Spectroscopic properties of Pr3+ and Er3+ -doped KPb2Br5 crystals were investigated for possible applications in eye-safe lasers as well as Ce3+-doped KPb2Cl5 and Eu2+-doped KPb2Cl5/KPb2Br5 for potential radiation detectors. The studied materials were synthesized through careful purification of starting materials including multi-pass zone-refinement and halogenation. The growth of the purified materials was then carried out through the vertical or horizontal Bridgman technique. Under resonant excitation, infrared (IR) emissions at ~1.5 μm and ~1.6 μm were observed from Er:KPb2Br5 and Pr:KPb2Br5 corresponding to the 4f-4f transitions of 4I13/2→4I15/2 and 3F4,3F3→3H4, respectively. Emission characteristics of the ~1.5 μm Er3+ and ~1.6 μm Pr3+ transitions including IR to visible upconversion emission studies were also discussed. Under xenon lamp excitation, spectroscopic results showed allowed 5d-4f Ce3+ emission centered at ~375 nm in Ce3+-doped KPb2Cl5. Fast photoluminescence decay time of ~30-50 ns was attained from Ce:KPb2Cl5, while X-ray excited emission at ~530 nm appeared to originate from the host KPb2Cl5 crystal. In addition, a commercial Ce:YAP (yttrium aluminum perovskite, YAlO3) crystal was included in this study for comparison. Eu2+ 5d-4f emissions were not observed from Eu2+-doped KPb2Cl5 and KPb2Br5 crystals.
© 2017 Optical Society of America
A significant amount of interest has been focused on the luminescent properties of rare-earth (RE) ion doped crystals for a wide variety of applications including eye-safe lasers, phosphors, IR-countermeasures, gas sensing, and scintillators [1–7]. Recent results have indicated that PbCl2 and PbBr2 based ternary lead halides (e.g. KPb2Cl5, RbPb2Cl5, KPb2Br5, RbPb2Br5) are promising novel host materials for infrared (IR) solid-state lasers [7–10]. The low maximum phonon energies of these ternary lead halides lead to reduced nonradiative decay rates and efficient emission from RE ions at IR wavelengths. IR-to-visible upconversion luminescence in RE-doped low-energy phonon crystals has been studied for potential applications in displays, sensors, and upconversion lasers [11,12]. In addition, great effort on the investigation of pure and RE activated halide crystals were made to develop efficient scintillators [6,13–16]. Recently, the development of high performing halide scintillator materials including Ce3+-doped LaBr3 and LaCl3 as well as Eu2+-doped SrI2 and BaI2 has led to significant improvements in the applications of radiation detectors [13–16].
Lead based binary and ternary lead halides (PbCl2, PbBr2, KPb2Cl5 and KPb2Br5) are regarded as wide-gap semiconductors with band gap energies ranging from ~2.5 eV to 4.8 eV [17,18]. They are also considered non-hygroscopic crystals with narrow phonon spectra of <140 cm−1 to ~203 cm−1 [4,7,19]. They are monoclinic crystals and transparent in the 0.3-20 μm for the chlorides and 0.4-30 μm spectral range for the bromides [7–12]. When ternary lead halides (e.g KPb2Br5) are doped with RE ions, it is assumed that the RE3+ ion substitute into the Pb2+ lattice sites, while the vacancy of K+ provides for the charge compensation. In this work, near-IR emission properties of Pr3+- and Er3+-doped KPb2Br5 (KPB) crystals were evaluated for possible applications in 1.5 μm light source development for eye-safe lasers and optical communications. Moreover, preliminary spectroscopic characterization on the UV-Vis luminescence of Ce3+-doped KPb2Cl5 (KPC), Eu2+-doped KPC and KPB crystals are evaluated for potential radiation detection applications.
2. Materials and experimental details
Ternary potassium lead chloride and potassium lead bromide were synthesized by reacting stoichiometric proportions of using ultra-dry and high purity (99.999%, Alfa Aesar) starting materials of PbCl2, KCl, PbBr2, and KBr beads. The synthesized material was subsequently purified through a horizontal zone-refinement. The purified host materials were then loaded with 0.5-2 wt.% of ultra-dry RE-halide powders (PrBr3, ErBr3, CeCl3, EuCl2, EuBr2) (99.99%, Alfa Aesar). Following halogenation of the synthesized RE doped KPb2X5 (X = Cl, Br) materials, crystal growth experiments were performed using a two-zone vertical or a modified horizontal Bridgman technique. More details of the purification and material preparation of RE doped KPC and KPB were described elsewhere . Samples of good optical quality with dimensions 3 × 5 × 5 mm3 were selected and polished for optical spectroscopy measurements.
