Optical Spectroscopy of Li₆Y(BO₃)₃ Single Crystals Doped with Dysprosium

Abstract

The energy levels of Dy³⁺ ions have been determined in lithium yttrium borate (Li₆Y(BO₃)₃) single crystals in a wide spectral range between 3000 and 40,000 cm⁻¹ using optical absorption and luminescence spectroscopy, which also allow for an analysis of the ground state. The crystal field splittings of the ⁶H_15/2 ground state and all excited states up to the ⁴F_7/2 manifold were obtained at a low temperature, based on luminescence (T = 4.2–78 K) and absorption (T = 8–100 K) measurements, respectively. The numbers of experimentally observed Stark sublevels are in agreement with those expected theoretically for Dy³⁺ ions occupying a single low symmetry (C₁) site.

1. Introduction

Borate crystals often demonstrate advantageous properties such as high optical quality, transparency in a wide spectral range including vacuum ultraviolet (VUV), and optical non-linearity. For this reason, they are recognized as good matrices for laser applications [1,2,3]. Some of these, e.g., lithium borates, are attractive materials for neutron detectors due to the presence of naturally abundant ⁶Li and ¹⁰B isotopes [4,5,6,7,8,9]. Lithium yttrium borate (Li₆Y(BO₃)₃, LYB) has been suggested as a phosphor, laser, or scintillator material depending on the host morphology and rare-earth (RE) ion doping [10,11,12,13]. This compound with monoclinic crystal structure of the P2₁/c space group (Z = 4) [14] and the unit cell parameters a = 7.157(5) Å, b = 16.378(4) Å, c = 6.623(4) Å, β = 105.32(5)° [15] is an excellent host for RE doping, since it contains yttrium sites of only one type which can incorporate any RE dopant without concentration restrictions, as all Li₆RE(BO₃)₃ crystals are isostructural with Li₆Y(BO₃)₃ [16]. The Y³⁺ sites form a zigzag chain along the crystallographic <001> direction with a distance of 3.85 Å between the closest neighbors and ~7 Å between the chains. This anisotropy of the Li₆RE(BO₃)₃ crystal structure causes a dominant one-dimensional energy transfer and prevents concentration quenching of RE luminescence [17]. In Li₆Y(BO₃)₃:Yb crystals, the absence of emission quenching has been proven for concentrations up to 20 mol% of Yb, whereas unlike powders, crystals demonstrate a remarkable lengthening of the decay time of the RE emission with concentration, which is related to the reabsorption and repeated emission process taking place within the impurity complex [18]. The trivalent Dy³⁺ is a promising RE dopant because of its relatively strong white-light emission which is important for laser, display panel, and telecommunication applications [19,20,21]. Some of the RE ions have already been extensively studied in various host crystals in view of their potential application as a medium for coherent quantum dynamics (e.g., praseodymium in yttrium silicate in resonant nonlinear optical experiments [22]). Dy³⁺ is a perspective candidate in this field, since it has shown highly efficient APTE (Addition de Photons par Transferts d’Energie) up-conversion luminescence in Yb³⁺ doped compounds containing Dy³⁺ ions even at trace levels [18,23]. This sensitive energy transfer to the Dy³⁺ ions through the excited Yb³⁺ states and a stable borate host matrix establish the potential of using dysprosium doped LYB crystals in quantum optical experiments. Preliminary spectroscopic studies of Dy-doped Li₆Y(BO₃)₃ have been performed for polycrystalline phosphors [10], glasses [12], and recently, single crystals [24]. These studies revealed the spectral positions and temporal characteristics of the main electronic transitions of Dy³⁺ in the material, however, did not provide information on fine structure of the transitions necessary to fully identify the crystal field splitting of Dy³⁺ ion states introduced in such an anisotropic matrix. In the present work, the results of high-resolution absorption and luminescence measurements aimed at identifying the Stark levels of Dy³⁺ ions in LYB crystals are presented.

