Full Length ArticleEmission of Tm2+ in alkaline-earth fluoride crystals
Introduction
Spectroscopy of Tm2+ in CaF2 crystals was first studied in Refs. [1,2] in connection with its possible application as a laser material [3]. The divalent thulium ion has the ground state 4f13 with atomic levels and split by 1 eV by spin orbit interaction. The splitting of Tm2+ energy levels in the CaF2 cubic field and the level scheme are considered in detail in papers [4,5].
The allowed 4f-5d transitions of Tm2+ in halide crystals are located in the region of 200–700 nm. Luminescence is observed for 5 d-4f transitions at 680–800 nm and for 4f-4f transitions at 1.0–1.3 μm for Tm2+ ions [6]. Vibronic spectra of 4f-4f transitions in Tm2+ ions have been studied and modeled in CaF2, SrF2 crystals [7].
Relatively recent studies have shown the promise of using crystals with Tm2+ as effective concentrators of solar radiation [8,9], which stimulates new research on Tm2+ in halide crystals [10,11].
Our research was motivated by the need for detailed studies of the Tm2+ spectra in nonhygroscopic crystals of alkaline earth fluorides as potential laser media and phosphors for solar concentrators.
Section snippets
Experimental
Crystals were grown in vacuum in a graphite crucible by the Stockbarger method in the Institute of Geochemistry SB RAS [12]. The graphite crucible contained six cylindrical cavities with a diameter of 10 mm and a length of 80 mm, which made it possible to simultaneously grow six samples with sizes of 10 × 50 mm with different amounts of impurities. A few percentages of CdF2 were added into the raw materials for purification from oxygen during growth. Cadmium was not found in the grown crystals
Calculations
The scheme of f-levels of Tm2+ ions in fluoride crystals was calculated using the ORCA 5.0.3 software package [17]. Complete active space self-consistent field (CASSCF + NEVPT) calculation was carried out according to the method described in the tutorial [18]. The active space included 13 electrons distributed in the seven 4f-based orbitals. In a recent paper, the applicability of the SARC2 bases for the ab-initio calculation of f-f transitions in free lanthanides Ln3+ is shown with good
Excitation spectra
In the excitation spectra of the infrared emission of Tm2+ ions, a number of bands is observed in the region of wavelengths less than 220 nm (Fig. 1). There are distinct bands at 197 and 182 nm both in the excitation and absorption spectra (Fig. 1). A band near 197 nm was also observed in the absorption spectra of SrF2–Tm2+, BaF2–Tm2+ crystals [21]. It is assumed that the absorption bands of Tm2+ in the region of 360–700 nm are due to the 4f-5d (eg) transitions, and the bands at 250–350 nm are
Discussion
Experimental results show that the integral intensity of the d-f emission decreases significantly in the series CaF2–SrF2–BaF2 (see Fig. 5). This should be expected, since the excited 5 d (eg) level in these series monotonically approaches the conduction band. According to Dorenbos’ model [24,28], the distance 5 d (eg) of the Tm2+ level relative to the bottom of the conduction band is 0.37, 0.1, −0.7 eV in CaF2–SrF2–BaF2, respectively. The excited 5 d (eg) level in BaF2 is estimated to be in
Conclusion
The f-f emission spectra at low temperatures consist of two sharp lines due to transitions from the lowest state to the ground states, split by the crystal field. Upon heating, the f-f emission decreases in two stages and finally disappears at 250, 300, 350 K in BaF2, SrF2, CaF2, respectively. The distance between the emission lines decreases in the series CaF2, SrF2, BaF2 by 240 cm−1, which is associated with a decrease in the strength of the crystal field.
In the region of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors gratefully acknowledge V.A. Kozlovskii for preparation of the crystals investigated in this paper.
This study was performed under the project 0284-2021-0004 “Materials and Technologies for the Development of Radiation Detectors, Luminophores, and Optical Glasses” of Russian Academy of Science.
We acknowledge MAX IV Laboratory for time on Beamline FinEstBeams under Proposal 20200308. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research
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