The room temperature absorption spectra were recorded using a Cary 5000 spectrophotometer. Infrared emission studies were carried out using a Tm-fiber laser operating at 1.907 µm, an Er-fiber laser operating at approximately 1532 nm or diode lasers operating at 1.45 µm. The ultraviolet (UV) luminescence spectra were obtained using Cary Eclipse Fluorescence Spectrophotometer, PTI TimeMaster Fluorometer or Jobin Yvon Fluorog-3 Spectrofluorometer. The visible/near-IR emissions were dispersed in a 0.3 m monochromator in conjunction with a photomultiplier tube (PMT)/InGaAs detector. The emission spectra were recorded using a standard lock-in technique. Excitation spectra and emission decay measurements were performed using the output (5 ns pulses, 10 Hz) of a Nd:yttrium aluminium garnet (YAG) pumped Optical Parametric Oscillator (OPO) system. Radioluminescence was obtained by X-ray irradiation through a Be window, using a Phillips 2274 X-ray tube with a tungsten target operated at 50 kV and producing an effective X-ray energy of ~25 keV with a dose rate of 1.75 Gy/s for 30 s. The luminescence decay time for UV-visible spectral region was obtained using a HORIBA Jobin Yvon Fluorolog-3 Spectrofluorometer paired with a time-correlated single photon counting (TCSPC) module. The excitation source was a NanoLED pulsed at 314 nm, each pulse was <1.5 ns in length.
3. Results and discussion
3.1 Comparative studies of Pr3+- and Er3+- doped potassium lead bromide for eye-safe laser applications
Efficient laser emission in the 1.5 – 1.6 µm eye-safe wavelength window continues to be of significant interest for civilian and military applications [10–12]. Resonantly pumped Er:KPB and Pr:KPB were explored for eye-safe laser applications given that Er3+ and Pr3+ have the metastable emission transitions 4I13/2 → 4I15/2 and 3F3,3F4 → 3H4, respectively, that fall within the eye-safe wavelength regime. Moreover, the low phonon-energy characteristic of KPB (~138 cm−1) implies that additional thermal loads via multiphonon relaxation between levels is weaker [20,21]. Fig. 1 depicts the emission cross-section spectrum of the 3F3,3F4 → 3H4 transition of Pr3+. This spectrum was generated by exciting the 3F3,3F4 band of Pr3+ with a 1440 nm diode laser. Figure 1 also shows the 4I13/2 → 4I15/2 emission band of Er3+, which was produced by exciting the 4I13/2 with a 1532 nm Er-fiber laser. To visualize the entire 4I13/2 → 4I15/2 Er3+emission band free from laser scattering, the emission was measured under both 970 nm and 1532 nm laser excitation and then the two spectra were joined together. This is necessary because the 1532 nm pump wavelength is within the 4I13/2 → 4I15/2 emission band of Er3+, as was displayed in the inset of Fig. 1. The peak emission cross-sections of Er:KPB and Pr:KPB were calculated to be ∼1.6 x 10−20 cm2 at 1540 nm and ∼6.1 x 10−20 cm2 at 1667 nm, respectively, using the well- known Fuchtbauer-Ladenburg relation . It is clear from Fig. 1 that the emission cross-section arising from Pr3+ is spectrally broad relative to Er3+possessing a full width at half maximum of ∼87 nm which provides a possibility of larger laser tunability in the eye-safe wavelength regime. Moreover, Pr3+ has an overall larger peak emission cross-section relative to Er3+-doped in KPB as well as the more traditional host YAG .
