Contents lists available at ScienceDirect Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro Tl 2 HfCl 6 and Tl 2 ZrCl 6 : Intrinsic Tl-, Hf-, and Zr-based scintillators R. Hawrami a, ⁎ , E. Ariesanti a , V. Buliga a , A. Burger a , S. Lam b , S. Motakef b a Fisk University, Nashville, TN 37208, USA b CapeSym, Inc., Natick, MA 01760, USA ARTICLE INFO Communicated by R.S. Feigelson Keywords: B2. Intrinsic scintillator A1. Cubic structure B2. Tl-, Hf-, Zr-based scintillator B2. High density scintillator B2. Bright scintillator crystals B2. Good proportionality ABSTRACT The re-discovery of Cs 2 HfCl 6 (CHC) as a scintillator has lately generated much interest in this material and its family, which belongs to the K 2 PtCl 6 cubic crystal structure [1]. CHC is an intrinsic scintillator that is non- hygroscopic, has no self-radioactivity, provides good energy resolution, and has good non-proportionality. In this paper we are reporting growth and scintillation properties of two new and Tl-, Hf- and Zr-based compounds of Tl 2 HfCl 6 and Tl 2 ZrCl 6 , with physical densities of 5.1 g/cm 3 and 4.5 g/cm 3 , respectively and effective atomic numbers (Z eff ) of 71 and 69, respectively. Samples from successfully grown 16 mm diameter boules were cut, processed, and characterized for their scintillation and radiometric properties. Energy resolutions of 3.7% (FWHM) for THC and 3.4% for TZC (FWHM) at 662 keV are measured. Primary luminescence decay times of 1.1 μs and 2.3 μs, for Tl 2 HfCl 6 and Tl 2 ZrCl 6 , respectively, as well as good proportionality for both materials have been observed. 1. Introduction Widespread use of scintillators as gamma-ray detectors is largely generated by their tunable properties and extensive availability. Currently there are needs for scintillators with targeted properties such as high light output, high stopping power (Z eff ), fast decay time, good linearity and low cost. A rediscovered family of scintillators, Cs 2 HfCl 6 [1], has recently been in one of the materials in the forefront of new scintillator development for gamma-ray detectors. CHC was re-dis- covered and reported as an example of a little-known class of non-hy- groscopic compounds having the generic cubic crystal structure of K 2 PtCl 6 [1]. Since its re-discovery as a new non-hygroscopic, intrinsic scintillator with a simple cubic crystal structure, several papers have reported their studies on their attractive properties as well as growth challenges. In an initial study CHC scintillation was reported to be centered at 400 nm, with a principal decay time of 4.4 μs and a light yield of up to 54,000 photons/MeV (when compared to BGO), and energy resolution of 3.3% at 662 keV using a 0.65 cm 3 cubic sample [2]. Another study on crystal growth and behavior CHC and its variant, Cs 2 HfCl 4 Br 2 (CHCB), also reported a clear CHC sample with a light yield and energy resolution of 30,000 ph/MeV (when compared to NaI:Tl) and 3.3%, respectively, and decay components of 0.39 and 3.9 μs. A sample of CHCB with a secondary phase present in the core had a light yield and energy resolution of 18,600 ph/MeV and 4.4%, and with decay components of 0.38 and 2.0 μs for CHCB. Both CHC and CHCB showed minimal moisture sensitivity [3]. Recently Tl 2 HfCl 6 and Tl 2 ZrCl 6 , compounds belonging to a new sub- family based on substitution of Tl + for Cs + in CHC, have been pub- lished [4]. These compounds, similarly to CHC and CHCB, were also grown by the vertical Bridgman–Stockbarger method. The Tl + ion substitution increases both the density (see Table 1) as well as the ef- fective atomic number Z eff (see Table 1 and [4]), factors which directly determine x-ray/γ-ray detection efficiency of materials. Both Tl 2 HfCl 6 and Tl 2 ZrCl 6 crystals showed an intrinsic emission band peak between 450 and 470 nm under UV light and X-ray excitations. Their reported energy resolutions at 662 keV were 17.7% for Tl 2 HfCl 6 and 5.6% for Tl 2 ZrCl 6 . Their reported scintillation decay times are 288 and 6340 ns for Tl 2 HfCl 6 , while for Tl 2 ZrCl 6 696 and 2360 ns [4]. Our paper is re- porting on initial growth of Tl 2 HfCl 6 and Tl 2 ZrCl 6 single crystal boules with improved radiometric and scintillation properties. Compared to previously published papers on these materials, with respect to crystal size and scintillating performance, our results demonstrate successful crystal growth runs resulting in superior crystal quality. 2. Experimental Methods The crystal growth process started by loading stoichiometric amounts of starting materials respectively according to compound (TlCl with 99.99% and HfCl 99.9% for Tl 2 HfCl 6 and TlCl with 99.99% and ZrCl 99.9% for Tl 2 ZrCl 6 ) into 16 mm diameter ampoules, completed https://doi.org/10.1016/j.jcrysgro.2019.125316 Received 7 September 2019; Received in revised form 15 October 2019; Accepted 21 October 2019 ⁎ Corresponding author. E-mail addresses: drh1980@gmail.com, rhawrami@fisk.edu (R. Hawrami). Journal of Crystal Growth 531 (2020) 125316 Available online 22 October 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved. T