. K.; Miller, S. R.; Klugerman, Y.; Squillante, M. R.; Entine, G., Structured CsI(Tl) scintillators for X-ray imaging applications. IEEE Trans. Nucl. Sci. 1998, 45 (3), 492-496. 5. Hawrami, R.; Glodo, J.; Shah, K. S.; Cherepy, N.; Payne, S.; Burger, A.; Boatner, L., Bridgman bulk growth and scintillation measurements of SrI2:Eu2+. J. Cryst. Growth 2013, 379, 69-72. 6. Seifert, S.; van Dam, H. T.; Huizenga, J.; Vinke, R.; Dendooven, P.; Lohner, H.; Schaart, D. R., Monolithic LaBr3:Ce crystals on silicon photomultiplier arrays for time-of-flight positron emission tomography. Physics in Medicine and Biology 2012, 57 (8), 2219-2233. 7. Wu, Y. T.; Lindsey, A. C.; Zhuravleva, M.; Koschan, M.; Melcher, C. L., Large-Size KCa0.8Sr0.2I3:Eu2+ Crystals: Growth and Characterization of Scintillation Properties. Cryst. Growth Des. 2016, 16 (7), 4129-4135. 8. Bizarri, G.; Bourret-Courchesne, E. D.; Yan, Z. W.; Derenzo, S. E., Scintillation and Optical Properties of BaBrI: Eu2+ and CsBa2I5: Eu2+. IEEE Trans. Nucl. Sci. 2011, 58 (6), 3403-3410. 9. Stand, L.; Zhuravleva, M.; Lindsey, A.; Melcher, C. L., Growth and characterization of potassium strontium iodide: A new high light yield scintillator with 2.4% energy resolution. Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equip. 2015, 780, 4044. 10. Wei, H.; Stand, L.; Zhuravleva, M.; Meng, F.; Martin, V.; Melcher, C. L., Two new cerium-doped mixed-anion elpasolite scintillators: Cs2NaYBr3I3 and Cs2NaLaBr3I3. Opt. Mater. 2014, 38, 154-160. 11. Wu, Y. T.; Li, Q.; Chakoumakos, B. C.; Zhuravleva, M.; Lindsey, A. C.; Johnson, J. A.; Stand, L.; Koschan, M.; Melcher, C. L., Quaternary Iodide K(Ca,Sr)I-3:Eu2+ Single-Crystal


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45


Scintillators for Radiation Detection: Crystal Structure, Electronic Structure, and Optical and Scintillation Properties. Adv. Opt. Mater. 2016, 4 (10), 1518-1532. 12. Wei, H.; Zhuravleva, M.; Yang, K.; Blalock, B.; Melcher, C. L., Effect of Ba substitution in CsSrI3:Eu2+. J. Cryst. Growth 2013, 384, 27-32. 13. Stand, L.; Zhuravleva, M.; Chakoumakos, B. C.; Johnson, J. A.; Loyd, M.; Koschan, M.; Melcher, C. L., Crystal Growth and Scintillation Properties of Eu2+ doped Cs4CaI6 and Cs4SrI6. J. Cryst. Growth 2018; https://doi.org/10.1016/j.jcrysgro.2018.01.017. 14. Beck, H. P.; Milius, W., STUDY ON A4BX6 COMPOUNDS .1. STRUCTURE REFINEMENT OF TERNARY CD HALIDES NH4(4)CDCL6, K4CDI, RBCDX6, INCDX6, TICDX6. Z. Anorg. Allg. Chem. 1986, 539 (8), 7-17. 15. Melcher, C. L., Perspectives on the future development of new scintillators. Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equip. 2005, 537 (1-2), 6-14. 16. Agilent CrysAlis PRO, 38.46; Oxford Diffraction/Agilent Technologies Ltd: Yarnton, Oxfordshire, England, 2017. 17. Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; Giacovazzo, C.; Mallamo, M.; Mazzone, A.; Polidori, G.; Spagna, R., SIR2011: a new package for crystal structure determination and refinement. Journal of Applied Crystallography 2012, 45, 357-361. 18. Farrugia, L. J., WinGX and ORTEP for Windows: an update. Journal of Applied Crystallography 2012, 45, 849-854. 19. Sheldrick, G. M., A short history of SHELX. Acta Crystallographica Section A 2008, 64, 112-122. 20. Lindsey, A. C.; Zhuravleva, M.; Stand, L.; Wu, Y. T.; Melcher, C. L., Crystal growth and characterization of europium doped KCaI3, a high light yield scintillator. Opt. Mater. 2015, 48, 1-6. 21. Sturm, B. W.; Cherepy, N. J.; Drury, O. B.; Thelin, P. A.; Fisher, S. E.; O'Neal, S. P.; Payne, S. A.; Burger, A.; Boatner, L. A.; Ramey, J. O.; Shah, K. S.; Hawrami, R.; Ieee, Characteristics of Undoped and Europium-doped SrI2 Scintillator Detectors. In 2011 Ieee Nuclear Science Symposium and Medical Imaging Conference, Ieee: New York, 2011; pp 7-11. 22. Lindsey, A. C.; Zhuravleva, M.; Wu, Y.; Stand, L.; Loyd, M.; Gokhale, S.; Koschan, M.; Melcher, C. L., Effects of increasing size and changing europium activator concentration in KCaI3 scintillator crystals. Journal of Crystal Growth 2016, 449, 96-103. 23. Dorenbos, P., Fundamental Limitations in the Performance of Ce3+-, Pr3+-, and Eu2+Activated Scintillators. IEEE Trans. Nucl. Sci. 2010, 57 (3), 1162-1167. 24. Alexandrov, B. S.; Ianakiev, K. D.; Littlewood, P. B., Branching transport model of NaI(Tl) alkali-halide scintillator. Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equip. 2008, 586 (3), 432-438. 25. Payne, S. A.; Moses, W. W.; Sheets, S.; Ahle, L.; Cherepy, N. J.; Sturm, B.; Dazeley, S.; Bizarri, G.; Choong, W. S., Nonproportionality of Scintillator Detectors: Theory and Experiment. II. IEEE Trans. Nucl. Sci. 2011, 58 (6), 3392-3402. 27. van Eijk, C. W. E., Inorganic scintillators in medical imaging. Physics in Medicine and Biology 2002, 47 (8), R85-R106. 28. Wilson, C. M.; Van Loef, E. V.; Glodo, J.; Cherepy, N.; Hull, G.; Payne, S.; Choong, W. S.; Moses, W.; Shah, K. S., Strontium iodide scintillators for high energy resolution gamma ray spectroscopy. In Hard X-Ray, Gamma-Ray, and Neutron Detector Physics X, Burger, A.; Franks, L. A.; James, R. B., Eds. Spie-Int Soc Optical Engineering: Bellingham, 2008; Vol. 7079.

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1

i

Page 28 of 29

Summary of the structural parameters determined via powder X-ray diffraction.

2 3

ii

Summary of the structural parameters determined via single crystal X-ray diffraction

4


28

Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Use Only

1

2 3

K1 Cs1

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Sr1

The results of Bridgman single crystal growth and visualization of the crystal structure determined via single crystal X-ray diffraction for Eu2+ doped [Cs3K]CaI6. This study showed that substitution of smaller monovalent ions in the hexagonal K4CdCl6-type crystal structure creates an ordered substitution by occupying the lower coordinated monovalent site.


29

Discovery of New Compounds and Scintillators of the A4BX6 Family

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