As a byproduct of resonant excitation of Pr:KPB at 1550 nm, visible upconversion emission was observed. Shown in Fig. 2(a) is the visible upconversion emission from Pr:KPB when excited at ∼1550 nm. For comparison, the visible emission was also generated by direct 474 nm and 590 nm laser excitation in Figs. 2(b) and 2(c), respectively. Similar to exciting Pr3+ at 474 nm, emissions from the 3P0 → 3H4, 3H6, 3F2, 3F3 and 3P1 → 3H5, 3F2 transitions were observed. An additional emission band not seen under 474 nm excitation corresponding to the 1D2 → 3H4 transition was observed and can be also be generated directly by exciting the 1D2 absorption band of Pr3+ (Fig. 2(c)). According to reference , the visible upconversion in a similar system, Pr:Rb2Pb2Cl5, is suggested to be attributed to the following scheme: (1) upconversion to the 1G4 level via this process 3F3 + 3F3 → 1G4 + 3H5 which contributes to the 3H5 population and (2) a series of excited state absorption (ESA) of the pump which leads to populating the 1D2 and 3P1,2,3 states via the following processes: Pump + 3H5 → 1G4, Pump + 1G4 → 1D2 and Pump + 1D2 → 3P1,2,3. However in Pr:KPB, the decay transients of the emission coming from the 1D2 state (600 nm) and 3P0 states (650 nm) suggests that energy transfer upconversion (ETU) is responsible for populating the visible emitting states under 1.5 μm excitation. The transients show a significant rise time after the laser pulse followed by a decay time of τ ~213 µs and τ ~140 µs for the 1D2 and 3P0 states, respectively (Figs. 2(d) and 2(e)). The emission decay times are significantly longer relative to direct excitation of the 1D2 (τ ~25 µs) and 3P0 (τ ~17 µs) states (Figs. 2(d) and 2(e)).
This feature is indicative of nonradiative ETU process taking place . To obtain population within the 1D2 and 3P0,1 levels, the 1G4 level must be populated. Initially, the 1.5 µm pump populates the 3F3,3F2 levels and, due to the resonance conditions amongst the 3F3 → 3H5 and 3F3 → 1G4 transitions, the ETU process 3F3 + 3F3 → 3H5 + 1G4 is possible  which provides a channel for populating the 1G4. Once there is population within the 1G4 state, the 1D2 state is populated through the ETU process 1G4 + 1G4 → 1D2 + 3H5 . Finally, once there is concurrent population within the 1G4 and 1D2 levels, the 3P0 and 3P1 levels may be populated via the ETU process 1G4 + 1D2 → 3H5 + 3P1 . In addition, if there is significant population in the 3F2 level, the 3P0 level could be populated via the ETU process 1D2 + 3F2 → 3P0 + 3H4  (Fig. 5).
Visible and infrared upconversion luminescence was observed in Er:KPB under resonant, 1532 nm laser excitation of the 4I13/2 excited state (Fig. 3). The dominant upconversion emission from Er:KPB is in the infrared and corresponds to the 4I9/2 → 4I15/2 transition with a peak at 807 nm (Fig. 3). A weaker infrared upconversion emission band at 858 nm corresponds to the 4S3/2 → 4I13/2 transition of Er3+. The dominant 4I9/2 → 4I15/2 infrared upconversion is typical in low phonon-energy hosts. For example, under similar pumping conditions, the dominant upconversion transition in Er-doped yttria (Er:Y2O3) is not the 4I9/2 →4I15/2 but rather the visible transitions. This difference is due to the fact that yttria has a larger phonon energy (450-550 cm−1) relative to KPB (∼138 cm−1), and therefore multiphonon emission from the 4I9/2 is more competitive. In even larger phonon energy hosts such as YAG the 4I9/2 →4I15/2 upconversion emission is negligible. The most intense visible upconversion emission arising from Er:KPB is green (~548 nm) and is due to the 4S3/2→4I15/2 transition of Er3+. A second, weaker green upconversion emission band 2H11/2→4I15/2 was also observed at 528 nm. The 2H11/2 excited state is only ~800 cm−1 above 4S3/2, an energy difference that can be bridged by high frequency phonons. Consequently, the 2H11/2 and 4S3/2 excited states are thermally coupled at room temperature. Thus, at 300 K these two states will be treated as one state and will be labeled as 2H11/2 + 4S3/2. A technique commonly used to decipher if ETU or ESA is populating upper energy levels involves comparing the excitation spectrum of the upconverted emission with the corresponding ground state absorption spectrum [23–25]. An indicator that ETU is active occurs when the excitation spectrum of the upconverted emission looks comparable to the corresponding ground state absorption spectrum [23–25]. Evidence of ESA occurs when the excitation spectrum of the upconverted emission possesses additional absorption peaks not seen in the corresponding ground state absorption band. These peaks represent additional absorption transitions that occur after the initial ground state absorption [23–25]. Depicted in Figs. 4(a) and 4(b) are the 300 K excitation spectrum of the 4I9/2 → 4I15/2 and 2H11/2 + 4S3/2 → 4I15/2 upconverted emissions from Er:KPB along with the 4I15/2 → 4I13/2 ground state absorption spectrum of Er:KPB. It is evident that the excitation spectra in Figs. 4(a) and 4(b) are very similar to the 4I15/2 → 4I13/2 ground state absorption spectrum of Er:KPb2Br5, indicating that ETU is likely responsible for the population of 4I9/2 and the thermally coupled 2H11/2 + 4S3/2 levels rather than ESA.
