Elsevier

Physica B: Condensed Matter

Volume 403, Issue 1, 1 January 2008, Pages 207-218
Physica B: Condensed Matter

Temperature dependence of the EPR lines in weakly doped LiNbO3:Yb—possible evidence of Yb3+ ion pairs formation

https://doi.org/10.1016/j.physb.2007.08.224Get rights and content

Abstract

The electron paramagnetic resonance (EPR) studies of LiNbO3 single crystal doped with 1 wt% of Yb3+ are reported. To put the EPR results in perspective, a brief discussion of optical absorption spectroscopy investigations of LiNbO3:Yb3+ is provided. The temperature behavior of the EPR lines intensity and linewidth for LiNbO3:Yb3+ reveals antiferromagnetic coupling between Yb3+ ions. The deconvolution of the EPR lines indicates that EPR signals arise from both the isolated Yb3+ ions as well as the Yb3+–Yb3+ ion pairs; the latter signals dominate. Based on this indication, EPR spectra are interpreted using a spin Hamiltonian for the Yb3+ dissimilar ion pairs. The negative sign of the isotropic parameter J confirms the existence of the antiferromagnetic interactions within Yb3+–Yb3+ pairs. The value of J obtained based on the proposed pair model, assuming the dipole–dipole interactions, is used to identify the positions of the Yb3+–Yb3+ pairs in the unit cell. Our results suggest the evenYb3+evenYb3+ pairs are located at the neighboring Li+ and Nb5+ positions, whereas the pair axis is not parallel to the optical c-axis. Some alternative explanations of the observed EPR spectra are also considered.

Introduction

Lithium niobate LiNbO3 (LN) is one of the best studied ferroelectric, electro-optic materials, see e.g. the reviews [1], [2]. Structural properties and phase transitions as well as luminescence, Raman and electron paramagnetic resonance (EPR/ESR) spectra of LN have been reviewed in Refs. [3], [4]. Interest in LN is driven by its unusual non-linear electric, magnetic, and acousto-optic properties, which have been utilized for various technological applications. Doping with trivalent transition ions, especially rare-earth (RE) elements, has pronounced effect on some properties of LiNbO3, e.g. structure, electro-optical coefficients, and light absorption. Fine-tuning these properties by doping opens wide possibilities of improving the important technological applications focused on electro-optic devices used for, e.g. information storage, holography, planar waveguide lasers, and amplifiers. In general, the optical properties of doped crystals are largely determined by the local site symmetry of the optically active ions.

The space group of LiNbO3 crystal is trigonal: R3c (C3v6). The unit cell of LN is rhombohedral with the lattice constants at 296 K, a=0.514829 nm and c=1.38631 nm [4], [5]. In the LN structure the oxygen atoms are arranged in planar sheets forming a network of trigonally distorted octahedra, which are adjoined by walls forming chains along the crystallographic c-axis being the optical axis. The sequence of octahedra repeats as {Nb, vacancy, Li} [4], [5], [6]. The RE impurity ions in LN may occupy one of the four sites, namely, three octahedral sites: Li+, Nb5+ or cation vacancy, and the tetrahedral interstitial site. It has been shown that the trivalent RE ions occupy mostly Li+ sites and are located off-center from the regular Li+ positions towards the structural vacancy octahedral sites along c-axis [7], [8]. Thus, the RE3+ ions can be distributed in the crystallographically inequivalent centers in this host, giving rise to different sets of optical transitions [8]. Several experimental and theoretical studies have indicated that RE ions in LN occupy also the Nb5+ sites [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. LiNbO3 is usually grown from congruent melt compositions with Li+ to Nb5+ concentration ratio of the order of 0.945 giving rise to the Li-deficient crystals that need intrinsic defects to satisfy the overall charge compensation. It is generally accepted [10], [11] that the Li-deficiency in LN single crystals is compensated by a certain amount of Nb5+ ions located at the Li+ sites, the so-called anti-sites. For a given RE3+ dopant ion, various crystallographically inequivalent centers may occur, whereas the relative concentration of such centers depends on the stoichiometry (i.e. the Li/Nb concentration ratio). This result indicates that the nature of the RE3+ centers in LN is directly related to the defects associated with the non-stoichiometric conditions.

