Crystal structure, electronic structure, optical and ...

Journal of Luminescence 201 (2018) 460–465

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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Crystal structure, electronic structure, optical and scintillation properties of self-activated Cs4YbI6☆

T

⁎

Yuntao Wua,b, , Bryan C. Chakoumakosc, Hongliang Shid, Mao-Hua Due, Ian Greeleya,b, Matthew Loyda,b, Daniel J. Rutstroma,b, Luis Standa,f, Merry Koschana, Charles L. Melchera,b,f,g a

Scintillation Materials Research Center, University of Tennessee, Knoxville, TN 37996, USA Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, USA c Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA d Key Laboratory of Micro-Nano Measurement-Manipulation and Physics (Ministry of Education), Department of Physics, Beihang University, Beijing 100191, China e Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA f Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN 37996, USA g Department of Nuclear Engineering, University of Tennessee, Knoxville, TN 37996, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Scintillator Single crystal Self-activated Electronic structure

A self-activated Cs4YbI6 single crystal was grown by the vertical Bridgman method. Crystal structure refinements verified the phase purity and the trigonal crystal system with a space group of R 3 c. By using differential scanning calorimetry, the melting and crystallization points were determined to be 550 and 510 °C, respectively. Luminescence and scintillation properties were systematically studied. Upon ultraviolet light (360 nm) excitation, the Cs4YbI6 crystal exhibits bluish-green emission centered at 450 and 480 nm due to spin-allowed and spin-forbidden transitions of Yb2+ activators. The lifetimes of the corresponding emission bands at room temperature are tens and hundreds of nanoseconds, respectively. X-ray excited radioluminescence spectrum is dominated by the spin-forbidden transition of Yb2+ at 480 nm. The absolute light yield is 2700 ± 200 photons/ MeV with a principal scintillation decay time of 33 ns. The physical explanation for the low light yield observed is proposed from experimental and theoretical insights.

1. Introduction Inorganic scintillators are widely used in the field of nuclear security, nuclear medical imaging, high energy physics, and oil well logging. Due to the increased requirements for targeted applications, intense research and development activities have taken place over last few decades, and many promising oxide and halide materials have been discovered and developed [1]. Self-activated and intrinsic scintillators have the advantage of natural luminescent homogeneity compared to externally activated scintillators. The intrinsic luminescence usually includes the following forms: electron-hole recombination; free-, selftrapped, and defect-trapped exciton luminescence; or core-valence band transitions [2]. For the self-activated materials, luminescence is usually achieved by a constituent of the crystal, such as molecular complexes [3–5] or ionic activators [6–11]. In the case of ionic activators, the materials with high scintillation efficiency are all self-

activated with Ce3+, Eu2+, or Tl+ ions, such as CeBr3 [6], Ce(Br,Cl)3 [7], CsCe2Br7 [8], Cs2LiCeCl6 [9], Cs3EuI5 [10], TlMgCl3 and TlCaI3 [11]. There are almost no reports on Yb2+ self-activated compounds, but, in fact, quite a few Yb2+ doped inorganic compounds have been studied, and their optical properties were summarized in Ref. 12. Quite often an anomalous luminescence or luminescence quenching is observed in Yb2+ doped inorganic compounds at room temperature. These phenomena have been determined to be caused by unwanted electronic structures, for example, if the 5d excited state is too close to the conduction band or the 5d1 excited state lies in the conduction band [12]. To date only two Yb2+ doped iodides with superior scintillation performance have been reported, namely SrI2:Yb2+ [13–15] and CsBa2I5:Yb2+ [13,14]. They have a light yield of over 50,000 photons/ MeV and an energy resolution of 4.4–6.8% at 662 keV [13,14]. Most recently, we have reported Eu2+ doped Cs4MI6 (M = Ca, Sr) [16] and self-activated Cs4EuX6 (X = Br, I) [17] single crystals with

☆ This manuscript has been co-authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). ⁎ Corresponding author at: Scintillation Materials Research Center, University of Tennessee, Knoxville, TN 37996, USA. E-mail addresses: [email protected], [email protected] (Y. Wu).

https://doi.org/10.1016/j.jlumin.2018.05.037 Received 5 April 2018; Received in revised form 9 May 2018; Accepted 12 May 2018 Available online 14 May 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.