To further confirm the nature of the upconversion mechanisms, the decay transients of the upconverted emissions were collected. Figure 4(c) displays the 4I9/2 → 4I15/2 decay transients of Er:KPB after 1532 nm pulsed laser excitation (~5 ns). The transients show a rise time after the laser pulse followed by a decay time of τ ~4.2 ms. The emission decay time is significantly longer relative to direct excitation of the 4I9/2 (τ ~1.9 ms ). This feature is indicative of nonradiative ETU process taking place . Furthermore, another characteristic of ETU is that the decay time of the upconverted emission corresponds to the decay time of the intermediate state . In the present case, the decay time of the upconverted 4I9/2 → 4I15/2 emission from Er:KPb2Br5 is 4.2 ms, which is similar to the 4.6 ms decay time of the intermediate state (4I13/2) . In the case of the radiative ESA process, the decay occurs immediately after the laser pulse given that ESA occurs only during the laser pulse. The decay time of the upconverted emission via ESA is similar to the decay time after direct laser excitation [24,26]. Displayed in Fig. 4(c) is the decay transient of the 2H11/2 + 4S3/2→4I15/2 emission, where there is a rise time after the laser pulse followed by a decay times of τ ~1.9 ms. The emission decay time is significantly longer relative to direct excitation of the 2H11/2 + 4S3/2, which has a value of τ ~180 µs . It has been reported that excited state absorption is likely the mechanism responsible for the green upconversion in Er:KPb2Cl5 under excitation of the 4I13/2 . However, the decay transient in Fig. 4(c) along with the excitation spectrum of the 2H11/2 + 4S3/2 → 4I15/2 upconverted emission from Er:KPB (Fig. 4(b)) suggests that ETU is responsible for populating the 2H11/2 + 4S3/2 → 4I15/2 levels. In addition, the decay time of the upconverted 2H11/2 + 4S3/2 → 4I15/2 emission is 1.9 ms, which is identical to the 1.9 ms decay time of the intermediate state (4I9/2) . Consequently, population within the 4I9/2 and 2H11/2 + 4S3/2 levels are assigned to the energy transfer upconversion processes (4I13/2, 4I13/2) → (4I15/2, 4I9/2) and (4I13/2, 4I9/2) → (4I15/2,2H11/2 + 4S3/2), respectively (Fig. 5).
3.2 Studies of Ce3+- and Eu2+- doped potassium lead halides for scintillator applications
Scintillator research has attracted much attention over the past years for various applications such as monitoring nuclear materials, medical imaging, high-energy particle physics, industrial inspections, and gas exploration [6, 13–16]. Several rare-earth activated halide scintillators have been established and showed highly favorable scintillation materials [6, 13–16]. Divalent Europium and trivalent Cerium have been widely exploited as activators in several solid-state phosphor and scintillator materials [13–16, 28]. In recent years, allowed 5d-4f transitions of Pr3+ and Nd3+ ions giving rise to luminescence in the UV to visible wavelength range have also gained attention owing to their promising scintillation characteristics [29,30]. Thus, the search for new materials as well as the optimization of current scintillators has continued to be an active area of research with important implications for various civilian and homeland security related applications. In the following, the spectroscopic characterization of Ce3+-doped KPC as well as Eu2+-doped KPC and KPB were explored for possible applications in radiation detection.
