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Temperature dependence of the EPR spectra and optical measurements of LiNbO3: Er, Tm single crystal
Journal of Alloys and Compounds 468 (2009) 581–585
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Temperature dependence of the EPR spectra and optical measurements of LiNbO3 : Er, Tm single crystal Tomasz Bodziony ∗ , Sławomir Maksymilian Kaczmarek Institute of Physics, Szczecin University of Technology, Al. Piast´ ow 17, 70-310 Szczecin, Poland
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Article history: Received 15 November 2007 Received in revised form 13 March 2008 Accepted 14 March 2008 Available online 28 April 2008 Keywords: Electron paramagnetic resonance Lithium niobate Er3+ Rare-earth ions Pairs of ions
a b s t r a c t Optical, Raman and EPR spectroscopy measurements of the congruent LiNbO3 : Er (0.2 wt.%), Tm (0.3 wt.%) single crystal have been carried out. The shape of the optical absorption suggests the presence of RE3+ ion pairs. Unusual behaviour of the temperature dependence of the EPR spectra was reported. The EPR spectra could be analysed as superposition of several paramagnetic centres originating from isolated even Er3+ ions and even Er3+ –even Er3+ pairs of ions. In low temperature region the main EPR signal is dominated by signals originating from even Er3+ –even Er3+ pairs of ions. The inverse intensity of the EPR line, in low temperature region, fulfils the Curie–Weiss law and enabled to determine the Curie–Weiss constant = 6.1 ± 1.1 K. The positive sign of the suggests that ferromagnetic interactions arise in the system of even Er3+ –even Er3+ ion pairs in LiNbO3 . Our results suggest that the presence of codopant ions play the most important role in unusual behaviour of the EPR spectra. They facilitate introduction of the Er ions into host lattice and creation of the even Er3+ –even Er3+ ion pairs in the LiNbO3 host crystal. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Erbium doped lithium niobate (LiNbO3 , LN) has excellent perspective of applications in integrated optics and has been utilized in various technological applications [1,2]. It is an especially suitable system for solid-state lasers due to non-linear properties of LN [3]. Recently, erbium doped LN has gained attention through the development of light amplifiers based on LN: Er3+ in optical fibers for optical communication. Optical properties of any doped crystal are largely determined by the local symmetry of the optically active ions. Electron paramagnetic resonance (EPR) is a very sensitive method to investigate the position and the defect structure of an impurity ion. Due to non-stoichiometry, LN contains several defects including antisite defects (Nb5+ ion in a Li+ site, namely NbLi ) and four sites. Three of them are octahedral sites: Li+ , Nb5+ or structural vacancy, and fourth is a tetrahedral interstitial site [4–6]. All paramagnetic sites positioned along the optical c-axis of LN crystal have the C3 point group symmetry, while the impurity ions at other positions have the lowest, C1 , symmetry [2]. LN crystal can be tuned to fit various experiment requirements. One of the method applied to realize this tuning is doping. Rareearth (RE) dopant ions, as e.g. Nd3+ or Er3+ , are introduced to LN host as optically active impurities [7]. The relative concentration of such
∗ Corresponding author. E-mail address: [email protected] (T. Bodziony). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.062
impurities depends on the stoichiometry (i.e. the Li/Nb concentration ratio). Properties of RE3+ centres in LN are directly related to the defects associated with the non-stoichiometric conditions [7]. LN crystals doped with erbium (Er) have been widely studied because of their optoelectronic applications [8]. Dierolf and Koerdt [9] and Milori et al. [10] have found a trigonal centre which they attributed to Er3+ ions occupying Li+ positions. Bravo et al. [11] have found a new trigonal centre with relatively small anisotropy of the g factor in comparison to previously observed one in LN: Er3+ codoped with MgO or ZnO. This centre was attributed to Er3+ ions located at Nb5+ site. Shao-Yi and Wen-Chen [12] certified that Er3+ ions do not occupy exactly the Li+ or Nb5+ sites in LN but are displaced along c-axis away from the centre of oxygen octahedron. And last but not least, Nolte et al. [13] has investigated a congruent LN: Er3+ . They reported that paramagnetic centre which may be attributed to Er3+ defect at Li+ site next to Li+ vacancy (V+ Li ) tilts the g-tensor orientation away from the c-axis. Our interest in the investigation of LN: Er3+ is connected with our previous investigations of LN weakly doped with ytterbium (LN: Yb) and LN doped with ytterbium and codoped with praseodymium (LN: Yb, Pr) single crystals [14–16]. The temperature dependence of the EPR lines for the above crystals was explained by supposing the existence of Yb–Yb ion pairs in the host lattice, which was confirmed by optical and Raman spectroscopy [17]. In one of our previous paper we have analyzed the anisotropy of the paramagnetic centres in weakly doped LN: Er (0.1 wt.%) single crystal and
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Fig. 2. Raman spectrum recorded for LN: Er, Tm (0.2 wt.%, 0.3 wt.%) single crystal. Fig. 1. The absorption coefficient of LN single crystals doped with—1: Tm (0.3 wt.%); 2: Er (0.2 wt.%), Tm (0.3 wt.%); 3: Tm (0.6 wt.%); 4: Er (0.2 wt.%), Tm (0.6 wt.%); 5: Er (0.1 wt.%); 6: Er (1 wt.%), Yb (0.5 wt.%) in the range of OH− absorption.