2. Experimental

Li₆Y(BO₃)₃ single crystals doped with 1 and 5 mol% dysprosium were grown by the Czochralski method detailed in [25] (Figure 1). To synthesize the starting charge, Li₂CO₃ (99.9%), Y₂O₃ (99.9%), B₂O₃ (Merck pro analysis) and Dy₂O₃ (99.9%) powders were mixed in the ratio of constituents expected for bulk crystal and placed into a silver crucible. The mixtures were annealed at 580 °C for 42 h and at 650 °C for 12 h. Between the reaction steps applied, the samples were weighed and reground. The synthesized material was finely ground and placed into a gold-coated platinum crucible. Crystals were grown from this crucible in air, using a resistance-heated furnace fitted with a diameter-controlled growth apparatus. The crystals were pulled along the b = <010> axis, with a rate of 0.16 mm/h and a rotation speed of 8–14 rpm. Samples oriented by X-ray diffraction were prepared in the form of thin slices with a thickness of about 1–2 mm with their large faces perpendicular or parallel to the growth direction. In the latter case, the plane of the slice coincides with the $\left(\overline{1}02\right)$ Miller plane which is nearly perpendicular to the dielectric z-axis, according to [26].

The absorption spectra were recorded by BRUKER IFS 66v and 120 FTIR Fourier transform spectrophotometers (Bruker, Ettlingen, Germany) with resolution of 0.2 cm⁻¹ and 0.05 cm⁻¹, respectively. A closed cycle He cryostat equipped with KRS-5 and quartz windows (Specac, Orpington, England) for measurements in different wavenumber regions was used to perform the low temperature (≈8 K) studies. KRS-5 and quartz polarizers (Specac, Orpington, England) were utilized to investigate the anisotropy of the Dy³⁺ electronic transitions in the monoclinic LYB lattice.

The steady-state and time-resolved photoluminescence studies were carried out at the Institute of Physics, University of Tartu, using a custom-made setup using a 400 W deuterium discharge lamp DDS-400 (Vladikavkaz electric lamp plant, Vladikavkaz, Russia) or a Xe flash-lamp PerkinElmer 150 W (Perkin Elmer Optoelectronics, Wiesbaden, Germany) for sample excitation, respectively. The excitation wavelength was selected with a double-quartz prism-monochromator DMR-4 (Kazan Optical-Mechanical Plant, Kazan, Russia). The samples were mounted in a Janis VPF-800 liquid nitrogen cryostat (78–800 K) (Janis Research Company, Woburn, England) or in a liquid helium cryostat (1.8–400 K) (Low Temperature Laboratory, Tartu, Estonia) by means of a specially designed spring-equipped copper finger or with conductive silver glue, respectively. Luminescence spectra were studied using an ARC Spectra Pro 308i (Acton Research Corporation, Acton, England) Czerny-Turner type grating monochromator equipped with a Princeton Instruments CCD camera (Teledyne Princeton Instruments, Krailing, Germany) or a Hamamatsu H6240-02 (Hamamatsu Photonics Norden AB, Solna, Sweden) photon counting head for the steady-state and time-resolved measurements, respectively. The spectral resolution was kept 1.08 nm for review luminescence spectra and 0.27 nm for detailed spectra. The luminescence detection technique is described in more detail in [27]. Photostimulated luminescence emission and excitation spectra were measured in the temperature interval 4.2–300 K, which was controlled by a LakeShore 335 device (Lake Shore Cryotonics, Inc., Westerville, OH, USA). Various SCHOTT (Edmund Optics, Mainz, Germany) or UQG Optics (UQG Optics, Cambridge, England) colour glass filters were used to additionally suppress possible effects of stray light or second orders of excitation and emission light. The excitation spectra were normalized to the reference signal of sodium salicylate to equalize the quantum intensities of incident light at different photon energies. The emission spectra were corrected for the monochromator spectral efficiency and spectral sensitivity of the detector.

