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Growth and optical properties of Yb3+ and Tb3+ codoped BaB2O4 crystals
Optics Communications 285 (2012) 5205–5209
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Growth and optical properties of Yb3 þ and Tb3 þ codoped BaB2O4 crystals V.P. Solntsev a,n, A.P. Yelisseyev a, T.B. Bekker a, A.E. Kokh a, S. Yu. Stonoga a, A.V. Davydov a, A. Maillard b a b
Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, 630090 Novosibirsk, Russia Lorrain University, Metz, France
a r t i c l e i n f o
abstract
Article history: Received 13 June 2012 Received in revised form 13 July 2012 Accepted 13 July 2012 Available online 28 August 2012
Optical absorption and luminescence spectra of ytterbium and terbium codoped BaB2O4 (b-BBO and aBBO) crystals grown in different conditions have been studied. Low-temperature absorption peaks were observed in all samples. Features related to rare earth ions were observed in absorption and luminescence spectra. Absorption and emission in the range 860–1000 nm are caused by 2F5/222F7/2 transitions in Yb3 þ ions. Emission peaks at 500, 550, 590 and 630 nm correspond to 5D4-7F6, 7F5, 7F4, and 7F3 transitions of Tb3 þ ions, respectively. The probable reasons of variations in spectroscopic features related to Yb in BBO host are discussed. It has been shown that the replacement of Ba2 þ by Yb3 þ in the lattice of BaB2O4 results in the decrease in the symmetry of oxygen surrounding of Yb3 þ . & 2012 Elsevier B.V. All rights reserved.
Keywords: Yb3 þ , Tb3 þ :BaB2O4 crystal Crystal growth Optical properties Optical absorption Luminescence
1. Introduction BaB2O4 (b-BBO) crystals of low-temperature modification are widely used in nonlinear-optical devices. These efficient crystals have large nonlinear optical coefficients, high laser damage thresholds, wide optical transparency range, high birefringence, good chemical stability and appropriate hardness. b-BBO crystals have a number of unique properties and are the only nonlinear crystals that enable the transformation of Nd:YAG laser emission into the fifth harmonic (5HG) at 213 nm [1–4]. Powerful infra-red lasers are used to generate coherent UV radiation in powerful solid-state laser systems and the requirements on optical quality of nonlinear crystals are very high. Doping of BaB2O4 matrix by rare-earth cations is a widespread approach to achieve new optical properties and to improve physical properties of undoped matrix. The introduction of 15.5 mol% Nd2O3 into BBO melt leads to the crystallization of b-BaB2O4 at 995 1C [5]. This result shows that it is possible to grow Nd3 þ :b-BBO crystals without any solvent. However, no data on the features of the crystals and their nonlinear properties are reported. In [6] introduction of 2 mol% Yb2O3 and 0.5 mol% SrO relative to BaO in the BBO melt is shown to lead to crystallization of a-BaB2O4. It is known that the most common state of rare-earth (RE) ions is trivalent though some RE ions tend to exist in an unusual valence state such as, for example, Sm2 þ , Eu2 þ , Yb2 þ , Ce4 þ , Pr4 þ ,
n
Corresponding author. Tel.: þ73833066408, þ 73833330523. E-mail address: [email protected] (V.P. Solntsev).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.102
and Tb4 þ , because all of them have more stable electronic configurations. So in [7] it is shown that two rare-earth ions with the number of 4e electrons equal to seven or fourteen have conjugate electronic configurations and these two conjugate electronic configurations of RE ions tend to transfer an electron to each other to produce a more stable electronic configuration. The introduction of Tb3 þ and Yb3 þ codopants to the BaB2O4 matrix provides the conditions for RE to exist in a more favorable charge state Yb2 þ as follows: Tb3 þ (4e8)þYb3 þ (4e13)Tb4 þ (4e7) þYb2 þ (4e14). Also, it has been shown that the relative intensity of Eu2 þ luminescence is increased when Eu3 þ and Tb3 þ are incorporated in BaB4O7 [7]. By analogy, in this work we expect that simultaneous introduction of Yb3 þ and Tb3 þ will lead to the change of a valency condition of Yb3 þ , that is, to charge transfer between Yb3 þ and Tb3 þ . Charge transfer luminescence of Yb3 þ became of interest due to possible applications of Yb-containing materials for scintillation detectors in neutrino physics. The main technique of growing b-BBO crystals is crystallization from high temperature solution. The most common solvents for growing these crystals nowadays are Na2O and compounds in the BaO–B2O3–Na2O ternary system [8], a common problem of which is the high viscosity of the solution. Growth of b-BBO crystals in fluoride systems makes it possible to decrease the viscosity of the melts. It is reported that the viscosity of the BaB2O4–NaF system is by 15% lower than that of BaB2O4–Na2O in the corresponding temperature range [9]. Investigations showed that the BaB2O4–NaF section is not quasi-binary and includes the primary crystallization field of a new compound Ba2Na3[B3O6]2F with symmetry P63/m and lattice parameters a ¼7.346(1), ˚ This is a result of chemical interaction between c¼12.636(2) A.