concluded that new low symmetry (C1 ) centres can explain the anisotropy [18]. In this work, we have focused on the investigation of temperature dependence of the EPR lines and optical measurements of LN: Er, Tm single crystals. 2. Experimental LiNbO3 single crystals doped with Er (0.1 wt.%), Tm (0.3 wt.%), Tm (0.6 wt.%), Er3+ (0.2 wt.%) and Tm3+ (0.3 wt.%), Er3+ (0.2 wt.%) and Tm3+ (0.6 wt.%), and, Er3+ (1 wt.%) and Yb3+ (0.5 wt.%) were grown along the c-axis from the congruent melt by the Czochralski method in the Institute of Electronic Materials Technology (IEMT, Warsow, Poland). The following starting materials were used: Nb2 O5 (4N purity) from Johnson-Matthey; Li2 CO3 (4N purity) from IEMT. After mixing of adequate amounts of reagents the mixture was calcined at 1373 K for 6 h. The Er2 O3 , Tm2 O3 of 4N purity were added to the charge of congruent melt with the xc = [Li]/([Li] + [Nb]) ratio equal to xc ≈ (48.5 ± 0.2)%, prior to synthesis at elevated temperatures. A more detailed description of the applied growth process is presented elsewhere [19]. LN samples for optical measurements were cut from the obtained crystals perpendicularly to c-axis and both sides were polished. Lambda-900 of PerkinElmer spectrophotometer was applied to room transmission spectra measurements, performed in the Institute of Optoelectronics, Military University of Technology. X(YZ)Y Raman spectra were recorded for LN: Er, Tm (0.2 wt.%, 0.3 wt.%) single crystals, for A and E modes within a resolution 0.56 cm−1 . SPEX 1403 Raman spectrometer was used applying right-angle scattering geometry and the 488 nm line of an Ar + laser. The samples for EPR measurements were in the form of parallel plates of dimensions (XYZ) 5.0 mm × 3.3 mm × 3.0 mm and were oriented using the X-ray diffraction method (XRD). The laboratory axis system (XYZ) was chosen as follows: the Z-axis is taken along the crystallographic (i.e. optical c-axis) z-axis, the X-axis along the one of the crystallographic a-axis and the Y-axis as perpendicular to both the Zand X-axes. EPR spectra were recorded using Bruker E 500 X-band spectrometer ( ∼ 9.4 GHz) with 100 kHz field modulation equipped with an Oxford flow cryostat for measurements at temperatures from liquid nitrogen temperature down to 4 K (EPR signals of RE ions can be observed mainly at liquid-helium temperatures). The samples were mounted on a goniometer to measure the angular dependence of the spectra.
3. Results 3.1. Optical spectroscopy Room temperature absorption coefficients of the all investigated lithium niobate samples are presented in Fig. 1. A band characteristic of OH− absorption, centred at about 2872 cm−1 (for LN crystal doped with Tm (0.3 wt.%), Tm (0.6 wt.%), Er (0.1 wt.%), and, Er (0.2 wt.%) and Tm (0.6 wt.%)) can be clearly recognized. As one can see the shape of the band is distinctly different for two of the crystals, being shifted for a few nm towards shorter wavelengths in case of Er (0.2 wt.%) and Tm (0.3 wt.%) doping, and towards longer wavelengths in case of Er (1 wt.%) and Yb (0.5 wt.%) doping.
From Fig. 1 it follows that the increase in the Tm concentration leads to decrease in the intensity of the 2872 cm−1 band both in single and double doping cases. Adding of the Er3+ to Tm3+ doped LN crystal leads, at first, to the strong (twofold) increase in the absorption, which maximum shifts towards shorter wavelengths (2868 cm−1 ). The band is build of at least two bands which are shifted 10 nm to each other. The shifting is also observed, but in reversed directions, in case of higher Er and Yb co-doping. The observation of an OH− absorption band at 2872 cm−1 was used to analyze the location of RE3+ ions in the LN lattice. It is known that OH− ions located at O2− sites generate 1+ charged defect in the LN lattice. Occupation of Li+ sites by X2+ or X3+ ions also generates 1+ or 2+ positively charged defects so that their incorporation should in some way be a competitive process for the generation of OH− ions. The location of a X2+ or X3+ ions at a Nb5+ site generates 3− or 2− negatively charged defects and should not retard the incorporation of OH− ions. The results obtained imply that the reduction in OH− ions creation observed under, e.g., increasing in the Tm concentration, suggests the location of RE3+ ions at Li+ sites in the lattice. Strong increase in the OH− absorption band, observed in case of Er (0.2 wt.%) and Tm (0.3 wt.%) co-doping, suggests their location also at other sites in the lattice, e.g. at Nb5+ sites or at interstitial positions with octahedral symmetry. Such a substitution may be a source of the RE3+ pairs.
3.2. Raman spectroscopy Raman spectroscopy can be sensitive to a small change in a structure due to the non-stoichiometry related to the intrinsic defects [20] or extrinsic impurities [21]. This can lead to a frequency shift or broadening of Raman lines or appearance of new lines [20,22,23]. The Raman spectrum shown in Fig. 2 reveals mainly the 152 cm−1 E(TO1 ) phonon mode, which corresponds to the deformation of the BO6 (NbO6 ) framework in the (X, Y) plane, is significantly affected by doping and resemble the strong photoluminescence of the crystal. Moreover, 100 cm−1 phonon mode we can recognize (see arrow), being probably an effect of extrinsic doping and RE co-doping (pairs).
3.3. The EPR measurements The spin Hamiltonian characterizing EPR spectra of all isotopes of Er3+ ions (with the effective spin Seff = 1/2) can be written simply
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Fig. 3. Examples of EPR spectra for LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal recorded at temperatures T = 12.0, 24.2, 30.7, 35.7 and 45.5 K. The magnetic field is parallel to z-axis (optical c-axis, B||c, = 0◦ ).
Fig. 4. Samples of EPR spectra for LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal recorded at temperatures T = 8.5, 15, 8.5, 21.1, 36.2 and 60.8 K measured at angle ϕ = 150◦ in the plane perpendicular to the crystal c-axis.