3. Results and Discussion

3.1. Absorption Spectra

To characterize the incorporation of dysprosium impurity into the host, the absorbances of LYB crystals doped with Dy³⁺ ions in concentrations of 1 and 5 mol% were compared using samples of identical thickness and orientation. The absorption bands in the 7000–10,000 cm⁻¹ wavenumber range corresponding to the electronic transitions from the ⁶H_15/2 ground state to the ⁶H_9/2, ⁶F_11/2, ⁶H_7/2, and ⁶F_9/2 terms of the excited state can be seen in Figure 2. The amplitude of the absorption bands measured in the LYB:Dy(5 mol%) crystal was by a factor of five higher than that in LYB:Dy(1 mol%). This confirms that the distribution coefficient of Dy³⁺ ions is the same in both cases and does not depend on the Dy concentration in the melt (see also [15]).

To determine the fine structure of Dy³⁺ electronic transitions, detailed high-resolution measurements were performed at a low temperature. The absorption spectra of LYB:Dy (5 mol%) crystal in the 3000–40,000 cm⁻¹ wavenumber range present a sequence of narrow bands with a minimum halfwidth of about 1 cm⁻¹ at T = 8 K. The absorption bands correspond to the transitions from the lowest sublevel of the ⁶H_15/2 ground state to the excited energy levels of the Dy³⁺ ions, which can be easily assigned using the Dieke diagram [28] up to the ⁴F_7/2 multiplet at about 25,000 cm⁻¹. Because of the monoclinic crystal structure of LYB, all energy levels of Dy³⁺ split into the maximum possible number of Stark components separated by about 100–600 cm⁻¹. As a consequence, the Stark components of various nearby levels are mixed and cannot be unambiguously assigned, especially at high energies above 29,000 cm⁻¹ (see, e.g., Figure 3g,h). Some doubtful assignments based on the excitation spectrum of a LYB:Dy crystal can be found in the paper by Saha et al. [24]. Difficulties in the determination of the proper energy terms have also been encountered for other monoclinic crystals, such as KY(WO₄)₂ and Lu₂SiO₅, doped with Dy³⁺ ions [29,30]. Most of the spectral lines experimentally observed in LYB:Dy in the present work are shown in Figure 3 and the most probable assignments are listed in

Table 1. Energy levels of Dy³⁺ ions in monoclinic Li₆Y(BO₃)₃ crystals (in cm⁻¹).

	1	2	3	4	5	6	7	8	9	10	Ntheory	ΔE
6H15/2	0	42	79	134	244	389	503	598			8	598
6H13/2	3564.7	3590.5	3631.8	3692.9	3745.8	3794.0	3861.7				7	297
6H11/2	5901.2	5941.9	5989.8	6022.8	6068.0	6115.0					6	214
6H9/2 + 6F11/2	7613.4	7698.9	7713.5	7766.0	7798.5	7870.0	7896.5	7947.2	7980.9	8151.1	11	619
	8232.1
6H7/2 + 6F9/2	8980.3	9045.2	9085.5	9237.6	9270.9	9299.0	9350.0	9382.0	9602.0		9	622
6H5/2	10,233.0	10,312.0	10,460.0								3	223
6F7/2	11,038.6	11,173.1	11,223.9	11,276.9							4	238
6F5/2	12,470.0	12,528.4	12,615.5								3	135
6F3/2	13,327.2	13,340.7									2	13.5
6F1/2	13,873.6										1	0
4F9/2	20,897	20,997	21,108	21,136	21,473						5	576
4I15/2	21,928.7	21,995	22,061	22,130	22,313	22,359	22,375	22,418			8	489
4G11/2	23,378	23,434	23,458	23,507	23,539	23,596					6	218
4F7/2	24,769	24,834	24,916	24,935							4	166
4I13/2–4M19/2	about 20 lines
6P5/2 + 6P3/2	27,342	27,384	27,415	27,435	27,504						5	162
4I11/2	27,831	27,880	27,906	27,962	28,036	28,074					6	243
4M15/2	28,382	28,394	28,442	28,467	28,480	28,512	28,541	28,565			8	183
6P7/2–2F7/2	about 40 lines
4H13/2	33,061	33,084	33,097	33,110	33,130	33,146	33,156				7	95
4K13/2 + 4F3/2	33,384	33,398	33,415	33,442	33,456	33,484	33,533	33,560	33,576		9	192
4D7/2	33,854	33,881	33,916	33,950							4	96
4F5/2–4F3/2	about 25–30 lines
4P5/2 + 4P3/2	38,705	38,875	39,019	39,050	39,074						5	369