3.2.1 Ce3+-doped KPb2Cl5
In this work, 2 wt. % Ce3+-doped KPC was grown using a self-seeded Bridgman technique employing a two-zone furnace. Additional chlorination (bubbled through high-purity HCl gas for further removal of oxide impurities) was carried out for the investigated 2 wt.% Ce:KPC. In the previous study on 1 wt.% Ce:KPC , a modified horizontal Bridgman growth technique was employed and no further purification was performed. A commercial Ce:YAP crystal (Scionix Holland) was included for a comparative study on the luminescence spectroscopy. The excitation and emission spectra of Ce:YAP and Ce:KPC crystals were obtained from room temperature luminescence measurements, as displayed in Fig. 6 and Fig. 7, respectively. The emission spectra of Ce:YAP were collected in the spectral range of 300-450 nm under 250 nm, 270 nm, and 300 nm excitations. The strongest Ce3+ emission centered ~365 nm under 300 nm pumping was observed from Ce:YAP crystal (Fig. 6). A broad excitation band covering 250-340 nm for the Ce3+ emission at 365 nm was observed from Ce:YAP as reported by other studies of this material [30,32,33]. In the case of Ce:KPC, the broad emission spectrum centered ~375 nm emission was obtained under 310 nm excitation (Fig. 7). The excitation spectrum of the Ce3+ emission monitored at 375 nm revealed a strong excitation peak at ~310 nm along with weaker excitation bands between 220 and 270 nm. On the other hand, the previous study on 1 wt.% Ce:KPC showed strong excitation bands between 200 and 270 nm along with very weak excitation bands between 300 and 330 nm (inset of Fig. 7, top) . Similar absorption features were also observed from Ce3+-doped LaCl3 hosts and, reference  reported one broad band centered at ~200 nm and one saturated band between 237 and 294 nm. It was suggested that the excitation bands at higher energy are most likely related to exciton absorption (host absorptions), and that the other broad band can be ascribed to the 4f → 5d absorptions of Ce3+ . The Ce3+ emission results from the electron transition from the lowest 5d crystal-field level to the 4f ground state of the Ce3+ ion. It was realized that a relatively narrow emission spectrum of 1 wt.% Ce:KPC was obtained under 250 nm pumping where the host absorption was dominant (inset of Fig. 7, top). It seems likely that the existence of different Ce3+ centers could probably be involved in the previous study of 1wt.% Ce:KPC.
The optical excitation and emission transitions of Ce:YAP are illustrated in the energy-level diagram as well as tentatively assigned 4f-5d transition for Ce:KPC (inset of Fig. 6) [29–33]. No Ce3+ emission was observed from the present sample 2 wt.% Ce:KPC under excitations between 220 and 270 nm suggesting that these are host-related excitation bands. Furthermore, the observed 5d-4f Ce3+ emission was confirmed by measuring the emission spectrum of undoped KPC under 310 nm pumping, which did not show any features between 350 and 550 nm (Fig. 7, dashed line). The 5d electrons are not efficiently shielded by other electrons, and the energy levels of the 4f-5d electronic configuration are therefore strongly influenced by the crystal field. Consequently, the energy levels in the 4f-5d electronic configuration can vary significantly between different hosts. It can be noted that the transmittance spectrum of the Ce:KPC crystal partially overlaps with the emission spectrum, indicating that some self-absorption is likely to occur (inset of Fig. 7, bottom). Studies have shown for several crystal scintillators (Ce:LaCl3, Ce:LaBr3, CeF3) that the light output of the crystals is reduced when the absorption edge overlaps with the emission spectrum .
The photoluminescence lifetime of Ce3+-doped KPC was determined to be ~30-50 ns, which is comparable to other Ce-doped halide crystals [15–18]. Figure 8 shows the X-ray excited emission spectra of Ce-doped KPC and undoped KPC recorded at room temperature. Radioluminescence (RL) shows a broad band centered at ~530 nm for Ce:KPC and ~600 nm for undoped KPC. The radioluminescence of Ce: KPC appears to be in a different spectral region as the photoluminescence (~375 nm) of Ce3+ ions. The radioluminescence of Ce-doped chloride based materials is typically obtained in the 300-400 nm spectral region [6,15,16]. This suggests that X-ray excitation of Ce:KPC is inefficient in exciting Ce3+ but rather leads to KPC host emission. Possible reasons include inefficient incorporation of Ce3+ into KPC, introduction of impurities, or defects-related trapping levels which then hinder the energy transfer and capture processes. In order to gain more insight into the Ce3+ incorporation in the KPC crystal, detailed studies of the temperature dependence of the excitation and emission spectra as well as the decay dynamics are required.
