CRYSTAL GROWTH AND SPECTROSCOPY OF Yb2+ DOPED CsI SINGLE CRYSTAL

1. Introduction

Scintillators based on CsI have been known for quite a long time [1]. The most popular impurities for doping CsI crystals are Tl [2,3], Na [4,5] and Eu²⁺[6,7,8]. Despite the fact that halide scintillators are emerging with greater efficiency than CsI, such as SrI₂-Eu [9,10,11], LaBr₃-Ce [12,13], BaBrI-Eu [14,15,16], BaBrCl-Eu [17], it does not lose its position on the market and remains one of the most popular scintillators at present. This is due to its relatively low price, as well as low hygroscopicity [18]. Currently, research is being conducted to increase the light output of CsI-Tl crystals using digital signal processing methods [19], modifying growth techniques [20], using photonic crystals [21], as well as co-doping with various cations [22]. Pure CsI crystals cooled to a temperature of 77 K exhibit high light output [23,24]. With increasing temperature, the decay of exciton excitations begins to occur non-radiatively, so CsI is doped with various impurities. The most studied process is the transfer of excitations to Tl⁺ luminescence centers[3,25,26,27]. Promising impurities for halide scintillators are divalent lanthanides, in particular CsI-Eu²⁺. However, the processes of energy transfer in these crystals have been studied in much less detail. However, effective energy transfer from self-trapped exciton to Eu²⁺ is noted[7].

Recently, other divalent lanthanides, such as Yb²⁺, have begun to be used as activators for scintillation halide crystals [28,29,30] and Sm²⁺[31,32,33]. Previously, the optical properties of Yb²⁺ in crystals with the NaCl structure were widely studied [34,35]. The work [36] notes that the intensity of X-ray luminescence increases under irradiation in NaCl-Yb²⁺ crystals. However, studies of CsI-Yb²⁺ crystals have not been previously carried out. Only some works [37,38] note that the addition of Yb to the charge when growing CsI-Tl crystals leads to a decrease in afterglow and an improvement in scintillation properties. However, confirmation that Yb²⁺ is present in the crystal structure was not obtained in these works; in particular, spectra were not given where absorption bands characteristic of the 4f-5d transitions would appear.

In this work, a crystal study of crystal growth CsI-Yb²⁺ crystals and study 4f-5d transitions in Yb²⁺ using optical absorption, luminescence and thermally stimulated depolarization techniques is carried out. The prospects for doping CsI crystals with Yb²⁺ ions for use as scintillators are also being assessed.

2. Materials and Methods

CsI:Yb²⁺ single crystals were grown from the melt using the Czochralski method within a specialized single crystal growth facility. The growth unit consists of a spherical water-cooled chamber constructed from stainless steel. The crystal pulling rod is cooled by thermostatically controlled water and is operated by two stepper motors, enabling rotation and movement of the rod. At the end of the rod there is a graphite holder where a quartz capillary is installed. This setup is furnished with a thermal unit crafted from pyrolytic graphite; the heater takes on a cylindrical shape with vertical cuts, and a Type K thermocouple is affixed to the bottom of the crucible (Figure 1). To initiate the growth of a CsI:Yb²⁺ single crystal, 160 grams of crystalline ultradry CsI (Lanhit) with a purity of 99.998% (for metal impurities), along with 3 grams of YbI₂ with a purity of 99.99% (Lanhit), were utilized as initial components. Thus, the concentration of Yb in the melt was approximately 1 mol.%. The raw materials were loaded into a glassy carbon crucible and subjected to drying directly within the growth installation. The drying process entailed gradual heating and holding of the raw materials for five hours at a temperature of 500°C under vacuum. By the final stage of drying, the residual vapor pressure in the chamber was reduced to less than 0.01 Pa, indicating successful removal of absorbed impurities. Cesium iodide exhibits rapid evaporation in a vacuum; therefore, after drying, high-purity argon was introduced into the chamber until a pressure of 110 kPa was attained. Subsequently, the raw material was melted and overheated to 710°C for 1 hour to ensure homogenization. The temperature was then lowered to 645°C, corresponding to 625°C at the surface of the melt. The growth of a single crystal was facilitated by seeding onto a capillary, with the formation of a crystal neck.