. Beside the electronic transitions of Dy³⁺ ions some additional weak absorption bands attributed to the stretching vibration of hydroxyl ion (OH⁻) impurities can be observed between 3450–3560 cm⁻¹ as shown in Figure 3a. According to our measurements, these bands are present in undoped and other RE ion doped LYB crystals as well. The uneven baseline in this wavenumber range is related to the absorption of higher harmonics of the B-O vibrations in the host crystal.

Figure 3a–c,e–i shows the high-resolution absorption spectra of LYB:Dy(5 mol%) crystal recorded for light propagating along the <010> axis of the crystal. The spectra for light propagating along the n$\left(\overline{1}02\right)$ direction with polarization parallel and perpendicular to the <010> direction are shown in Figure 3d. All lines related to the fully split Stark components are clearly visible for both polarizations, only their relative intensities vary slightly. The number of experimental bands is in full agreement with theoretical predictions; the exact positions of the lines obtained from experiment are given in Table 1. Since no additional absorption lines appear in the low-temperature spectra, only one type of site can be assumed for Dy³⁺ ions in the LYB lattice, as expected due to the isostructural property of Li₆RE(BO₃)₃ crystals [16]. The same conclusion was drawn also from the optical and electron paramagnetic resonance (EPR) spectroscopy of Er³⁺ doped LYB crystals [31].

It is remarkable that upon gradually increasing the temperature from 8 to 100 K, additional lines appear in the absorption spectrum, which can be related to the transitions from thermally populated higher states of the ⁶H_15/2 ground state of Dy³⁺ to various terms of the excited state. Figure 4 demonstrates this on the example of the ⁶H_15/2 → ⁶F_5/2 transition. At the lowest temperatures nearly exclusively the lines at 12,470, 12,528.4, and 12,605.5 cm⁻¹, corresponding to transitions from the lowermost Stark level of the ⁶H_15/2 ground state, are seen. As the higher sublevels of the ground state become populated at elevated temperatures, additional repetitions for these transitions appear at wavenumbers less by the same amounts (42, 79, and 134 cm⁻¹) corresponding to transitions from some of the higher Stark levels of the ground state to unchanged excited states. The positions of these hot replicas are helpful for identifying the lowest energy Stark levels of the ground state to be situated at 42, 79, and 134 cm⁻¹. The higher energy crystal field components of the ground state were determined from luminescence measurements (see below).

3.2. Luminescence

Additional information on the position of high-energy excited Dy³⁺ states was obtained from the studies of low-temperature luminescence and excitation spectra for the LYB:Dy(1 mol%) crystal. To record the luminescence spectra, the excitation wavelength was chosen at 356 nm (28,068 cm⁻¹), which populated the ⁴I_11/2 multiplet and provided the highest emission intensity. Upon the excitation of this term, electrons relax to the ⁴F_9/2 multiplet via nonradiative transitions, therefore, the transitions originating from the latter were further investigated in detail (see Figure 5b). The excitation spectrum was recorded for the most intense line at λ_em = 577 nm (17,331 cm⁻¹) of the ⁴F_9/2 → ⁶H_13/2 transition. The emission and excitation spectra measured at temperatures 4.2 K or 78 K are presented in Figure 5, Figure 6, Figure 7 and Figure 8, respectively. Figure 5a shows the overview of the emission spectrum in the region of 11,000–22,000 cm⁻¹. Two major bands centered at about 20,620 and 17,240 cm⁻¹ related to the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions, respectively, dominate the spectrum. Three weaker groups of bands observed approximately at 15,040, 13,160, and 11,770 cm⁻¹ in the near-infrared region are assigned to the ⁴F_9/2 → ⁶H_11/2, ⁴F_9/2 → ⁶H_9/2,⁶F_11/2 and ⁴F_9/2 → ⁶H_7/2,⁶F_9/2 transitions, respectively (Figure 5a,b). The positions of the main emission bands are in good agreement with those reported earlier for Li₆Y(BO₃)₃:Dy [10,12,24] and various other systems, such as phosphate glasses, molybdates and vanadates [32,33,34].