The EPR spectroscopy of trivalent ytterbium (Yb3+) in LN was investigated by Burns et al. [10] and Bonardi et al. [11], whereas Dong et al. [17] studied theoretically the spin-Hamiltonian (SH) parameters for Yb3+ ions in LN:Yb3+, Mg2+ single crystals. The studies [10], [11], [17] indicate that the site symmetry of Yb3+ centers is C3 and Yb3+ ions occupy mainly the Li+ sites. In LiNbO3:Mg, Yb, the Yb3+ ions may also occupy Nb5+ sites due to MgO co-dopant [17]. Choh et al. [18] arrived at similar conclusion for Er3+ in Er and Mg co-doped LN single crystals. The structural analysis by Malovichko et al. [20], [21] reveals that the RE ions enter octahedral sites in LiNbO3, which may exhibit either the C3 or the C1 site symmetry. Montoya et al. [22] study of cooperative luminescence in LiNbO3:Yb3+ provided a firm evidence of the Yb3+ pairs in this matrix. Subsequently, Montoya et al. [23] proposed three models for Yb3+ ions distribution in LiNbO3:MgO and related each model to the observed features of cooperative luminescence. LN crystals studied in Refs. [22], [23] concerned low concentration of Yb doping. The experimental results [23] favor the Yb3+ distribution model in which a fraction of dopant ions (about 10.4 wt% of all Yb3+ ions) forms pairs with one Yb3+ ion placed at the Li+ site and another one at the Nb5+ site, while the rest of the Yb3+ ions are randomly distributed at the Li+ sites [23].

Our previous studies [24], [25], [26] focused on spectroscopic investigations of LN single crystal doped with Yb ions and/or co-doped with Pr ions. No RE impurity centers with C3 site symmetry in LN host were clearly observed by us. We have found that Yb3+ ions predominantly occupy sites with lower C1 symmetry in both LN: Yb, Pr and LN:Yb crystals [25], [26]. In this paper EPR study of Yb3+ ions in LN crystal doped with Yb (1 wt%) are reported. In Section 2 the sample details and general experimental conditions are outlined. Optical spectroscopy studies are briefly discussed in Section 3 to put in perspective the EPR results. Analysis of the EPR spectra and their temperature dependence in terms of a model of Yb ion pairs is carried out in Section 4. This enables identification of the positions of the Yb3+–Yb3+ pairs in the unit cell. Since changes of the EPR line profiles may also be related to several inequivalent Yb3+ centers, the counterarguments against this alternative explanation are discussed. In Section 5 we provide summary and conclusions.

Section snippets

Sample and experimental details

LiNbO3 single crystal doped with 1 wt% of Yb3+ was grown along c-axis from the congruent melt by the Czochralski method in the Institute of Electronic Materials Technology (IEMT, Warsow, Poland). The Yb concentration was 1% molar fraction in the melt. The following starting materials were used: Nb2O5 (4 N purity) from Johnson-Matthey; Li2CO3 (4 N purity) from IEMT. After mixing of adequate amounts of reagents the mixture was calcined at 1373 K for 6 h. The Yb2O3 was added to the charge of congruent

Optical spectra

The optical spectra of LN:Yb3+ single crystal, presented in Fig. 1, consist of intense sharp absorption bands, due to the 2F7/22F5/2 electronic transitions, and weak bands, due to the vibronic transitions or low symmetry effects superimposed onto wide continuous absorption of the host lattice. Pair absorption of the Yb3+ ions is not observed due to too low concentration of impurity ions. Nevertheless, the main 980 nm line has a maximum at about 423 K (see Fig. 2). The shape of the temperature

EPR spectroscopy measurements

Representative EPR spectra of the LN:Yb3+ (1 wt.%) sample [24], [25] taken at various angles in the plane (denoted XY) perpendicular to the crystal c-axis and the plane (ZX) containing the crystal c (Z)-axis are presented in Fig. 3, Fig. 4, respectively. The spectra were also measured in a third plane (ZY) yielding similar angular dependencies as in Fig. 3. Depending on a given angle, two or three strong lines and additionally several weak lines are observed, which partially cannot be fully

Summary and conclusions

Optical and EPR spectroscopy measurements of LN single crystal weakly doped with Yb3+ ions (1 wt%) have been carried out. Analysis of the optical spectra reveals, apart from the lines originating from the isolated Yb3+ ions observed before, appearance of an additional bands. This observation may be considered as an indirect evidence of local perturbations arising around Yb ions.

Analysis of the temperature dependence of the EPR linewidth confirms the presence of at least two distinct paramagnetic

Acknowledgments

Prof. Taiju Tsuboi from Kyoto Sangyo University, Japan is deeply acknowledged for low temperature absorption measurements. This work was partially supported by the research grant from the Polish Ministry of Science and Tertiary Education in the years 2006–2009 (TB) and SMK acknowledge gratefully a DSc (habilitation) grant and a Professorial grant, respectively, from the SUT.

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