The CsI:Yb²⁺ crystal was grown at a rate of 1 mm per hour with a constant rotation of 1.2 revolutions per minute. The growth process lasted 42 hours, following which the crystal underwent annealing in a chamber for 30 hours, during which the temperature gradually decreased to eliminate internal stresses. The resulting single crystal measures 14 by 40 mm and exhibits no color; however, it displays intense blue luminescence when excited by a 395 nm UV diode (see Figure 2).

The samples have been cut in 2 and 10 mm thick plates and polished for optical experiments. Optical absorption spectra were measured using a Lambda 950 spectrophotometer (Perkin-Elmer, NY, USA). The luminescence time-resolved spectra were registered by an LS-55 spectrofluorimeter (Perkin-Elmer, NY, USA) and a spectrometer based on a SLD-1 and a MDR-2 grating monochromators (LOMO, Saint-Petersburg, Russia) equipped with the grating 1200 and 600 lines per mm. A photomultiplier module Hamamatsu H6780-04 was used as a photodetector. Ionic thermocurrent was measured using a picoampermeter A2-4 (MNIPI Minsk). Measurements were done in vacuum with Pt electrode and heating rate as 20 K min⁻¹. The sample (about 14 mm in diameter and about 1.6 mm thick) was attached to the cryostat with a spring-loaded platinum electrode, the polarization of the dipoles was carried out at room temperature with a voltage of 1.4 kV for 2–3 min, and then cooled to 90 K. Then the electrodes were connected to a picoammeter and the current was recorded during the heating process. The luminescence experiments under VUV excitations were carried out using synchrotron radiation from 1.5 GeV storage ring of MAX IV synchrotron facility (Lund, Sweden). The luminescence experiments under synchrotron radiation excitations are a powerful tool for the study of scintillators[39,40,41]. The experiments have been performed at the photoluminescence endstation of FinEstBeAMS beamline. The parameters of the beamline and the experimental setup are given in [42,43,44,45,46].

2.1. Theoretical Calculations

Geometry optimization for CsI-Yb²⁺ crystals was performed using density functional theory (DFT) within the VASP software package [47] and on the "Akademik V.M. Matrosov" computing cluster [48]. A 3x3x3 supercell with 216 atoms was constructed, with one Yb²⁺ ion replacing a lattice cation. Atomic positions and crystal symmetry were obtained from the ICSD database [49]. The PBEsol exchange-correlation functional and a G-centered grid of 8 k-points in the irreducible Brillouin zone were used for the gradient approximation geometry optimization, while preserving the cell shape and volume. Convergence was determined if the difference in total energies between iterations did not exceed 10⁻⁶eV. The plane wave expansion energy cutoff was set as 500 eV.

Optical absorption spectra were calculated using the Orca software package [50]. A cluster consisting of a Yb²⁺ ion, 12 cesium ions, and 14 iodine ions was extracted from the optimized supercell. This quantum cluster was surrounded by several hundred cesium ions described by SDD pseudopotentials and several thousand point charges. The def2-TZVP pseudopotential was used for quantum domain calculations [51]. The TD-DFT (time dependent DFT) approach was employed for calculating optical transitions.

In the first step, the geometries of the lattice with a Yb²⁺ ion and a charge-compensating vacancy in the nearest or distant environment were calculated. In the case of a nearby vacancy, the absence of a cation resulted in significant displacements of ions surrounding the ytterbium ion. The maximum displacement of iodine ions was found to be 0.72 Å, which corresponds to 18% of the Cs-I distance in a defect-free crystal. The absence of one cation also led to a displacement of Cs ions, with a maximum displacement of 0.62 Å(15% of the Cs-I distance). Consequently, the defect caused a local reduction in symmetry. On the other hand, when the vacancy was situated far from the ytterbium ion (at the supercell boundary), the local environment retained octahedral symmetry. The maximum displacement of iodine ions towards the Yb²⁺ ion was 0.69 Å(17%), while the displacements of Cs ions were negligible.

3. Results

The CsI-Yb²⁺ crystal exhibits luminescence in the blue-green spectral range when excited at 3.4 and 4.55 eV. At 290 K, the luminescence is characterized by two bands centered at 2.6 and 2.8 eV (Figure 3, curve 2).

The higher energy band decays exponentially with a time constant of approximately 120 ns (Figure 3, curve 3), while the lower energy band at 2.6 eV also decays exponentially with a time constant of 325 µs (Figure 3, curve 4). As the crystal cools, the intensity of the higher energy band at 2.8 eV decreases, while the intensity of the 2.6 eV band increases and slightly shifts to a lower energy region (Figure 3, curve 1). At lower temperatures, a new luminescence band centered at 3.6 eV appears under excitation at 5.9 eV (Figure 3, curve 5).