The shape of the bands responsible for the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions varies in the above-cited works, and for that reason, zoomed spectrum parts measured with higher spectral resolution are shown for these transitions in more detail in Figure 6 and Figure 7. The fine structure of these transitions is represented by 8 lines in the regions of 20,200–20,950 cm⁻¹ (Figure 6) and 7 lines in the region of 17,000–17,350 cm⁻¹ (Figure 7), respectively, in agreement with the number of crystal field components of the given multiplets ⁶H_15/2 and ⁶H_13/2. Weaker lines at higher wavenumber values in both Figure 6 and Figure 7 are due to transitions from a hot state of the ⁴F_9/2 multiplet. The Stark components of the ⁶H_15/2 ground state can be determined from the ⁴F_9/2 → ⁶H_15/2 luminescence spectrum (see black scale (1) on top of Figure 6). The three lowest energy levels are in good agreement with those obtained from the hot bands in the absorption spectra measured at about T = 100 K (see, e.g., Figure 4). A few lines at wavenumber values higher than 20,900 cm⁻¹ may be related to the hot transitions originating from the second Stark component of the ⁴F_9/2 multiplet populated at 78 K (see red scale (2) on top of Figure 6).

The Stark components of the ⁶H_13/2 multiplet can be determined from both the absorption and luminescence measurements. It is remarkable, that the detailed structure of the ⁴F_9/2 → ⁶H_13/2 emission corresponds almost perfectly to the reversed absorption spectrum shown for the ⁶H_15/2 → ⁶H_13/2 transition at 8 K in Figure 3a. The reversion point was defined by the overlay of the emission line at 17,331 cm⁻¹ with the absorption line at 3564.7 cm⁻¹. This comparison demonstrates the consistency of the results obtained by the two spectroscopic methods.

The position of higher energy multiplets of the Dy³⁺ ion in Li₆Y(BO₃)₃ may be obtained from the excitation spectra of the LYB:Dy(1 mol%) single crystal measured for the 17,331 cm⁻¹ emission line, one of the most intense ones in the series of the ⁴F_9/2 → ⁶H_13/2 transitions (Figure 8). The general features of the recorded excitation spectrum are in good agreement with the excitation spectra reported earlier for glass [12] and crystalline material [24], however, by using a better spectral resolution it was possible to ascribe the observed bands to specific levels of the higher-energy multiplets situated above 22,000 cm⁻¹.