as a sum of two terms [24]:
ture, so we can conclude that each of the EPR lines appears to be a superposition of several resonance lines originating from different non-equivalent paramagnetic centres (see Fig. 4). The linewidth of the main EPR line, recorded at T = 12 K, is equal to 8.8 ± 0.2 mT (B||z, see Fig. 3) while the linewidths of the EPR lines recorded at T = 8.5 K are equal to 464 and 791 mT, for the EPR lines centred at 250 and 450 mT (B⊥z, see Fig. 4), respectively. Such broad resonance lines may indicate that in this plane we have recorded the EPR lines attributed mainly to even Er3+ –even Er3+ ion pairs in LN host lattice. The maximal amplitude (and the maximal intensity) of the EPR lines were observed at a temperature of T = 4.8 K. The temperature dependences of the total intensity, the inverse intensity and the linewidth of the main, EPR line are gathered in Fig. 5. The total intensity is estimated by double integrating of the EPR main line originating from Zeeman transition (see Eq. (1)) [24]. One can see that with increasing temperature, beginning from liquid helium temperature, the total intensity of the main EPR line decreases and reaches a minimum at a temperature of T = 36 K. On further temperature increase, the total intensity of the main EPR line slightly increases (see Fig. 5, upper panel—solid squares, left
H = B SgB + SAI
(1)
where first is a electronic Zeeman, second is a hyperfine term, B is the external magnetic field, B is the Bohr magneton, S and I are the electron and nuclear spin of the paramagnetic centres, respectively. g and A are the anisotropic (and generally non-coincident) tensors characterizing Zeeman and hyperfine interactions, respectively. EPR signals for rare-earth ions can only be observed at liquid-helium temperatures. The EPR spectrum of Er-doped LN crystal shows a strong, central line for the even Er isotopes (I = 0, no nuclear magnetic moments for 166 Er, 168 Er and other even isotopes, natural abundance 77.06%) and the eight hyperfine transition (lines) distributed about the central line corresponding to different values of the nuclear magnetic number (I = 7/2, natural abundance 22.94%). Fig. 3 shows the EPR spectra of LiNbO3 : Er (0.2 wt.%), Tm (0.3 wt.%) single crystal recorded at various temperatures, when magnetic field is parallel to z-axis (optical c-axis, B||c, = 0◦ ). The main, central line—corresponding to fine transition is clearly visible in the centre of Fig 3. One can see also three low field lines corresponding to hyperfine transitions. Remaining five lines are hidden by the strong, main EPR line. In our analysis we focused on the investigation of the behaviour of the main EPR line because the hyperfine EPR lines have much less intensity and quickly vanish with increasing temperature. The above mentioned EPR spectra were recorded at temperatures: T = 12, 24.2, 30.7, 35.7 and 45.5 K. It should be mentioned a distorted shape of the main EPR line, especially well visible at low temperatures, e.g. at T = 12.0 K (see Fig. 3). The shape of the EPR line may indicate that the line is a superposition of several, at least two, resonance lines originating from magnetically non-equivalent paramagnetic centres (NEC). The examples of the EPR spectra of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal in the plane perpendicular to the crystal zaxis (at angle ϕ = 150◦ ) are presented in Fig. 4. The resonance spectra were recorded at temperatures T = 8.5, 15, 8.5, 21.1, 36.2 and 60.8 K. As one can see the EPR spectra look quite differently in this plane. One can notice a wide and complex EPR spectrum consists of three broad and strong resonance lines centred at about 250, 450 and 650 mT. The fourth resonance line centred at about 950 mT, and visible only in the low temperature region (T = 8.5 K) can take origin from resonance chamber. Temperature dependence shows that the broad EPR lines change their shape with increasing tempera-
Fig. 5. The temperature dependence of the total intensity (solid squares, the left axis—upper panel), inverse intensity (solid circles, the right axis—upper panel) and the temperature dependence of the linewidth (lower panel) of the main EPR line of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal measured when a magnetic field is parallel to the crystal z-axis (optical c-axis, B||c, = 0◦ ).
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Fig. 6. The temperature dependence of the total intensity (solid squares, left axis) and inverse intensity (solid circles, right axis) of the EPR spectrum of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal measured at an angle of ϕ = 150◦ in the plane XY perpendicular to the crystal (optical c) z-axis.
axis). The intensity of EPR lines originating from rare-earth ions should decrease with increasing temperature and vanish above 40 K, so the increase in the total intensity of the EPR line with increasing temperature is unexpected. The temperature dependence of the inverse total intensity reveals complex behaviour (see Fig. 5, upper panel—solid circles, the right axis). One can distinguish three regions in Fig. 5: in the first (12–24 K) and the second one (24–36 K) region the inverse intensity of the main EPR line increases approximately linearly with temperature but with different slopes. In the third (above 36 K) region the inverse intensity of the main, EPR line decreases with increasing temperature. The temperature dependence of the linewidth is presented in the lower panel in Fig. 5. As one can see, the linewidth of the main, EPR line decreases with decreasing temperature. Fig. 6 presents the temperature dependence of the total intensity and inverse total intensity of the EPR spectrum of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal measured at an angle of ϕ = 150◦ in the plane perpendicular to the crystal (optical c) z-axis. One can see that the total intensity of the EPR spectrum increases with decreasing temperature in Fig. 6. However, at the low temperature region two maxima of the intensity are observed, at T = 8.5 and 4.8 K, respectively. On further temperature increase, the total intensity quickly decreases. The temperature dependence of the inverse intensity shows that the EPR spectrum vanishes above 60 K (see Fig. 6, solid circles, the right axis). In the low temperature region, the inverse intensity fulfils the Curie–Weiss law (see Fig. 6, right axis), similarly as for the direction parallel to the crystal (optical c) z-axis (see Fig. 5, upper panel). Finally let we consider crystallographic position of the Er3+ ions and possible vacancies in LN single crystal. Fig. 7 presents crystallographic arrangement of constituent atoms of LN crystal projected on the (0 0 0 1) plane (perpendicular to the c-axis). The charge compensation is required when Er3+ ion is incorporated into Li+ site (in the middle of the picture). Six possibilities of the cation vacation are marked in Fig. 7: 1, 2, 3 – Li+ vacancies and 1 , 2 , 3 – Nb5+ vacancies in the first shell. All paramagnetic sites being positioned along the optical c-axis of LN crystal (perpendicular to the plane of Fig. 7) have the C3 point group symmetry, while the paramagnetic sites at other positions have the lowest, C1 , symmetry. We should restrict our concern to non-equivalent crystallographic sites of low C1 point group symmetry. The differently positioned Vi – vacancies in the first shell cause
Fig. 7. Crystallographic arrangement of constituent atoms of LN: Er crystal projected on the (0 0 0 1) plane (perpendicular to the c-axis); 1, 2, 3—Nd5+ sites in the first shell below the Er ion, 1 , 2 , 3 —Li+ site in the first shell above the Er ion; A, B, C, D, E, F—Li+ in the second shell in the plane contain Er.