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2.5. Optical property measurements

promising scintillation performance. As self-activated scintillators, the light yield of Cs4EuBr6 and Cs4EuI6 can reach 78,000 and 53,000 photons/MeV, respectively. In this work, we aim to substitute Yb2+ ions for Eu2+ ions in Cs4EuI6, and to investigate its crystal structure, luminescence, and scintillation properties. In this paper, the work is organized as follows: first, the crystal quality, thermal property, and crystal structure of a Bridgman-grown Cs4YbI6 single crystal are examined; second, the optical and scintillation properties, including photoluminescence emission and excitation spectra, photoluminescence and scintillation decay time, and light yield, are studied; finally, we propose physical explanations for the low light yield observed in Cs4YbI6.

Optical absorption spectra were acquired with a Varian Cary 5000 UV–VIS–IR spectrophotometer in the range between 300 and 800 nm. Photoluminescence emission (PL) and excitation (PLE) spectra were obtained with a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer. The excitation light went through an excitation monochromator with a 1 nm bandpass to ensure monochromaticity. Similarly, the emission monochromator was set at a 1 nm bandpass to select emission light of a specific wavelength. In the case of emission and excitation spectra, a 450 W continuous xenon lamp was used as the excitation source. Photoluminescence decay was measured on the same spectrofluorometer using a time-correlated-single-photon counting module. HORIBA Jobin Yvon NanoLEDs (pulsed light-emitting diodes) were used as the excitation source. The duration of the light pulse was shorter than 2 ns and therefore was not deconvoluted from the much longer decay profiles.

2. Experimental methods 2.1. Crystal growth Anhydrous, high-purity beads of (99.99%) CsI and (99.95%) YbI2 (Sigma-Aldrich) were used as starting materials. The compositions were mixed according to the stoichiometry of Cs4YbI6. The mixtures were then loaded into a quartz ampoule. The ampoule was evacuated to 10−6 mbar and heated to 200 °C and kept for 10 h at this temperature to remove residual water and oxygen impurities. After baking, the ampoule was sealed and transferred to the Bridgman growth furnace. It passed through a temperature gradient of about 40 °C/cm at a pulling rate of 1 mm/h. Finally, the furnace was cooled to room temperature at 20 °C/h. The size of the ingot is about ∅12 mm × 6 mm. The sample size used for measurements is about 5 mm × 5 mm × 1 mm.

2.6. Scintillation property measurements The scintillation decay time was measured using a time-correlated single-photon counting (TCSPC) setup under a 137Cs gamma-ray source excitation. The absolute light yield measurement was recorded by using a pulse processing chain consisting of a Hamamatsu R2059 photomultiplier tube (PMT) operated at − 1500 Vbias, an Ortec 672 Amp, a Canberra model 2005 pre-Amp and a Tukan 8k multi-channel analyzer. Each sample was directly coupled to the PMT using mineral oil, and a PTFElined dome-shaped reflector with a 50 mm radius was used to maximize the collection of light. The photoelectron yields were estimated by using the single photoelectron peak method. Measurements of the samples were made with 10 μs shaping time to provide light integration. Each sample was measured under irradiation with a 15 μCi 137Cs source. The reproducibility of the LY measurements is ± 5%. An X-ray tube operated at 35 kV and 0.1 mA was used as the excitation source for X-ray excited luminescence measurements. For the afterglow measurement, a crystal was coupled to a Hamamatsu R2059 PMT covered with a Tetratex TX3104 PTFE membrane. The crystal was irradiated with X-rays at room temperature for 15 min, after which a Uniblitz XRS6S2P1–040 shutter was used to cut off the X-ray beam within 3 ms and the luminescence emitted from crystal was recorded as a function of time.

2.2. Single-crystal X-ray diffraction study Crystal fragments (~0.001 mm3) were isolated from the slowlygrown boules and suspended in paratone oil, after which each was mounted on a plastic loop attached to the copper pin/goniometer. Single-crystal diffraction data were collected at 250 K using a Rigaku XtaLAB PRO diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) equipped with a Dectris Pilatus 200 K detector and an Oxford N-HeliX cryocooler. Peak indexing and integration were done using the Rigaku Oxford Diffraction CrysAlisPro software [18]. An empirical absorption correction was applied using the SCALE3 ABSPACK algorithm as implemented CrysAliPro. The SIR-2011 in WinGX and SHELXL-2013 software packages were used for data processing and structure solution and refinement [19,20]. Crystal structure projections were made with VESTA [21].