The absorption spectrum of CsI-Yb²⁺ reveals four bands at 3.05, 3.4, 3.9, and 4.55 eV (Figure 4 a, dashed curve). The excitation spectrum, measured at 2.6 and 2.8 eV, shows six bands at 3.05, 3.4, 3.9, 4.55, 5.2, and 5.6 eV. Additionally, there are weak, sharper structured bands observed near 5.9 eV (Figure 4 a, black curve).

The excitation spectrum of the self-trapped exciton luminescence is presented in Figure 4 (a, blue curve). It shows a peak at 5.9 eV and a sharp dip structure in the range between 5.75 eV and 6.3 eV. The dips in the spectrum are located at 5.83, 5.93, 6.00, and 6.25 eV (Figure 4, b).

The comparison of experimental and calculated absorption spectra is given in Figure 5. The black vertical lines show the 4f¹⁴-4f¹³5d¹ transitions in the Yb²⁺ ion compensated by the cation vacancy in the next-nearest-neighbor position (nnn), while the red vertical lines show the 4f¹⁴-4f¹³5d¹ transitions in the Yb²⁺ ion when the cation vacancy is located in the nearest-neighbor (nn) position.

The excitation spectra, monitored at 2.6 eV and 3.6 eV, in the higher energy range of 7-45 eV are depicted in Figure 6. It is evident from the spectra that there is an increase in intensity at energies exceeding 12 eV.

The polarized CsI crystals doped with Yb²⁺ ions, cooled to 80 K, display prominent peaks in the thermally stimulated depolarization current at 190 and 205 K (Figure 7), in contrast to nominally pure CsI. The intensity of these peaks is observed to rise with an increase in the concentration of Yb²⁺ ions.

4. Discussion

Due to the closed 4f shell in Yb²⁺ doped materials, only broad luminescence bands attributed to transitions from the excited 4f¹³-5d¹ state to the ground 4f¹⁴ state are observed. Generally, the 5d-4f emission of Yb²⁺ ions exhibits two bands due to the half-filled 4f shell. These bands correspond to a spin-forbidden high-spin transition, where excitation occurs with a reversal of spin, and a spin-allowed low-spin transition. The high-spin excited states have lower energy than the low-spin transitions, following Hund’s rules. Therefore, the observed luminescence band at 2.6 eV corresponds to the high-spin 5d-4f transition in Yb²⁺ ions, while the higher energy band at 2.8 eV is attributed to the low-spin 5d-4f transition. Yb²⁺ doped alkali halides and alkali earth halides have been extensively studied [52]. Both spin-forbidden and spin-allowed emissions are observed in NaCl-Yb²⁺ and SrI₂:Yb²⁺[34]. The kinetic of luminescence was calculated for NaCl-Yb in [53].

Optical absorption spectra associated with Yb²⁺ in KI, NaI, KBr, NaBr, KCl, NaCl, and KF were obtained at both room and liquid nitrogen temperatures. In NaCl [34] and KCl, NaBr [54], nine groups of lines attributed to 4f¹⁴-4f¹³5d¹ transitions were identified. The excited 4f¹³5d¹ configuration of Yb²⁺ decomposes into a total of 58 levels under an octahedral crystal field, with 20 free-ion levels associated with it [55]. The theoretical analysis of the absorption spectra for the Oh point group was performed in [34], while an increase in absorption bands was observed for the C_2v point group in the case of cation vacancy charge compensation [56]. The position of absorption bands was found to strongly depend on the strength of the crystal-field parameter B in different types of charge compensation scenarios. The ab initio calculation without spin-orbital coupling predicts 4 groups of levels corresponding to 4f¹³(²F_7/2)5d¹(t2g). However, the inclusion of spin-orbital coupling leads to the appearance of a higher energy transition attributed to 4f¹³(²F_5/2)5d¹(t2g).

The ab initio calculation shows that the lowest energy bands in the absorption spectra correspond to transitions to triple degenerated 5d T_2g states, but higher energy transitions are attributed to transitions to double degenerated E_g states (Figure 5) in the nnn configuration when the cation vacancy is located in the next-nearest neighbor position. The position of the 4f-5d absorption bands for the divalent Yb ion compensated by a cation vacancy in the nearest neighbor configuration has higher energy and larger intensity. The observed absorption spectrum contains the contributions from both Yb²⁺ centers.