differently tilted Er3+ g – tensor z-axis and consequently lower local site symmetry of the Er3+ ion. Each of these paramagnetic centres ErLi+ –Vi (ErLi+ connected with one of the vacancies Vi , i = 1, 2, 3, 1 , 2 , 3 ) give six non-equivalent paramagnetic centres that characterize low C1 point group symmetry. These centres should dominate in the EPR spectrum [13]. The paramagnetic centres ErLi+ –Vi , Vj (ErLi+ connected with two of vacancies present in the first shell i, j = 1, 2, 3, 1 , 2 , 3 ) characterize much less intensity. Each of these paramagnetic centres has the same surrounding, so the temperature dependence of the intensity of EPR line originating from these centres should be approximately the same (see Fig. 7). It allows us conclude that the change of the intensity of EPR lines versus temperature could not be determined by the existence of several non-equivalent crystallographic sites of single Er3+ ions in LN: Er, or LN: Er, Tm. A model of Er3+ –Er3+ ion pairs could also be visible in Fig. 7. One Er3+ ion is incorporated into Li+ site, the second Er3+ ion is incorporated into Li+ site or Nb5+ site (1, 2, 3, 1 , 2 , 3 positions) in the first shell. Each of these Er3+ –Er3+ ion pairs should present C1 symmetry. 4. Discussion and conclusions The results of the optical and EPR measurements of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal have been reported. The temperature dependence of the intensity, inverse intensity and linewidth revealed complexity of the EPR spectra. We observed some unusual behaviour in temperature dependence of the EPR spectra. Three different regions in the temperature dependence of the main EPR line of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal, when a magnetic field is parallel to a crystal (optical c) z-axis (see Fig. 4) were observed. However, it can be understand if we assume that the main, EPR line is a superposition of several (at least three) paramagnetic lines and the different paramagnetic centres corresponding to the lines dominated in these three regions. The EPR signal originating from even Er3+ ions dominated below 36 K. But above 36 K the EPR signal is dominated by resonance signals originating from some others paramagnetic ions yet unidentified. We suppose that they may enter into the LN structure as uncontrolled impurities from the chemical ingredients during the crystal growth. These paramagnetic centres produce a very weak and very broad resonance signal, which vanishes above 46 K. Similar phenomena were observed in case of LN: Yb3+ and LN: Er3+ single crystals [17,18]. The change of the slope of the intensity indicates
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that the EPR signal originating from even Er3+ ions can dominate only in one of the regions; in the other one, the EPR signal is dominated by signals arising from different kinds of even Er3+ species, possibly even Er3+ –even Er3+ ion pairs. Such possibility was confirmed by optical spectroscopy and Raman measurements presented in Sections 3.1 and 3.2 of the paper. The two peaks pattern, which we observed in the temperature dependence of the total intensity of the EPR spectrum measured at an angle of ϕ = 150◦ in the plane XY perpendicular to crystal (optical c) z-axis, can be explained by assumption that in this temperature range the EPR spectrum is dominated by two different paramagnetic centres originating from even Er3+ –even Er3+ ion pairs (see Fig. 6). The inverse intensity versus temperature fulfils the Curie–Weiss law in both crystallographic planes and enables us to determine the Curie–Weiss constant = 6.1 ± 1.1 K. The positive sign of the constant suggests that the ferromagnetic interactions arise in the system of even Er3+ –even Er3+ ion pairs in LN: Er, Tm single crystal. The assumption on the presence of even Er3+ –even Er3+ ion pairs in LN: Er, Tm single crystal can explain a decrease of the total intensity observed below 4.8 K, too (see Fig. 6). On the basis of the structural consideration we can conclude that observed temperature dependence of the intensity and inverse intensity of the EPR line could not be related to the existence of several nonequivalent crystallographic sites of single Er3+ ions in LN: Er single crystal. The EPR spectrum of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal seem to be a superposition of several lines originating from different kinds of paramagnetic centres: originating from isolated even Er3+ ions, the second one even Er3+ –even Er3+ pair of ions and third one—arising from unidentified paramagnetic ions. It seems that the most important role in the unusual behaviour of the EPR spectra plays a codopant—thulium and unidentified impurities. The presence of additional impurities may act in a similar way as the co-doping by MgO, i.e. forcing a certain proportion of the Er3+ ions to take the Nb5+ positions [11]. Those ErNb ions locating nearby the ErLi sites create the even Er3+ –even Er3+ ion pairs, which are apparently observed in our optical spectra and in temperature dependence of the intensity and linewidth of the main EPR line. The proposed model quite well explains formation of such ErLi –ErNb pairs. The similar ferromagnetic interactions were identi-
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fied in the system of even Er3+ –even Er3+ ion pairs in LN singly doped with Er [18] but in case of our sample this phenomenon is much better recognizable. Previously, the LN: Er crystal was only weakly doped with Er (0.1 wt.%). Now, we are investigating the LN single crystal doped with Er (0.2 wt.%) and codoped with Tm (0.3 wt.%). We have got approximately two times much larger number of Er ions and additional Tm codopant ions. The results of our research indicate that the presence of codopant ions play the most important role in unusual behaviour of the EPR spectra of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal, e.g. facilitates introducing of the Er ions and forces creating of the even Er3+ –even Er3+ ion pairs in the LN host crystal. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Properties of Lithium Niobate, EMIS Datareviews Series No. 5, Inspec, 1989. G. Malovichko, V. Grachev, V. Kokanyan, O. Shirmer, Phys. Rev. B 59 (1999) 9113. L.F. Johnson, A.A. Ballman, J. Appl. Phys. 40 (1969) 297. D. Xue, X. He, Phys. Rev. B 73 (2006) 064113. G. Burns, D.F. O’Kane, R.S. Title, Phys. Rev. 167 (1968) 314. C. Bonardi, C.J. Magon, E.A. Vidoto, M.C. Terrile, L.E. Bausa, E. Montoya, D. Bravo, A. Martin, F.L. Lopez, J. Alloys Compd. 323–324 (2001) 340. S.H. Choh, J.H. Kim, I.W. Park, H.J. Kim, D. Choi, S.S. Kim, Appl. Magn. Reson. 24 (2003) 313. J. Amin, B. Dussardier, T. Schweizer, M. Hempstead, J. Lumin. 69 (1996) 17. V. Dierolf, M. Koerdt, Phys. Rev. B 61 (2000) 80. D.M.B.P. Milori, I.J. Moraes, A.C. Hernandes, R.R. de Souza, M. Siu Li, M.C. Terrile, Phys. Rev. B 51 (1995) 3206. D. Bravo, A. Martin, F.J. Lopez, Solid State Commun. 112 (1999) 541. S.-Y. Wu, W.-C. Zheng, Phys. Rev. B 65 (2002) 224107. Th. Nolte, Th. Pawlik, J.M. Spaeth, Solid State Commun. 104 (1997) 535. T. Bodziony, S.M. Kaczmarek, Res. Chem. Intermed. 33 (8–9) (2007) 885. T. Bodziony, S.M. Kaczmarek, Opt. Mater. 29 (2007) 1440. T. Bodziony, S.M. Kaczmarek, J. Hanuza, J. Alloys Compd. 451 (1/2) (2008) 240. T. Bodziony, S.M. Kaczmarek, C. Rudowicz, Physica B 403 (2008) 207. T. Bodziony, S.M. Kaczmarek, Physica B 400 (2007) 99. ˛ I. Pracka, M. Malinowski, A.L. Bajor, B. Surma, Z. Gałazka, M. Swirkowicz, M. Mozdzonek, Proc. SPIE 3178 (1997) 295. A. Ridah, P. Bourson, M.D. Fontana, G. Malovichko, J. Phys. Condens. Matter 9 (1997) 55. R. Mouras, M.D. Fontana, P. Bourson, A.V. Postnikov, J. Phys. Condens. Matter 12 (2000) 5053. R. Mouras, P. Bourson, M.D. Fontana, G. Boulon, Opt. Commun. 197 (2001) 439. K. Chen, M.S. Zhang, W.C. Chen, Q. Chen, 2000, Progress in Crystal Growth and Characterization of Materials, Pergamon, p. 41. A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon Press, Oxford, 1970.
