Luminescence and Charge Trapping in Cs2HfCl6 Single Crystals

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Luminescence and Charge Trapping in CsHfCl Single Crystals: Optical and Magnetic Resonance Spectroscopy Study Robert Král, Vladimir Babin, Eva Mihokova, Maksym Buryi, Valentyn V. Laguta, K. Nitsch, and Martin Nikl J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

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The Journal of Physical Chemistry

Luminescence and Charge Trapping in Cs2HfCl6 Single Crystals: Optical and Magnetic Resonance Spectroscopy Study R. Král1, V. Babin1, E. Mihóková1, M. Buryi1, V. V. Laguta1, K. Nitsch1, and M. Nikl*1 1

Institute of Physics, CAS v.v.i., Cukrovarnická 10, 162 00 Prague, Czech Republic

*Corresponding author: [email protected], +420220318445 (tel), +420233343184 (fax)

Abstract Single crystals of the undoped Cs2HfCl6 were grown using the vertical Bridgman method. Photoluminescence spectra and decays were measured in the extended temperature range of 8 – 500 K and approximated by a phenomenological model. Self-trapped exciton and defect-related emissions were clearly distinguished with the onset of thermal quenching above the room temperature. Electron paramagnetic resonance experiment revealed formation of two configurations of Vk center ( Cl 2− molecular ion) at low temperatures. One of the Vk centers is created by a hole self-trapping at two neighboring Cl ions. In the temperature range 70−80 K, this state is thermally disintegrated and reappears in another configuration formed most probably by an interstitial Cl ion (an analog of the H-type center in alkali halides).

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Introduction Single crystals of Cs2HfCl6 have recently been identified as promising scintillating material.1,2 Furthermore, extended first principles calculations of Cs2HfCl6 electronic and optical properties have been reported which, among others, indicate the self-trapping of holes and electrons in the form of VK and polaron centers, respectively.3 Renewed interest in halide scintillators appeared in late 1990s when rare earth-based halides were studied and later on the Ce-doped LaCl3, LaBr3, and CeBr3 were discovered as scintillators with both the high light yield and excellent energy resolution.4,5,6,7 Further optimization of the LaBr3:Ce by divalent ion codoping provided the material with the best energy resolution within single crystal scintillators.8,9 Moreover, a growing interest in materials from the elpasolite group, such as Cs2LiYCl6:Ce, Cs2LiLaCl6:Ce, Cs2NaLaBr6:Ce, Cs2NaLaI6:Ce, and Cs2LiLaBr6:Ce and those with mixed anions has arisen.10,11 Among them, the Li-containing compositions can also be considered for thermal neutron detection. Most recently, SrI2:Eu single crystal was rediscovered, having ultrahigh light yield and excellent energy resolution as well.12,13 Similarly, the LuI3:Ce single crystals have been repeatedly addressed and found very competitive in the view of achievable light yield.14,15 All above mentioned materials are highly hygroscopic which makes their practical application difficult and expensive. Single crystals of Cs2HfCl6 were already prepared in 1970s serving as the host for electron paramagnetic resonance (EPR) studies of the doped molybdenum and tungsten centers.16,17 Furthermore, sharp emission lines of d-d transitions of Mo3+ and Re4+ doped in Cs2HfCl6 were studied in detail.18,19 The Cs2HfCl6 powder was reported as an efficient X-ray phosphor with a broad emission band in the blue spectral region.20 Recent experimental studies of its single crystals evidenced its high


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intrinsic scintillation efficiency giving rise to high light yield value within 37 000 – 54 000 photons/MeV measured in the undoped material.1,2,21 In fact, two emission bands were clearly resolved: (i) the 375 nm emission band belonging to the Cs2HfCl6 structure itself and (ii) the 435 nm emission band arising due to Zr impurity originating from the contamination of the hafnium chloride (HfCl4) raw material. Scintillation decays of these emissions obtained at the room temperature (RT) were approximated by two exponential functions with decay times of 0.3 + 4.4 µs and 2.2 + 8.4 µs, respectively.1 Cs2HfCl6 single crystals appear to be very interesting for scintillator applications due to their preliminary characteristics together with cubic structure of high symmetry and low hygroscopicity.1,2 Its cubic crystallographic structure could enable manufacturing of the transparent ceramics since the ceramic technology has already been explored in halides.22 At the same time, the theoretically forecasted self-trapping of charge carriers needs experimental verification and such a property might have a serious impact on the luminescence mechanism.3 Consequently, the study of the temperature dependence of the luminescence characteristics (so far only room temperature studies were addressed) and revealing the electron paramagnetic resonance (EPR) active centers is of high interest. Thus, in the present work, we focus on these two aspects using the correlated steady-state and time-resolved luminescence spectroscopy as well as EPR experiments applied in a broad temperature range.