According to [10,35,36] the ⁴F_9/2 → ⁶H_15/2 transition is an allowed magnetic dipole transition and the ⁴F_9/2 → ⁶H_13/2 transition belongs to a forced electric dipole transition being allowed only in the case when the Dy³⁺ ion is located at a lattice site missing inversion center symmetry. Therefore, the fact that the group of yellow emission bands around 577 nm (17,331 cm⁻¹) is the most intense one in the spectrum confirms that Dy³⁺ ions occupy Y³⁺ sites in Li₆Y(BO₃)₃ single crystals, resulting in the hypersensitive (⁴F_9/2 → ⁶H_13/2) transition. This is in accordance with the results reported earlier for the Li₆Y(BO₃)₃:Dy³⁺ phosphor [10]. Indicatively, the ratio of the intensities of the ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_15/2 emissions is much lower in glass samples [12] than in crystals. This difference originates from the simple fact that single crystals possess an established structure characterized by a well-defined low-symmetry yttrium substitution site while the glass material allows a varying local crystal structure in the environment of Y³⁺ (Dy³⁺) ions. This conclusion is supported also by our studies of the decay kinetics of the emission at 17,331 cm⁻¹ under pulse excitation into the ⁴M_15/2 multiplet at 28,382 cm⁻¹ (Figure 9). We found two exponential components with decay times around 310 and 840 μs at 69 K. The integrated intensity of the faster component is very low, amounting only to 5% of the total emission intensity, so the emission decay is close to a single exponential which one can see also from Figure 9 presenting the decay curve in a semi-logarithmic scale. The decay time of the main component slightly increases to 910 μs at room temperature, while that of the shorter one increases approximately to 350 μs, whereas the ratio of their intensities remains the same. The nearly single exponential decay of the Dy³⁺ emission confirms that the Dy³⁺ ions occupy only one type of lattice sites as also expected in the Li₆Y(BO₃)₃ compound. A small admixture of a faster component is presumably related to a small fraction of Dy³⁺ ions perturbed by a lattice defect, neighboring Dy³⁺ ion or crystal surface. These results are similar to those obtained earlier for polycrystalline powder [10] and single crystal [24] samples, whereas in the latter, the increase of the average decay time with temperature was also observed. In glass samples, on the contrary, a non-exponential decay with characteristics times depending on Dy concentration was observed reflecting non-equivalent positions of Dy³⁺ ions and energy transfer between them in this material [12].

A full set of energy states determined from the absorption, emission, and excitation spectra in the present work is listed in Table 1. An energy diagram of the energy states observed is presented in Figure 5b and Figure 7b accounting also for the splitting of the ground ⁶H_15/2 state. This splitting is well observed in the structure of the blue emission between 20,300–20,900 cm⁻¹ (Figure 5b) and in the temperature dependent absorption spectra (Figure 4).

The data shown in Table 1 present much more details on the splitting of excited state multiplets than reported in Refences [10,24]. The reason lies in the much higher resolution of our absorption measurements compared to that achieved in the measurements of excitation spectra in the cited papers. In addition to allowing for the determination of the Stark splitting of almost every multiplet, this was also helpful to better identify the multiplets themselves, especially in the energy regions where excited states are located at high densities (e.g., around 21,000, 28,000, and 37,000 cm⁻¹). Nevertheless, even the resolution achieved in our measurements was in some cases insufficient to resolve all lines in such energy regions. Therefore, in Table 1 we only present the estimated number of lines for the ⁴I_13/2–⁴M_19/2, ⁶P_7/2–²F_7/2, and ⁴F_5/2–⁴F_3/2 terms. The relative positions of the multiplets detected in the present paper are in good agreement with those predicted for free Dy³⁺ ions [28,37], and those determined for Dy³⁺ ions in solutions [38,39].

4. Conclusions

Electronic transitions of Dy³⁺ ions incorporated in the Li₆Y(BO₃)₃ crystals were successfully identified by absorption measurements in the wavenumber region of 3000–40,000 cm⁻¹ and luminescence excitation spectra measurements in the energy region of 18,500–40,000 cm⁻¹. The effect of crystal-field splitting in this low-symmetry crystal was investigated in detail by temperature and polarization dependent absorption and luminescence measurements. An intensive luminescence emission related to the ⁴F_9/2 → ⁶H_13/2 transitions was detected at 577 nm (17,331 cm⁻¹), testifying about the well-defined environments of Dy³⁺ ions homogeneously substituted at Y³⁺ sites lacking inversion symmetry. The splitting of the ⁶H_15/2 ground state of Dy³⁺ was established and the energy diagram of the excited levels derived.