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Temperature dependence of the EPR spectra and optical measurements of LiNbO3 : Er, Tm single crystal Tomasz Bodziony ∗ , Sławomir Maksymilian Kaczmarek Institute of Physics, Szczecin University of Technology, Al. Piast´ ow 17, 70-310 Szczecin, Poland
a r t i c l e
i n f o
Article history: Received 15 November 2007 Received in revised form 13 March 2008 Accepted 14 March 2008 Available online 28 April 2008 Keywords: Electron paramagnetic resonance Lithium niobate Er3+ Rare-earth ions Pairs of ions
a b s t r a c t Optical, Raman and EPR spectroscopy measurements of the congruent LiNbO3 : Er (0.2 wt.%), Tm (0.3 wt.%) single crystal have been carried out. The shape of the optical absorption suggests the presence of RE3+ ion pairs. Unusual behaviour of the temperature dependence of the EPR spectra was reported. The EPR spectra could be analysed as superposition of several paramagnetic centres originating from isolated even Er3+ ions and even Er3+ –even Er3+ pairs of ions. In low temperature region the main EPR signal is dominated by signals originating from even Er3+ –even Er3+ pairs of ions. The inverse intensity of the EPR line, in low temperature region, fulfils the Curie–Weiss law and enabled to determine the Curie–Weiss constant = 6.1 ± 1.1 K. The positive sign of the suggests that ferromagnetic interactions arise in the system of even Er3+ –even Er3+ ion pairs in LiNbO3 . Our results suggest that the presence of codopant ions play the most important role in unusual behaviour of the EPR spectra. They facilitate introduction of the Er ions into host lattice and creation of the even Er3+ –even Er3+ ion pairs in the LiNbO3 host crystal. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Erbium doped lithium niobate (LiNbO3 , LN) has excellent perspective of applications in integrated optics and has been utilized in various technological applications [1,2]. It is an especially suitable system for solid-state lasers due to non-linear properties of LN [3]. Recently, erbium doped LN has gained attention through the development of light amplifiers based on LN: Er3+ in optical fibers for optical communication. Optical properties of any doped crystal are largely determined by the local symmetry of the optically active ions. Electron paramagnetic resonance (EPR) is a very sensitive method to investigate the position and the defect structure of an impurity ion. Due to non-stoichiometry, LN contains several defects including antisite defects (Nb5+ ion in a Li+ site, namely NbLi ) and four sites. Three of them are octahedral sites: Li+ , Nb5+ or structural vacancy, and fourth is a tetrahedral interstitial site [4–6]. All paramagnetic sites positioned along the optical c-axis of LN crystal have the C3 point group symmetry, while the impurity ions at other positions have the lowest, C1 , symmetry [2]. LN crystal can be tuned to fit various experiment requirements. One of the method applied to realize this tuning is doping. Rareearth (RE) dopant ions, as e.g. Nd3+ or Er3+ , are introduced to LN host as optically active impurities [7]. The relative concentration of such
∗ Corresponding author. E-mail address: [email protected] (T. Bodziony). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.062
impurities depends on the stoichiometry (i.e. the Li/Nb concentration ratio). Properties of RE3+ centres in LN are directly related to the defects associated with the non-stoichiometric conditions [7]. LN crystals doped with erbium (Er) have been widely studied because of their optoelectronic applications [8]. Dierolf and Koerdt [9] and Milori et al. [10] have found a trigonal centre which they attributed to Er3+ ions occupying Li+ positions. Bravo et al. [11] have found a new trigonal centre with relatively small anisotropy of the g factor in comparison to previously observed one in LN: Er3+ codoped with MgO or ZnO. This centre was attributed to Er3+ ions located at Nb5+ site. Shao-Yi and Wen-Chen [12] certified that Er3+ ions do not occupy exactly the Li+ or Nb5+ sites in LN but are displaced along c-axis away from the centre of oxygen octahedron. And last but not least, Nolte et al. [13] has investigated a congruent LN: Er3+ . They reported that paramagnetic centre which may be attributed to Er3+ defect at Li+ site next to Li+ vacancy (V+ Li ) tilts the g-tensor orientation away from the c-axis. Our interest in the investigation of LN: Er3+ is connected with our previous investigations of LN weakly doped with ytterbium (LN: Yb) and LN doped with ytterbium and codoped with praseodymium (LN: Yb, Pr) single crystals [14–16]. The temperature dependence of the EPR lines for the above crystals was explained by supposing the existence of Yb–Yb ion pairs in the host lattice, which was confirmed by optical and Raman spectroscopy [17]. In one of our previous paper we have analyzed the anisotropy of the paramagnetic centres in weakly doped LN: Er (0.1 wt.%) single crystal and
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Fig. 2. Raman spectrum recorded for LN: Er, Tm (0.2 wt.%, 0.3 wt.%) single crystal. Fig. 1. The absorption coefficient of LN single crystals doped with—1: Tm (0.3 wt.%); 2: Er (0.2 wt.%), Tm (0.3 wt.%); 3: Tm (0.6 wt.%); 4: Er (0.2 wt.%), Tm (0.6 wt.%); 5: Er (0.1 wt.%); 6: Er (1 wt.%), Yb (0.5 wt.%) in the range of OH− absorption.