2.7. Computational methods All calculations were based on density functional theory (DFT) implemented in the VASP code [22]. The interaction between ions and electrons was described by projector augmented wave method [23]. The kinetic energy cutoff for the plane-wave basis is 325 eV. Experimental lattice parameters of Cs4YbI6 were used while the atomic positions were fully relaxed until the residual forces were less than 0.02 eV/ Å. Hybrid PBE0 functional [24], which includes 25% non-local Fock exchange, was used in all calculations. The inclusion of a fraction of Fock exchange significantly improves the calculations of the band gap energy, defects, dopants, and excitons in insulators [25–30].

2.3. Differential scanning calorimeter (DSC) measurements DSC measurements were carried out on a Labsys EVO instrument. A single crystal weighing approximately 50 mg was placed within an alumina crucible and heated and cooled at 5 K/min under a flow of ultra-high purity argon. The sample was measured twice using the same heating and cooling profile each time. For a baseline subtraction to the heat flow, the sample crucible was run empty in identical conditions prior to measuring the sample.

3. Results and discussion 2.4. Hygroscopicity measurements

3.1. Crystal quality, thermal behavior, and crystal structure

Moisture sorption profiles were recorded using a Dynamic Vapor Sorption technique with a DVS Intrinsic instrument by Particulate Systems. The measurements were carried out at 25 °C for 120 min at a relative humidity of 40%. The temperature and humidity selected here are to reproduce the actual operational conditions of radiation detectors.

The as-grown crystal appears yellow, inclusion-free, and transparent. A 1 mm thick Cs4YbI6 crystal sample is shown in the Fig. 1 inset. The optical transmission spectrum is plotted in Fig. 1. The existence of multiple strong absorption bands between 500 and 800 nm leads to a relatively low transmittance of 50–70%. The onset of the absorption edge is at about 460 nm (2.70 eV), which is almost the same as that of 461


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Table 1 Crystal structure data and single-crystal X-ray diffraction refinement results for Cs4YbI6.

Fig. 1. Optical transmission spectrum of a 1 mm thick Cs4YbI6 crystal sample. The crystal sample is shown in inset.

Formula

Cs4YbI6

fw (g) T (K) Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z Dcalc (mgm−3)

1466.09 250(1) Trigonal R3 c 14.4912(6) 14.4912(6) 18.2567(9) 90 90 120 3320.1802 6 4.40

[16]. The moisture absorption rates of Cs4YbI6 and NaI single crystals were measured by using the dynamic vapor sorption technique. Their DVS curves are plotted in Fig. 2(b). The moisture absorption rate of Cs4YbI6 is about eight times higher than that of NaI. It indicates the strong hygroscopic nature of Cs4YbI6. Based on the X-ray diffraction single crystal study, the crystal structure of Cs4YbI6 has trigonal crystal symmetry with the space group R 3 c. It is identical to that of Cs4EuI6 but with a slight smaller lattice constants due to the lanthanide contraction [17]. The citation is "The crystal structure refinement results are shown in Table 1. The Fraction atomic coordinates and equivalent isotropic displacement parameters are presented in Table 2. A crystal structure projection of Cs4YbI6 along the c-axis is shown in Fig. 3(a). Isolated and slightly distorted [YbI6]4octahedra alternate with Cs along the c-axis, and form one-dimensional chains. Cs atoms have two crystallographically independent sites, namely seven-coordinated Cs(1) site and six-coordinated Cs(2) site. The [YbI6]4- octahedra are face-sharing with distorted [Cs(2)I6]5- trigonal prisms in spiral columns. The enlarged view of the Cs(1)I7, Cs(2)I6, and the YbI6 polyhedra are presented in Fig. 3(b-d). 3.2. Optical and scintillation properties The photoluminescence emission and excitation spectra of Cs4YbI6 are depicted in Fig. 4(a) and (b). Unlike fully spin-allowed transitions of Ce3+ and Eu2+ activators, the lowest Yb2+ 4f135d excited state has a higher spin than the Yb2+ 4f14 ground state, and transitions between them are spin-forbidden. Observed from Fig. 4(a), the dominant emission band located at 20,833 cm−1 can be assigned to the spin-forbidden transition 4f13(2F7/2)5d1(t2g, HS) to 4f14(1S0) of Yb2+. As expected, it is approximately the same energy (21,551 cm−1) as the spin-allowed emission band of Eu2+ in Cs4EuI6. In Fig. 4(b), the emission band with relatively lower intensity at 22,727 cm−1 can be assigned to the spinallowed transition 4f13(2F7/2)5d1(t2g, LS) to 4f14(1S0) of Yb2+. The HS and LS are designated for the high-spin state (spin-forbidden) and lowspin state (spin-allowed). These two emission bands have an energetic separation of 1894 cm−1 because of the exchange interaction effect. The value is within a reasonable order of magnitude compared to the values found for other iodides, such as 2168 cm−1 for SrI2:Yb2+ [15].