Experimental details The Cs2HfCl6 compound was prepared by the direct synthesis from cesium chloride (CsCl) and HfCl4 raw materials mixed together in stoichiometric ratio 2:1. Synthesized material was


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subsequently purified by combination of gaseous halogenation agents introduced into its melt and zone melting technique. The Cs2HfCl6 single crystals were grown in sealed quartz ampoules by unseeded vertical Bridgman method at the pulling rate of 0.3-0.4 mm/h and temperature gradient of ca. 40 K/cm. The dimensions of the as-grown crystals defined by the shape of the ampoule were 12 mm in diameter and ca. 40 mm in length. The crystals showed twining and contained several secondary grains in the tip of the ampoule. Nevertheless, only few of them were preferred and allowed growth of homogeneous grains of sufficient size from 5 to 15 mm. The Cs2HfCl6 samples of dimensions of ca. 4 × 4 × 3 mm3 for optical characterizations were cleaved from the grown crystals along the (111) plane. No cutting or polishing of specimens was required. The Cs2HfCl6 crystallizes in simple cubic structure (space group Fm3m ) with the lattice constant of 1.042 nm.23 It is isostructural to K2PtCl6 and analogous to anti-fluorite CaF2 structure in which Cs occupies all tetrahedral sites and Hf all face centered cubic sites in the lattice. The Cl anions equally surround the Hf cation forming undistorted [HfCl6]2- octahedron.1,24 The obtained Cs2HfCl6 crystal was oriented by the Laue X-ray diffraction method and cut in the (110) plane for measurements of all EPR spectra. The EPR characterizations were performed at 9.25-9.26 GHz with the standard 3 cm wavelength of the EPR spectrometer in the 15-20 K temperature range using an Oxford Instrument liquid helium (LHe) cryostat. At first, the crystal was X-ray irradiated (50 kV, 15 mA, Co target, 10−15 min) at the temperature 77 K and then quickly moved into the LHe cryostat of the spectrometer for measurements. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured using the custom made 5000 M Horiba Jobin Yvon fluorescence spectrometer equipped with a TXB-04 photon counting detector (IBH Scotland). In the steady state spectra measurements, the


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sample was excited by deuterium lamp (Heraus Gmbh). All spectra were corrected for the spectral distortions of the setup. Temperatures between 8 and 500 K were controlled by a Janis Instruments closed cycle refrigerator. The Xe microsecond flashlamp (IBH Scotland) was used for the luminescence decay measurements using the multichannel scaling method. True decay times were obtained using the convolution of the instrumental response function with an exponential function and the least-square sum fitting procedure (Spectra Solve software package, Ames Photonics Inc.).

Results and discussion A. Optical characteristics The PL and PLE spectra of Cs2HfCl6 are displayed in Figures 1 and 2. Under excitation around the absorption edge, at 200 nm, the PL spectrum features a broad emission band with the maximum at about 375 nm both at RT and 8 K, Figure 1. Given the position of the PLE peak and the calculated value of the band gap of ~6.3 eV one may associate this band with the self-trapped exciton (STE) emission.3 Excitation below the absorption edge results in a defect or impurityrelated emission, at RT most effectively excited at 260 nm, see Figure 2. The PLE spectra of STE emission at RT point to the onset of absorption edge at about 230 nm. As expected, at low temperatures the edge shifts towards shorter wavelengths, at 8 K it appears at about 220 nm, see Figure 1. Defect-related emission peaking at 460 nm is mostly excited at longer wavelengths (with respect to the STE emission) up to about 260 nm, as shown in Figure 2. In addition, the Cs2HfCl6 crystals exhibited wide transmission range extending from ultraviolet to middle infrared region, from ca. 245 to 2500 nm at room temperature.


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Figure 1. PL and PLE spectra of STE in Cs2HfCl6 at 8K and RT. Excitation and emission wavelengths are reported in the figure.