The Yb²⁺ ions replace Cs⁺ ions in the material, necessitating charge compensation. This compensation can occur through the presence of interstitial iodine ions or cation vacancies in nearest-neighbor (nn) and next nearest-neighbor (nnn) positions. The experimental absorption and excitation spectra are in good agreement with the calculated Yb²⁺ centers compensated by cation vacancies in the nn and nnn configuration. The Yb²⁺-iodine vacancy and Yb²⁺-cation vacancy combinations form electrical dipoles. As a result, they can be polarized at low temperatures and subsequently depolarized during heating. This phenomenon gives rise to a thermally stimulated depolarization (TSD) curve. The TSD curve exhibits two closely located peaks at 195 and 205 K, which can be well described by a first-order kinetic equation for thermally stimulated processes [57](Eq. 1).

$$I_{TL}=n_{0}sexp\left\{-\frac{E_t}{kT}\right\}exp\left\{-\left(\frac{s}{\beta }\right){\int }_{T_0}^{T}exp\left\{-\frac{E_t}{k\theta }\right\}d\theta \right\},$$

(1)

The activation energies are 0.514 eV and 0.558 eV. The frequency factor, s = 5 THz, corresponds to the phonon modes present in CsI. The observed peaks are attributed to Yb²⁺-cation vacancy dipoles, also known as I-V complexes. The lower energy peak is related to the reorientation of nearest-neighbor (nn) dipoles, while the peaks at 205 K correspond to the reorientation of next nearest-neighbor (nnn) dipoles.

Similar thermally stimulated depolarization (TSD) phenomena have been observed in NaCl-type crystals doped with divalent cations within the temperature range of 190-230 K. The observed peaks in these crystals are also attributed to I-V complexes [58,59] in nn and nnn positions. In NaCl, the nn peak is located at 210 K, while the nnn peak is located at 230 K.

The presence of I-V centers in the TSD curve confirms that the charge compensation for Yb²⁺ occurs through cation vacancies. The intensity of TSD peaks attributed to I-V complexes in nn coordination is higher, indicating that the charge compensation mainly occurs through nearest-neighbor cation vacancies. The luminescence band at 3.6 eV is excited at approximately 5.8-6.0 eV and exhibits a sharp structure. The dips at 5.83, 5.93, 6.00, and 6.25 eV correspond to the n1 and n2 levels of the $\Gamma$ (3/2, 1/2) exciton [33,60]. The observed structure is consistent with theoretical calculations and the experimental absorption spectrum of thin films of CsI [61]. The luminescence band at 3.6 eV corresponds to luminescence from off-center self-trapped excitons (STE) in CsI [62]. This band coincides with the wavelength range of the 4f-5d absorption of Yb²⁺ ions, indicating the possibility of exciton energy transfer to Yb²⁺ ions. This is supported by the presence of similar bands in the excitation spectrum of Yb²⁺ within the exciton excitation region. Similar energy transfer mechanisms have been observed in other effective halide scintillators such as CsI-Tl [63] and CsI-Eu [3], BaBrI-Eu [16], BaBrI-Ce [64], BaBrCl-Eu [65]. The excitation spectrum in the vacuum ultraviolet (VUV) region demonstrates the multiplication of electronic excitations. Although not as pronounced as in STE, increasing the concentration of Yb²⁺ up to 0.3-1 mol.% is expected to enhance the multiplications due to an increase in exciton energy transfer deposition.

The Yb²⁺ luminescence region coincides with the peak sensitivity of SiPM (Silicon Photomultiplier) detectors. Therefore, Yb²⁺ ions are promising dopants for CsI scintillators and X-ray phosphors when combined with SiPM photodetectors.

5. Conclusions

The first time CsI doped Yb²⁺ single crystal was given. The charge compensation of Yb²⁺ occurs in the nearest neighbor and next nearest neighbor cation vacancies. The luminescence attributed to 5d-4f spin-allowed and spin-forbidden transitions was observed at 300 K. In the cooled-down samples, only spin-forbidden luminescence was detected. The excitation and absorption spectra of CsI-Yb²⁺ crystals demonstrate strong bands attributed to the 4f-5d transition in the energy range between 2.7-5.7 eV. The luminescence of Yb²⁺ is also excited in the excitonic range and demonstrates multiplication of electronic excitation in the range 10-45 eV. That allows us to expect the promising scintillation properties of Yb²⁺ in CsI.