Research ArticleWide-band EPR spectroscopy and relaxation study of Tm3+ ions in PbGa2S4 crystal
Introduction
PbGa2S4 crystals doped with rare-earth ions (RE) are promising laser materials that can be used in the atmospheric transparency window between 3.8 and 5.0 μm. At present, lasing near 4.5 μm is obtained at the 6H11/2 ↔ 6H13/2 transition of the Dy3+ ion in CaGa2S4 and PbGa2S4 crystals [1,2]. The luminescent properties of crystals with an admixture of Ce3+ and Nd3+ ions were also studied in the visible and mid-IR range [3,4]. These compounds have rhombic symmetry (Sp. group. Fddd), with unit cell dimension: a = 20.706 Å, b = 20.380 Å, c = 12.156 Å [5]. The unit cell contains 32 formula units. Assuming that the ion replaces the Pb2+ ion during the doping of the rare earth metal, one has to take into account that there are three types of various lead positions with different local symmetry. In addition, there are ordered structural vacancies in the lattice, into which the RE ion can also embed. Therefore, determining the position of an optically active ion is not an easy task. The direct method for the identification of centers is EPR spectroscopy, which has already been used to study the Dy3+ and Ce3+ ions in lead thiogallate [6,7]. In particular, for the dysprosium ion, only one the center is registered, and for the cerium ion, at least four different centers with close values of g-factors were observed. At the same time, the presence of several centers with similar values of spectral parameters can be the basis for the creation of tunable lasers. Therefore, a detailed study of such systems seems to be highly relevant. We have undertaken a study of crystals of lead thiogallate with an admixture of Tm3+ ions by the method of wideband tunable EPR spectroscopy. Since the first excited Stark level of thulium turned out to be close to the ground one, the crystals seemed suitable for the implementation of the Λ-scheme. Therefore, we measured the times of the spin-spin and spin–lattice relaxation of the ground state of Tm3+ in PbGa2S4.
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Materials and experimental details
The crystal was grown in the Kuban State University (Krasnodar, Russia) using the Bridgman – Stockbarger method. Samples of the following composition were investigated: PbGa2S4: Tm (1.4%), Na (1.4%). In addition to the impurity of RE ions, sodium was introduced to provide charge compensation during growth. Note that this produced crystals of better optical quality (Fig. 1).
Measurements of the EPR spectra were taken in X-band and using a broadband EPR spectrometer [8]. The operating range of the
Results
In Fig. 2 the EPR spectra at frequencies of 9 GHz and 143.3 GHz with crystal rotation in the (bc) plane are shown. The resolved hyperfine structure, consisting of two lines (169Tm, I = 1/2, natural abundance 100%), unambiguously indicates that the spectra belong to Tm3+ ions. From the shape of the spectra, it can be concluded that there are two types of centers of different intensities in the crystal. The more intense center has no magnetically nonequivalent positions. For the other, the
Discussion
Analyzing the experimental results obtained, it is first of all necessary to explain the observation of stationary EPR signals and echo signals in the X-band. Indeed, for a non-Kramers ion, the ground state of which can be represented as a two-level system, there is only one nonzero component of the magnetic moment directed along the z axis. Accordingly, a magnetic dipole transition is possible only in parallel (B0 ||B1) fields [11]. The Elexsys E580 spectrometer uses an ER4118MD5 dielectric
Conclusion
The magnetic and relaxation characteristics of impurity Tm3+ ions in a lead thiogallate single crystal were studied by wide-band EPR spectroscopy. It was found that thulium ions are in two different crystallographic positions P1 and P2. For P1, the magnetic multiplicity (KM) of the spectra is equal to one, in the position P2 KM = 2. The CF splitting between the ground and the first excited singlet electronic levels is measured for these positions: Δ = 6.93 GHz (P1) and Δ = 1.42 GHz (P2). The gz
CRediT authorship contribution statement
G.S. Shakurov: Project administration, Investigation, Supervision, Writing – review & editing. R.B. Zaripov: Investigation. V.V. Badikov: Crystal growth. D.V. Badikov: Crystal growth.
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.
Acknowledgements
The measurements at Zavoisky Physical-Technical Institute were performed with the financial support from the government assignment for FRC Kazan Scientific Center of RAS (AAAA-A18-118030690040-8). We also thank V.A. Shustov for the orientation of the crystal on an X-ray diffractometer and V.F. Tarasov for helpful discussions.
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