concluded that new low symmetry (C1 ) centres can explain the anisotropy [18]. In this work, we have focused on the investigation of temperature dependence of the EPR lines and optical measurements of LN: Er, Tm single crystals. 2. Experimental LiNbO3 single crystals doped with Er (0.1 wt.%), Tm (0.3 wt.%), Tm (0.6 wt.%), Er3+ (0.2 wt.%) and Tm3+ (0.3 wt.%), Er3+ (0.2 wt.%) and Tm3+ (0.6 wt.%), and, Er3+ (1 wt.%) and Yb3+ (0.5 wt.%) were grown along the c-axis from the congruent melt by the Czochralski method in the Institute of Electronic Materials Technology (IEMT, Warsow, Poland). The following starting materials were used: Nb2 O5 (4N purity) from Johnson-Matthey; Li2 CO3 (4N purity) from IEMT. After mixing of adequate amounts of reagents the mixture was calcined at 1373 K for 6 h. The Er2 O3 , Tm2 O3 of 4N purity were added to the charge of congruent melt with the xc = [Li]/([Li] + [Nb]) ratio equal to xc ≈ (48.5 ± 0.2)%, prior to synthesis at elevated temperatures. A more detailed description of the applied growth process is presented elsewhere [19]. LN samples for optical measurements were cut from the obtained crystals perpendicularly to c-axis and both sides were polished. Lambda-900 of PerkinElmer spectrophotometer was applied to room transmission spectra measurements, performed in the Institute of Optoelectronics, Military University of Technology. X(YZ)Y Raman spectra were recorded for LN: Er, Tm (0.2 wt.%, 0.3 wt.%) single crystals, for A and E modes within a resolution 0.56 cm−1 . SPEX 1403 Raman spectrometer was used applying right-angle scattering geometry and the 488 nm line of an Ar + laser. The samples for EPR measurements were in the form of parallel plates of dimensions (XYZ) 5.0 mm × 3.3 mm × 3.0 mm and were oriented using the X-ray diffraction method (XRD). The laboratory axis system (XYZ) was chosen as follows: the Z-axis is taken along the crystallographic (i.e. optical c-axis) z-axis, the X-axis along the one of the crystallographic a-axis and the Y-axis as perpendicular to both the Zand X-axes. EPR spectra were recorded using Bruker E 500 X-band spectrometer ( ∼ 9.4 GHz) with 100 kHz field modulation equipped with an Oxford flow cryostat for measurements at temperatures from liquid nitrogen temperature down to 4 K (EPR signals of RE ions can be observed mainly at liquid-helium temperatures). The samples were mounted on a goniometer to measure the angular dependence of the spectra.
3. Results 3.1. Optical spectroscopy Room temperature absorption coefficients of the all investigated lithium niobate samples are presented in Fig. 1. A band characteristic of OH− absorption, centred at about 2872 cm−1 (for LN crystal doped with Tm (0.3 wt.%), Tm (0.6 wt.%), Er (0.1 wt.%), and, Er (0.2 wt.%) and Tm (0.6 wt.%)) can be clearly recognized. As one can see the shape of the band is distinctly different for two of the crystals, being shifted for a few nm towards shorter wavelengths in case of Er (0.2 wt.%) and Tm (0.3 wt.%) doping, and towards longer wavelengths in case of Er (1 wt.%) and Yb (0.5 wt.%) doping.
From Fig. 1 it follows that the increase in the Tm concentration leads to decrease in the intensity of the 2872 cm−1 band both in single and double doping cases. Adding of the Er3+ to Tm3+ doped LN crystal leads, at first, to the strong (twofold) increase in the absorption, which maximum shifts towards shorter wavelengths (2868 cm−1 ). The band is build of at least two bands which are shifted 10 nm to each other. The shifting is also observed, but in reversed directions, in case of higher Er and Yb co-doping. The observation of an OH− absorption band at 2872 cm−1 was used to analyze the location of RE3+ ions in the LN lattice. It is known that OH− ions located at O2− sites generate 1+ charged defect in the LN lattice. Occupation of Li+ sites by X2+ or X3+ ions also generates 1+ or 2+ positively charged defects so that their incorporation should in some way be a competitive process for the generation of OH− ions. The location of a X2+ or X3+ ions at a Nb5+ site generates 3− or 2− negatively charged defects and should not retard the incorporation of OH− ions. The results obtained imply that the reduction in OH− ions creation observed under, e.g., increasing in the Tm concentration, suggests the location of RE3+ ions at Li+ sites in the lattice. Strong increase in the OH− absorption band, observed in case of Er (0.2 wt.%) and Tm (0.3 wt.%) co-doping, suggests their location also at other sites in the lattice, e.g. at Nb5+ sites or at interstitial positions with octahedral symmetry. Such a substitution may be a source of the RE3+ pairs.
3.2. Raman spectroscopy Raman spectroscopy can be sensitive to a small change in a structure due to the non-stoichiometry related to the intrinsic defects [20] or extrinsic impurities [21]. This can lead to a frequency shift or broadening of Raman lines or appearance of new lines [20,22,23]. The Raman spectrum shown in Fig. 2 reveals mainly the 152 cm−1 E(TO1 ) phonon mode, which corresponds to the deformation of the BO6 (NbO6 ) framework in the (X, Y) plane, is significantly affected by doping and resemble the strong photoluminescence of the crystal. Moreover, 100 cm−1 phonon mode we can recognize (see arrow), being probably an effect of extrinsic doping and RE co-doping (pairs).
3.3. The EPR measurements The spin Hamiltonian characterizing EPR spectra of all isotopes of Er3+ ions (with the effective spin Seff = 1/2) can be written simply
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Fig. 3. Examples of EPR spectra for LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal recorded at temperatures T = 12.0, 24.2, 30.7, 35.7 and 45.5 K. The magnetic field is parallel to z-axis (optical c-axis, B||c, = 0◦ ).