Fig. 2. (a) A DSC curve and (b) a DVS curve of Cs4YbI6. The DVS curve of a NaI standard sample is also plotted for comparison.

Table 2 Fractional atomic coordinates and equivalent isotropic displacement parameters for Cs4YbI6.

Cs4EuI6 [17]. A differential scanning calorimetry scan of a Cs4YbI6 sample is shown in Fig. 2. Except for an unknown exothermic peak at 555 °C, this compound has singular melting (Tm) and crystallization (Tc) peaks. Its Tm and Tc are 550 and 510 °C, respectively. The unknown exothermic peak is possibly ascribed to the crystallization of a foreign phase during the measurements, similar to that observed in Cs4SrI6 462

Atom

Site

Symmetry

x

y

z

Ueq(Å2)

Yb Cs(1) Cs(2) I

6b 18e 6a 36f

3 2 32 1

0 0.04179(4) 2/3 0.36260(3)

0 1/3 1/3 0.50488(3)

0 1/12 1/12 0.066642(19)

0.018291 0.030709 0.026669 0.039509


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Fig. 5. Photoluminescence decay profiles of a Cs4YbI6 crystal sample: (a) λex= 360 nm and λem= 450 nm; (b) λex= 360 nm and λem= 480 nm. The monitored excitation and emission wavelengths and the fitted decay components and fractions are all presented in the inset.

Fig. 3. (a) Crystal structure projection of Cs4YbI6 along the c-axis, where the dark green polyhedra are isolated [YbI6]4- octahedra. The cyan, blue, and purple spheres represent Cs, Yb, and I atoms, respectively. Also shown are an enlarged views of the (b) Cs(1)I7, (c) Cs(2)I6, and (d) YbI6 polyhedra.

assigned to the transitions of 4f14(1S0) to 4f13(7F7/2)5d1(t2g), and the peak at 32,216 cm−1 to the 4f14(1S0) to 4 f13(7F5/2)5d1(t2g). Similar assignments can be applied to the excitation spectrum by monitoring the Yb2+ spin-allowed emission at 22,727 cm−1. The X-ray excited radioluminescence spectrum shown in Fig. 4(c) is dominated by the spin-forbidden transition of Yb2+ at 20,730 cm−1, slightly redshifted in comparison to that observed in the PL spectrum. A broad and weak emission band is centered at 17,500 cm−1. The origin is not understood yet, but a similar emission in SrI2:Yb2+ was ascribed to near-defect exciton [33] or impurity mediated electron-hole recombination [34]. The photoluminescence decay profiles at room temperature are plotted in Fig. 5. The photoluminescence decay profile monitoring at 450 nm (22,222 cm−1), corresponding to the wavelength of the spinallowed Yb2+ emission, is well fitted with a single exponential function. Its decay constant is 65 ns. The profile monitoring 480 nm (20,833 cm−1), corresponding the spin-forbidden emission, is well fitted by triple-exponential functions due to the admixture of spin-allowed and spin-forbidden transitions. The derived decay constants are 67 ns (1%), 1.2 μs (0.3%), and 12.4 μs (98.7%). The dominant decay time of 12.4 μs is responsible for the spin-forbidden transition of Yb2+ in Cs4YbI6. In general, the decay times of spin-forbidden and spin-allowed transitions of Yb2+ are usually 10–600 μs and 700–2500 ns at room temperature, respectively [15,35–39]. The relatively shortened decay constants of either spin-allow or spin-forbidden emissions of Yb2+ in Cs4YbI6 suggests the luminescence quenching at Yb2+ activators. The scintillation decay curve of a Cs4YbI6 crystal sample is plotted in Fig. 6. The best fit of the experimental data was obtained using a sum of two exponential functions. The two decay components are 33 ns and 400 ns, respectively. The latter one only accounts for 0.01% of total scintillation emission. For Yb2+ doped iodide scintillators with high light yield (over 50,000 photons/MeV), the principal scintillation decay time is generally in several hundred nanoseconds, such as 840 ns for CsBa2I5:1%Yb2+ and 610 ns for SrI2:0.5%Yb2+ [14]. The pulse height spectra shown in Fig. 7 indicates that the as-grown Cs4YbI6 crystal has extremely low light yield despite the fact that it has good uniformity. Taking into consideration the emission weighted quantum efficiency of the Hamamatsu R2059 PMT for Cs4YbI6, its absolute light yield is 2700 ± 200 photons/MeV at room temperature, which is only about 5% of that of Cs4EuI6. The energy resolution of Cs4YbI6 is about 23% at 662 keV. The low energy resolution is caused by a poor statistical contribution that a very limited number of photons was detected in the PMT.