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Figure 2. PL and PLE spectra of the defect in Cs2HfCl6 at 8K and RT. Excitation and emission wavelengths are reported in the figure. Decay kinetics of both STE and defect-related centers was monitored within the temperature range of 8-500 K. Decay times of the STE emission as a function of temperature are displayed in Figure 3a. They show a temperature evolution quite typical for the slow decay component of the STE emission observed in other halides or various hosts.25,26,27 Typical fastening of the decay times below 10 K points to the excited state dynamics governed by two closely spaced excited states with the thermal exchange of their populations. They usually represent split excited triplet state. The third singlet state lying above and thermal exchange of populations within all three states then govern the decay time fastening above 100 K. The decay time fastening starting at around 300 K is not due to classical thermal quenching to the ground state as shown by the temperature evolution of the spectral intensities in Figure 3b. The fast component of the decay (corresponding to the three-level structure of the excited state) of the STE emission is not observed probably due to their low intensity. As mentioned above, the temperature evolution of the STE spectral intensity (Figure 3b) show the onset of thermal quenching just above room temperature. The intensity up to the onset of thermal quenching is expected to be constant. Interestingly, intensity increase up to 260 K was observed, however, this phenomenon needs further study. Possible reasons could be due to temperature dependence of exciton state creation, i.e. processes preceding the exciton relaxation and emission process itself. At temperatures above 330-350 K thermal quenching process noticeably accelerates the decay and decreases the emission intensity.


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Figure 3. STE emission as a function of temperature. In (a) decay times and (b) normalized intensity of emission spectrum (spectra were integrated within 250-600 nm range) are displayed. The inset of (b) shows the same graph with the linear abscissa for better visualization. Empty symbols are experimental data, solid lines are best fits of the model (see eqs. (1) to (5) in the text) to the data. Parameters used in calculations are as follows: k1=6.7×104 s-1, k2=1.45×105 s-1, k3=3.5×106 s-1, K=1×105 s-1, D=1.5 meV, Ki=3×106 s-1, E=70 meV, K1x=K2x=8×108 s-1, E1x=E2x =400 meV.


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The decay kinetics of the defect-related emission exhibited two decay components (Figure 4a), slow and fast, the latter of much lower intensity than the former. The temperature evolution of the defect-related emission (see Figures 4a and 4b) indicates the similar structure of the excited state as described above. However, the onset of decay time fastening at higher temperatures points to the significantly larger energy split of the lowest states than in the case of STE emission. This situation is typical of the ns2 impurity emission in various hosts, see e.g. Zazubovich et al.,26. The temperature evolution of the defect-related spectral intensity (Figure 4b) shows the onset of thermal quenching at around 350 K. A non-monotonic intensity increase up to 300 K is more pronounced than in the case of the STE emission in Figure 3b. Explanation of this phenomenon is uneasy and requires further investigation. The intensity maxima at around 280-320 K for the STE and defect emissions correlate with temperature range of thermal disintegration of Vk –type hole centers observed by EPR, see below.


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Figure 4. Defect-related emission as a function of temperature. In (a) decay times and (b) normalized intensity of emission spectrum (spectra were integrated within 300-600 nm) are displayed. The inset of (b) shows the same graph with the linear abscissa for better visualization. Empty symbols are experimental data, solid lines are best fits of the model (see eqs. (1)-(5) in the text) to the data. Parameters used in calculations are k1=1.3×103 s-1, k2=1.7×104 s-1, k3=4×105 s-1, K=1.3×104 s-1, D=35 meV, Ki=1×107 s-1, E=140 meV, K1x=K2x=5×108 s-1, E1x=E2x =430 meV.


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The excited state dynamics of the luminescence centers responsible for the STE and defectrelated emissions of Cs2HfCl6 can be described within the phenomenological model sketched in Figure 5. In this model the metastable (1) and radiative (2) levels of the triplet state and the singlet state, level (3), are taken into consideration.26,27

Figure 5. Energy level diagram used for the description of the excited states dynamics of STE and defect-related luminescence centers in Cs2HfCl6. The time evolution of the populations N1, N2, N3 of the excited levels 1, 2, and 3, respectively, can be described by the following rate equations: dN1/dt = − k1N1 − k12N1 − k13N1 + k21N2+ k31N3 − k1xN1

(1)

dN2/dt = − k2N2 − k21N2 − k23N2 + k12N1+k32N3− k2xN2 dN3/dt = − k3N3 − k31N3 − k32N3 + k13N1+k23N2

.


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Here k1, k2, k3 are radiative transition rates from levels 1, 2, and 3, respectively, k12, k21 are non-radiative rates of phonon assisted transitions between the radiative level 2 and metastable level 1, k3(2)1, k1(2)3 are non-radiative rates of transitions between the levels 3(2) and 1. Nonradiative transitions between levels 1 and 2 can be written as: k21= K(n+1), k12= Kn, n=1/[exp(D/kBT)−1]

(2)

and non-radiative rates of transitions between the levels 3 and 1: k31= Ki (n’+1), k13= Ki n’, n’=1/[exp(E/kBT)−1] .

(3)

Here K, Ki, n, n’, D, E are the zero-temperature transition rates between the excited levels, the Bose-Einstein factors and energy distances between the excited levels, respectively. Since D

Luminescence and Charge Trapping in Cs2HfCl6 Single Crystals

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