Fig. 4. Samples of EPR spectra for LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal recorded at temperatures T = 8.5, 15, 8.5, 21.1, 36.2 and 60.8 K measured at angle ϕ = 150◦ in the plane perpendicular to the crystal c-axis.
as a sum of two terms [24]:
ture, so we can conclude that each of the EPR lines appears to be a superposition of several resonance lines originating from different non-equivalent paramagnetic centres (see Fig. 4). The linewidth of the main EPR line, recorded at T = 12 K, is equal to 8.8 ± 0.2 mT (B||z, see Fig. 3) while the linewidths of the EPR lines recorded at T = 8.5 K are equal to 464 and 791 mT, for the EPR lines centred at 250 and 450 mT (B⊥z, see Fig. 4), respectively. Such broad resonance lines may indicate that in this plane we have recorded the EPR lines attributed mainly to even Er3+ –even Er3+ ion pairs in LN host lattice. The maximal amplitude (and the maximal intensity) of the EPR lines were observed at a temperature of T = 4.8 K. The temperature dependences of the total intensity, the inverse intensity and the linewidth of the main, EPR line are gathered in Fig. 5. The total intensity is estimated by double integrating of the EPR main line originating from Zeeman transition (see Eq. (1)) [24]. One can see that with increasing temperature, beginning from liquid helium temperature, the total intensity of the main EPR line decreases and reaches a minimum at a temperature of T = 36 K. On further temperature increase, the total intensity of the main EPR line slightly increases (see Fig. 5, upper panel—solid squares, left
H = B SgB + SAI
(1)
where first is a electronic Zeeman, second is a hyperfine term, B is the external magnetic field, B is the Bohr magneton, S and I are the electron and nuclear spin of the paramagnetic centres, respectively. g and A are the anisotropic (and generally non-coincident) tensors characterizing Zeeman and hyperfine interactions, respectively. EPR signals for rare-earth ions can only be observed at liquid-helium temperatures. The EPR spectrum of Er-doped LN crystal shows a strong, central line for the even Er isotopes (I = 0, no nuclear magnetic moments for 166 Er, 168 Er and other even isotopes, natural abundance 77.06%) and the eight hyperfine transition (lines) distributed about the central line corresponding to different values of the nuclear magnetic number (I = 7/2, natural abundance 22.94%). Fig. 3 shows the EPR spectra of LiNbO3 : Er (0.2 wt.%), Tm (0.3 wt.%) single crystal recorded at various temperatures, when magnetic field is parallel to z-axis (optical c-axis, B||c, = 0◦ ). The main, central line—corresponding to fine transition is clearly visible in the centre of Fig 3. One can see also three low field lines corresponding to hyperfine transitions. Remaining five lines are hidden by the strong, main EPR line. In our analysis we focused on the investigation of the behaviour of the main EPR line because the hyperfine EPR lines have much less intensity and quickly vanish with increasing temperature. The above mentioned EPR spectra were recorded at temperatures: T = 12, 24.2, 30.7, 35.7 and 45.5 K. It should be mentioned a distorted shape of the main EPR line, especially well visible at low temperatures, e.g. at T = 12.0 K (see Fig. 3). The shape of the EPR line may indicate that the line is a superposition of several, at least two, resonance lines originating from magnetically non-equivalent paramagnetic centres (NEC). The examples of the EPR spectra of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal in the plane perpendicular to the crystal zaxis (at angle ϕ = 150◦ ) are presented in Fig. 4. The resonance spectra were recorded at temperatures T = 8.5, 15, 8.5, 21.1, 36.2 and 60.8 K. As one can see the EPR spectra look quite differently in this plane. One can notice a wide and complex EPR spectrum consists of three broad and strong resonance lines centred at about 250, 450 and 650 mT. The fourth resonance line centred at about 950 mT, and visible only in the low temperature region (T = 8.5 K) can take origin from resonance chamber. Temperature dependence shows that the broad EPR lines change their shape with increasing tempera-
Fig. 5. The temperature dependence of the total intensity (solid squares, the left axis—upper panel), inverse intensity (solid circles, the right axis—upper panel) and the temperature dependence of the linewidth (lower panel) of the main EPR line of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal measured when a magnetic field is parallel to the crystal z-axis (optical c-axis, B||c, = 0◦ ).
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Fig. 6. The temperature dependence of the total intensity (solid squares, left axis) and inverse intensity (solid circles, right axis) of the EPR spectrum of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal measured at an angle of ϕ = 150◦ in the plane XY perpendicular to the crystal (optical c) z-axis.
axis). The intensity of EPR lines originating from rare-earth ions should decrease with increasing temperature and vanish above 40 K, so the increase in the total intensity of the EPR line with increasing temperature is unexpected. The temperature dependence of the inverse total intensity reveals complex behaviour (see Fig. 5, upper panel—solid circles, the right axis). One can distinguish three regions in Fig. 5: in the first (12–24 K) and the second one (24–36 K) region the inverse intensity of the main EPR line increases approximately linearly with temperature but with different slopes. In the third (above 36 K) region the inverse intensity of the main, EPR line decreases with increasing temperature. The temperature dependence of the linewidth is presented in the lower panel in Fig. 5. As one can see, the linewidth of the main, EPR line decreases with decreasing temperature. Fig. 6 presents the temperature dependence of the total intensity and inverse total intensity of the EPR spectrum of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal measured at an angle of ϕ = 150◦ in the plane perpendicular to the crystal (optical c) z-axis. One can see that the total intensity of the EPR spectrum increases with decreasing temperature in Fig. 6. However, at the low temperature region two maxima of the intensity are observed, at T = 8.5 and 4.8 K, respectively. On further temperature increase, the total intensity quickly decreases. The temperature dependence of the inverse intensity shows that the EPR spectrum vanishes above 60 K (see Fig. 6, solid circles, the right axis). In the low temperature region, the inverse intensity fulfils the Curie–Weiss law (see Fig. 6, right axis), similarly as for the direction parallel to the crystal (optical c) z-axis (see Fig. 5, upper panel). Finally let we consider crystallographic position of the Er3+ ions and possible vacancies in LN single crystal. Fig. 7 presents crystallographic arrangement of constituent atoms of LN crystal projected on the (0 0 0 1) plane (perpendicular to the c-axis). The charge compensation is required when Er3+ ion is incorporated into Li+ site (in the middle of the picture). Six possibilities of the cation vacation are marked in Fig. 7: 1, 2, 3 – Li+ vacancies and 1 , 2 , 3 – Nb5+ vacancies in the first shell. All paramagnetic sites being positioned along the optical c-axis of LN crystal (perpendicular to the plane of Fig. 7) have the C3 point group symmetry, while the paramagnetic sites at other positions have the lowest, C1 , symmetry. We should restrict our concern to non-equivalent crystallographic sites of low C1 point group symmetry. The differently positioned Vi – vacancies in the first shell cause
Fig. 7. Crystallographic arrangement of constituent atoms of LN: Er crystal projected on the (0 0 0 1) plane (perpendicular to the c-axis); 1, 2, 3—Nd5+ sites in the first shell below the Er ion, 1 , 2 , 3 —Li+ site in the first shell above the Er ion; A, B, C, D, E, F—Li+ in the second shell in the plane contain Er.