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BaB2O4 and NaF, another product of this interaction being BaF2. The compound belongs to the ternary reciprocal system Na, Ba// BO2, F [10,11]. The BaB2O4–Ba2Na3[B3O6]2F section is quasi-binary with the eutectic coordinates 810751C, 85 mol% Ba2Na3[B3O6]2F, 15 mol% BaB2O4. This system was reported to be appropriate for growing bulk b-BaB2O4 crystals [12]. The aim of our work is to study the influence of growth conditions of BBO crystals doped with Tb and Yb RE ions on the RE charge state, using optical absorption and luminescence spectroscopy.
2. Crystal growth The compositions used for growing barium borate crystals BaB2O4 are given in Table 1. All growth experiments were carried out in air. a-BBO crystals were grown from a stoichiometric melt containing 6 wt% Yb2O3 and 0.8 wt% Tb2O3 (sample A). b-BaB2O4 crystals were grown in the system BaB2O4–NaF from the composition 65 mol% BaB2O4, 35 mol% NaF, by adding 6 wt% Yb2O3 and 0.8 wt% Tb2O3 into the melt (B) and in the system BaB2O4–Ba2Na3[B3O6]2F from the composition 30 mol% BaB2O4, 70 mol% Ba2Na3 [B3O6]2F, by adding 2 wt% Yb2O3, 0.8 wt% Tb2O3 into the melt (C). Growth experiments were carried out in a precisely controlled furnace with a high symmetry and stability of thermal field. High temperature solution in the amount of 40 g was prepared in a platinum crucible with a diameter of 40 mm through the intermediate stage of solid-phase synthesis. BaCO3, H3BO3, Na2CO3, NaF, Tb2O3, and Yb2O3 of high purity grade were used as the starting materials. The crystals were grown on a single crystal seed of 5 5 mm2 cross section oriented along the optical axis. After determining the equilibrium temperature the seed was allowed to grow at the rate of temperature decrease of 1 1C/day.
Fig. 1. Raman spectra of the BBO samples. A: a-BaB2O4, B, C: b-BaB2O4. A–C correspond to the compositions shown in Table 1.
3. Experimental details and results Fig. 2. Absorption spectra of a- (A) and b-BBO (B, C) crystals at T ¼80 K. A–C correspond to the compositions shown in Table 1.