Fig. 4. (a)(b) Photoluminescence emission and excitation spectra, and (c) X-ray excited radioluminescence spectrum of a Cs4YbI6 crystal sample. Gaussian fitted peaks are presented in green dash dot.

By monitoring the Yb2+ spin-forbidden emission at 20,833 cm−1, multiple Yb2+ excitation bands can be observed between 21,000 and 36,000 cm−1. The groups of excitation levels of Yb2+ activated compounds are formed by the coupling between a crystal-field split 5d electron and a spin-orbit split 4f13 core. The characteristic separation of the spin-orbit split 4f13 core levels (2F7/2 and 2F5/2) is approximately 10,000 cm−1 [31]. Each group has maximum of five levels due to the crystal field interaction of the 5d electron [32]. Therefore, the five Gaussian fitted peaks from 21,000 and 31,500 cm−1 can be tentatively 463


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Fig. 6. A scintillation decay profile of a Cs4YbI6 crystal sample under irradiation. The fitting curve is shown in green line.

137

Cs Fig. 8. X-ray induced afterglow profiles of Cs4YbI6 and Cs4EuI6 single crystals as well as a BGO reference sample. Inset is the enlarged region within the first 200 s after X-ray cutoff.

states. Consequently, the extremely low afterglow observed cannot be ascribed to low concentrations of defect states but rather to the luminescence quenching of Yb2+ activators. The luminescence quenching of activators at room temperature is commonly determined by the unwanted electronic structure [42,43]. The drastic thermal quenching of the d-f emission of Yb2+ ions is usually explained by the proximity of the lowest 5d excited state to the host conduction band [44]. It is reported that the 5d1 of Yb2+ is commonly 0.5 eV closer to the conduction band than that of Eu2+ in the same site in the same compound [44]. From the observation of Yb2+ luminescence, a rough estimate is that the quenching temperature for Yb2+ luminescence will be lower by about 200 °C compared to the corresponding value for Eu2+ [45]. Therefore, for a host composition, when its Eu2+ d-f emission does not quench below 500 K, the Yb2+ ion is likely to become an efficient activator because its d-f emission will not quench at room temperature. To illustrate the luminescence quenching mechanism in Cs4YbI6, the electronic band structure was calculated and plotted in Fig. 9. The electronic band structures of Cs4YbI6 exhibits Yb-4f-derived valence band maximum (VBM) and the Cs-6s-derived conduction band minimum (CBM). The fully occupied Yb-4f band is above the I-5p band; the flatness of this band is consistent with the strong localization of the Yb-4f orbitals. Although the Yb-5d states are above the CBM at the ground state as shown in Fig. 9, upon the 4f-5d excitation, the Yb-5d level can be brought below the CBM by the strong Coulomb attraction between the electron on the 5d level and the hole on the 4f level. This is confirmed by the experimentally observed Yb2+ induced optical absorption and emission. This behavior is

Fig. 7. Pulse height spectra of Cs4YbI6 samples under 137Cs gamma-ray source irradiation acquired by Hamamatsu R2059 PMT. Samples #1 and #2 were extracted from the first-to-freeze section and middle section of the as-grown boule, respectively. The spectrum of a 4 mm3 BGO sample is plotted for comparison. Note: the gain setting for BGO is half that used for Cs4YbI6.