3.2.2 Eu2+-doped KPb2Cl5 and KPb2Br5
The transmission spectra of undoped KPC and Eu2+-doped KPC as well as and undoped KPB and Eu2+-doped KPB were recorded in the 200-800 nm wavelength range at room temperature as shown in Fig. 9(a) and 9(b), respectively. The transmittance of undoped KPC and KPB was ~40% with absorption edges of ~330 nm and ~380 nm, respectively. A reduced transmission was observed for the Eu-doped crystals with an onset of the optical absorption band starting at ~500 nm, indicating the incorporation of Eu ions into the host materials . The optical absorption band below 430 nm can be tentatively attributed to the 4f → 5d transitions of Eu2+ ions , as shown in Fig. 9(c). Under xenon lamp excitation, no blue luminescence was observed from the investigated crystals probably due to concentration quenching of the Eu2+ ions and/or the presence of Eu-related defects which can lead to quenching . It was reported that Eu2+ concentrations of greater than ~0.5 mol% exhibited concentration quenching effects in Eu2+-doped fluoride and chloride crystals [35,36]. The Eu2+ emissions from Eu-doped scintillators are typically obtained in the spectral range of ~400-450 nm arising from the 5d → 4f transition to the ground state (8S7/2) of the 4f7 configuration [35–37]. As described in Fig. 9(d), the ground state 4f7 electronic configuration of the free Eu2+ ion is 8S7/2, and the next 4f7 manifold (6PJ) lies roughly around 28000 cm−1 above the ground state [38,39]. It can be noticed that the Eu2+ 4f7 levels are lower than the lowest-lying 4f6-5d levels for a free Eu2+ion that starts near ~34000 cm−1. A tentative energy level scheme of the Eu doped potassium lead halides is shown in Fig. 9(d), which indicates the onset of absorption at ~430 nm (~23255 cm−1).
The optical properties of Pr3+- and Er3+-doped in the low phonon-energy halide host KPb2Br5 were explored for their potential as an eye-safe wavelength laser material. Measurements show that the peak emission cross-section for Pr:KPb2Br5 is determined to be ∼6.1x10−20 cm2 at 1667 nm, which is greater than the peak emission cross-section of Er: KPb2Br5 (1540 nm, ∼1.6x10−20 cm2). Moreover, emission cross-section spectra of Pr3+ are much broader relative to Er:KPb2Br5, providing an opportunity for greater laser tunability in the eye-safe wavelength regime. While Pr3+ possesses attractive absorption and emission cross-sections, it has been observed that resonant pumping of the 3F3,3F4 band of Pr3+ provides a channel for visible upconversion which accompanies the 1.5-1.6 µm infrared emission. These upconversion processes depopulate the 3F3,3F4 levels and lead to population of the higher lying 3P0,3P1, and 1D2 states. The upconversion processes populating these states have been identified as energy transfer upconversion (ETU). They appear to be less efficient in Pr3+ relative to the ETU processing taking place in Er3+ systems under resonant pumping of the 4I13/2 state. However, cross relaxation was identified as an additional quenching process of the 3F3,3F4 levels, indicating that a lower Pr3+ concentration is necessary in order to suppress cross relaxation and maximize the radiative decay. In the case of Er:KPb2Br5, 4I9/2 → 4I15/2 infrared upconversion and the 2H11/2 + 4S3/2 → 4I15/2 are the most dominant upconversion channels in this low phonon energy host. The excitation spectra along with the emission decay transients of the infrared and green upconversion were collected and strongly suggests that ETU is the mechanism responsible for the 4I9/2 → 4I15/2 and 2H11/2 + 4S3/2 → 4I15/2 upconversion emissions in Er:KPb2Br5.
A spectroscopic characterization of UV-induced luminescence from Ce3+-doped KPb2Cl5, as well as Eu2+-doped KPb2Cl5 and KPb2Br5 crystals was presented. Upon Xenon lamp excitation, allowed 5d-4f emission centered at ~375 nm was observed in Ce3+-doped KPb2Cl5. The luminescence excitation measurements have provided the 4f-5d absorption band of Ce3+ centered ~310 nm along with the host related absorptions at ~220-270 nm. A fast luminescence decay time of ~30-50 ns was obtained from Ce:KPb2Cl5, however, radioluminescence at ~530 nm appeared to originate from the host KPb2Cl5. The onset of the optical absorption below 430 nm can be tentatively attributed to the 4f-5d transitions of the Eu2+ ions in Eu:KPb2Cl5 and Eu:KPb2Br5 crystals, however, 5d-4f luminescence was not observed. Additional studies of the time-resolved excitation and emission as well as the decay dynamics of the investigated crystals are needed in order to understand the luminescence characteristics and to evaluate the scintillation properties of these materials.
National Science Foundation (NSF) (HRD-1401077, HRD-1649150, and HRD-1137747); Army Research Office (ARO) (W911NF-16-1-0530).
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