3.1. Raman spectra The Raman scattering spectra of a- and b-BBO cryatals were recorded using a Horiba Dilor Jobin Yvon spectrometer. Spectral resolution was 2 cm 1 (FWHM). No polarization selection was applied in the Raman experiment. The Raman spectrum A of a-BBO is typical of R3c space group (Fig. 1). The Raman spectra B, C (Fig. 1) of b-BBO crystals grown in NaF and Ba2Na3[B3O6]2F fluxes, respectively, suggest that the crystals belong to the space group R3c. The spectra can be divided into two well-resolved parts. Low-frequency lines (0–300 cm 1) are related to outer external modes, whereas the high-frequency peaks (300–1000 cm 1) are due to the internal vibrational modes of the metaborate rings [13,14].The samples were also tested using X-ray diffraction (DRON-3 diffractometer, radiation CuKa). 3.2. Absorption spectra The samples with dimensions of 4 4 1.5 mm3 were cut from grown crystals and polished for spectral measurements. The Table 1 Compositions used for the growth of BaB2O4 crystals. Solution composition (mol%)
A a-BaB2O4 B b-BaB2O4 C b-BaB2O4
Dopants (wt%)
BaO
Na2O
B2O3
NaF
Yb2O3
Tb2O3
50.0 39.4 30
– – 13
50.0 39.4 44
21.2 13
6 6 2
0.8 0.8 0.8
absorption spectra were recorded in unpolarized light at 80 K and 300 K using a SF-20 (LOMO) spectrophotometer. The absorption spectra in the UV spectral range at 80 K are presented in Fig. 2. When the temperature decreased to 80 K, aand b-BBO crystals showed three additional absorption bands at 190–215, 220–250 and 250–295 nm, which were not observed at room temperature. Weak dependences of position of these bands on crystal phase and type of the flux used for growth suggest that this absorption is connected, mainly, with native defects [15]. These defects are supposed to be ionized at room temperature and therefore, are invisible in absorption. At low temperature they capture electrons from the conduction band and become neutral donors. The latter are able to bind excited electron–hole pairs to form bound excitons at UV-excitation of electrons of the valence band [16]. The levels of the bound exciton are located close to the fundamental absorption edge. Because the intrinsic absorption in BBO crystal is at the indirect edge, the bound excitons in the crystal may be excited by photons with the participation of phonons [16]. Zhang et al. [17] suggest that these absorption bands are due to the transitions in the indirect excitons bound to neutral impurities which are present in the BBO samples. It is worth noting that in Pb:BBO crystals the absorption band at 240 nm is related to Pb2 þ [18–20]. This agrees well with the absorption edge of PbB4O7 crystal (l ¼235 nm) [21]. In addition to the above-mentioned absorption bands typical of undoped BBO crystals, a new weak band at 335–340 nm (Fig. 2) was observed in the doped crystals at 80 K. This band may be due
V.P. Solntsev et al. / Optics Communications 285 (2012) 5205–5209
Fig. 3. Fragment of absorption spectra of a-BBO (1) (crystal was grown from composition A, Table 1) and b-BBO (2) (crystal was grown from composition B, Table 1), doped with Yb and Tb, T¼ 80 K. Spectrum (1) is shifted upward for clarity.
to structural defects or f–d transitions in Yb2 þ located at the Ba2 þ site. In BBO crystals doped with Yb3 þ ions the dominating features are absorption bands in the 860–1000 nm spectral range which are associated with 2F7/2-2F5/2 in the Yb3 þ ion [22,23]. In Fig. 3 one can see six bands at 904, 920, 936, 942, 957 and 968 nm for a-BBO crystals and three bands at 901, 958 and 964 nm for b-BBO. Yu et al. [6] report a complex two-peak band in the absorption and luminescence spectra of Yb:a-BBO crystals at 300 K with maximums at 940 and 979 nm. The peak at about 940 nm was attributed to the transition 2F7/2-2F5/2 of Yb3 þ . And the peak at about 979 nm should be involved with Yb and/or native point defects [6]. We do not think that this interpretation of absorption bands is correct because the ionic radius of Yb3 þ (0.86 10 10 m) is considerably smaller that the ionic radius of Ba2 þ (1.34 10 10 m) and, hence, Yb3 þ can easily occupy both the position of Ba1 (coordination number 9) and the position of Ba2 (coordination number 6).
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Fig. 4. PL spectra for crystals a-BBO (1) (crystal was grown from composition A, Table 1) and b-BBO (2) (crystal was grown from composition B, Table 1), doped with Yb and Tb at excitations 250 nm and 300 K. Arrow shows the excitation wavelengths.
Fig. 5. PL spectra at 365 nm excitation for crystals a-BBO (1) (crystal was grown from composition A, Table 1) and b-BBO (2) (crystal was grown from composition C, Table 1), doped with Yb and Tb. T¼ 80 K. Fine structure is associated with Re3 þ . Spectral resolution is 1.5 nm. Spectrum (1) is shifted upward for clarity.