differently tilted Er3+ g – tensor z-axis and consequently lower local site symmetry of the Er3+ ion. Each of these paramagnetic centres ErLi+ –Vi (ErLi+ connected with one of the vacancies Vi , i = 1, 2, 3, 1 , 2 , 3 ) give six non-equivalent paramagnetic centres that characterize low C1 point group symmetry. These centres should dominate in the EPR spectrum [13]. The paramagnetic centres ErLi+ –Vi , Vj (ErLi+ connected with two of vacancies present in the first shell i, j = 1, 2, 3, 1 , 2 , 3 ) characterize much less intensity. Each of these paramagnetic centres has the same surrounding, so the temperature dependence of the intensity of EPR line originating from these centres should be approximately the same (see Fig. 7). It allows us conclude that the change of the intensity of EPR lines versus temperature could not be determined by the existence of several non-equivalent crystallographic sites of single Er3+ ions in LN: Er, or LN: Er, Tm. A model of Er3+ –Er3+ ion pairs could also be visible in Fig. 7. One Er3+ ion is incorporated into Li+ site, the second Er3+ ion is incorporated into Li+ site or Nb5+ site (1, 2, 3, 1 , 2 , 3 positions) in the first shell. Each of these Er3+ –Er3+ ion pairs should present C1 symmetry. 4. Discussion and conclusions The results of the optical and EPR measurements of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal have been reported. The temperature dependence of the intensity, inverse intensity and linewidth revealed complexity of the EPR spectra. We observed some unusual behaviour in temperature dependence of the EPR spectra. Three different regions in the temperature dependence of the main EPR line of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal, when a magnetic field is parallel to a crystal (optical c) z-axis (see Fig. 4) were observed. However, it can be understand if we assume that the main, EPR line is a superposition of several (at least three) paramagnetic lines and the different paramagnetic centres corresponding to the lines dominated in these three regions. The EPR signal originating from even Er3+ ions dominated below 36 K. But above 36 K the EPR signal is dominated by resonance signals originating from some others paramagnetic ions yet unidentified. We suppose that they may enter into the LN structure as uncontrolled impurities from the chemical ingredients during the crystal growth. These paramagnetic centres produce a very weak and very broad resonance signal, which vanishes above 46 K. Similar phenomena were observed in case of LN: Yb3+ and LN: Er3+ single crystals [17,18]. The change of the slope of the intensity indicates
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that the EPR signal originating from even Er3+ ions can dominate only in one of the regions; in the other one, the EPR signal is dominated by signals arising from different kinds of even Er3+ species, possibly even Er3+ –even Er3+ ion pairs. Such possibility was confirmed by optical spectroscopy and Raman measurements presented in Sections 3.1 and 3.2 of the paper. The two peaks pattern, which we observed in the temperature dependence of the total intensity of the EPR spectrum measured at an angle of ϕ = 150◦ in the plane XY perpendicular to crystal (optical c) z-axis, can be explained by assumption that in this temperature range the EPR spectrum is dominated by two different paramagnetic centres originating from even Er3+ –even Er3+ ion pairs (see Fig. 6). The inverse intensity versus temperature fulfils the Curie–Weiss law in both crystallographic planes and enables us to determine the Curie–Weiss constant = 6.1 ± 1.1 K. The positive sign of the constant suggests that the ferromagnetic interactions arise in the system of even Er3+ –even Er3+ ion pairs in LN: Er, Tm single crystal. The assumption on the presence of even Er3+ –even Er3+ ion pairs in LN: Er, Tm single crystal can explain a decrease of the total intensity observed below 4.8 K, too (see Fig. 6). On the basis of the structural consideration we can conclude that observed temperature dependence of the intensity and inverse intensity of the EPR line could not be related to the existence of several nonequivalent crystallographic sites of single Er3+ ions in LN: Er single crystal. The EPR spectrum of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal seem to be a superposition of several lines originating from different kinds of paramagnetic centres: originating from isolated even Er3+ ions, the second one even Er3+ –even Er3+ pair of ions and third one—arising from unidentified paramagnetic ions. It seems that the most important role in the unusual behaviour of the EPR spectra plays a codopant—thulium and unidentified impurities. The presence of additional impurities may act in a similar way as the co-doping by MgO, i.e. forcing a certain proportion of the Er3+ ions to take the Nb5+ positions [11]. Those ErNb ions locating nearby the ErLi sites create the even Er3+ –even Er3+ ion pairs, which are apparently observed in our optical spectra and in temperature dependence of the intensity and linewidth of the main EPR line. The proposed model quite well explains formation of such ErLi –ErNb pairs. The similar ferromagnetic interactions were identi-
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fied in the system of even Er3+ –even Er3+ ion pairs in LN singly doped with Er [18] but in case of our sample this phenomenon is much better recognizable. Previously, the LN: Er crystal was only weakly doped with Er (0.1 wt.%). Now, we are investigating the LN single crystal doped with Er (0.2 wt.%) and codoped with Tm (0.3 wt.%). We have got approximately two times much larger number of Er ions and additional Tm codopant ions. The results of our research indicate that the presence of codopant ions play the most important role in unusual behaviour of the EPR spectra of LN: Er (0.2 wt.%), Tm (0.3 wt.%) single crystal, e.g. facilitates introducing of the Er ions and forces creating of the even Er3+ –even Er3+ ion pairs in the LN host crystal. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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