3.3. Origins of the low light yield of Cs4YbI6 The light yield of inorganic scintillators is mainly influenced by the following factors: i) the optical quality of the crystals; ii) charge carrier traps; and iii) electronic structures. Regarding optical quality, the colorless Cs4EuI6 crystal can reach 70–80% transmittance at its emission wavelengths [17], but Cs4YbI6 has a lower transmittance of 50% at its emission wavelengths. The lower optical quality can result in a lower measured light yield. Moreover, the light yield can also be reduced by the charge carrier traps with energetically deep depth acting as nonradiative quenching centers. To indirectly probe the trap states, we investigated the room temperature afterglow of Cs4YbI6 and compared it with that of Cs4EuI6. Their afterglow profiles are plotted in Fig. 8 as well as that of a BGO reference. The afterglow level of Cs4YbI6 is orders of magnitude lower than that of Cs4EuI6, even lower than that of the BGO reference. In general, iodide scintillators grown by the Bridgman method are apt to have a long afterglow due to the existence of iodine vacancies acting as deep electron traps [40,41]. Iodide crystals grown by the Bridgman method are usually processed in a sealed quartz ampoule under high vacuum (~10−6 mbar). This makes iodide volatilization likely when the ampoule is heated. Due to a similar processing procedure, Cs4EuI6 should not be an exception. We recall the strong optical absorptions (color centers) observed in its transmission spectrum. This suggests that this crystal should have quite a few defect

Fig. 9. The electronic band structure of Cs4YbI6. 464


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similar to that observed in Cs4EuI6 [17]. However, the Yb-5d level relative to the CBM of Cs4YbI6 is higher than the Eu-5d level relative to the CBM of Cs4EuI6 (0.35 eV higher at the gamma point). This can be understood by the stronger screening of the nucleus charge in Yb2+ due to the fully filled 4f level. Thus, it is reasonable to believe that the Yb2+ luminescence quenching in Cs4YbI6 is caused by the strong non-radiative thermal ionization of electrons from 5d1 into the CBM due to a small energy separation between Yb2+ 5d1 and the CBM.

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4. Conclusions The Cs4YbI6 single crystal grown by the Bridgman method crystallized into the trigonal crystal system with a space group of R 3 c. The melting point and crystallization point are 550 and 510 °C, respectively. The Cs4YbI6 crystal exhibits blue-green emission centered at 450 and 480 nm due to spin-allowed and spin-forbidden transitions of Yb2+ activators. The RL spectrum is dominated by the spin-forbidden transition of Yb2+ peaking at 480 nm. The absolute light yield is 2700 ± 200 photons/MeV with a principal scintillation decay time of 33 ns. The electronic band structures of Cs4YbI6 exhibits Yb-4f-derived valence band maximum (VBM) and the Cs-6s-derived conduction band minimum (CBM). The low light yield is mainly ascribed to the strong thermal ionization of electrons from Yb2+ 5d1 states to the conduction band at room temperature. This work demonstrates the importance of the electronic structure for optical materials, i.e., the relative position of activators with respect to the CBM and VBM. Acknowledgements This work was supported by Siemens Medical Solutions. A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. The work at ORNL were supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. H. Shi was supported by the National Natural Science Foundation of China (NSFC) under Grants No.11604007 and the start-up funding at Beihang University. References [1] M. Nikl, A. Yoshikawa, Adv. Opt. Mater. 3 (4) (2015) 463–481. [2] M.J. Weber, Nucl. Instrum. Methods Phys. Res. A 527 (2004) 9–14. [3] V.B. Mikhailik, H. Kraus, S. Henry, A.J.B. Tolhurst, Phys. Rev. B 75 (2007) 1–6 (184308). [4] M.J. Weber, R.R. Monchamp, J. Appl. Phys. 44 (1973) 5495–5499. [5] W.W. Moses, S.E. Derenzo, IEEE Trans. Nucl. Sci. 36 (1989) 173–176. [6] F.G.A. Quarati, P. Dorenbos, J. van der Biezen, A. Owens, M. Selle, L. Parthier, P. Schotanus, Nucl. Instrum. Methods Phys. Res. A 729 (2013) 596–604. [7] H. Wei, V. Martin, A. Lindsey, M. Zhuravleva, C.L. Melcher, J. Lumin. 156 (2014) 175–179.

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