3.3. Luminescence spectra The photoluminescence (PL) spectra of BBO crystals codoped with Tb and Yb were recorded at 80 and 300 K with a luminescence spectrometer SDL1 and a 1 kW Xe lamp used as the excitation light source. Necessary excitation wavelength was separated using a diffraction MDR2 monochromator combined with an appropriate color glass filter. A short wave excitation near 250 nm produces PL emission in a broad band centered at 360 nm (Fig. 4). This emission is observed for both undoped and RE3 þ –doped BBO samples and is likely associated with native defects in BBO. These defects may be oxygen vacancies V0 [20,24–26], which generate deep levels of electron type in the forbidden zone and the vacancies of barium (VBa) forming holetype centers. The peaks of thermostimulated luminescence (TSL) at 123, 200 and 240 1C in the samples with sodium content of 0.02 wt% and more were observed in [27], whereas other TSL peaks in the 80–90 1C range of TL were present when the sodium content was less than 0.002 wt% [28,29]. The authors found that TSL emission takes place in the wide 400 nm band in the first case and in the 360 nm band in the other one. This implies that bands at 190–215 nm, 220–250 nm, and 250–295 nm in the luminescence excitation spectra connected with PL emission in the 300– 450 nm range are due to different luminescence centers in BBO [15,18–20,30–32]. Indeed we see that the shape of a wide PL band
in the 270–400 nm range is rather complicated (Fig. 4) and several individual components could be separated. The emission of BBO crystals codoped with Tb and Yb have a greenish tint and a set of narrow lines associated with Tb3 þ luminescence are observed in the 400–700 nm range (Fig. 5). The main group near 550 nm is due to 5D4-7F5 transitions whereas weaker groups at 500, 590 and 630 nm correspond to 5D4-7F6, 7 F4, 7F3 transitions, respectively (Fig. 5). A group of lines in the 960–1100 nm range are associated with the 2F5/2-2F7/2 transitions in the Yb3 þ ion (Fig. 6). Such transitions are typical of the Yb3 þ ion in low-symmetric eightfold oxygen surrounding [18,33]. For the Yb3 þ ion in low symmetric (point symmetry–C1) eightfold oxygen surrounding the ground 2F7/2 and excited 2F5/2 multiplets are split into four levels and three levels, respectively. In this case the spectra are expected to consist of three and four individual components in absorption and emission, respectively. The analysis of PL results for BBO samples with Yb3 þ þTb3 þ and with Yb3 þ shows that eight peaks of emission (at 976, 983, 992, 998, 1006, 1014, 1026 and 1042 nm) in a–-BBO crystals and four peaks of emission (at 969, 978, 991 and 1009 nm) in b–BBO are observed on the background of a wide band at 80 K (Fig. 6). As follows from Fig. 3 six and three bands related to Yb3 þ in a-BBO and b-BBO crystals are observed at absorption respectively.
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To confirm our assumption, we would like to refer to the work of Yoo et al. [34], where the authors observed four absorption bands at 237, 272, 317, and 379 nm for Yb2 þ in the PL spectrum with l ¼400 nm excitation for the Ba5(PO4)3Cl:Yb crystal. The peak at 379 nm almost coincides with those observed in our research. The remaining peaks could not be distinguished on the background of the wide band associated with the native defects due to the weakness of the spectrum of Yb2 þ (Figs. 2 and 7). Thus one can conclude that electronic excitation is transfered effectively from the BBO matrix to Re3 þ ions.
4. Discussion It is well known that sodium ions are the main impurity in Fig. 6. PL spectra at 365 nm excitation for crystals a- (1) (A, Table 1) and b-BBO (2) (B, Table 1), doped with Tb and Yb. T¼ 80 K. Fine structure is associated with Yb3 þ . Spectral resolution is 1.5 nm. Spectrum (1) is shifted upwards for clarity.
Fig. 7. X-ray excited luminescence spectrum of Tb, Yb:a-BBO sample at 300 K.
These data evidence that Yb3 þ ions replace Ba1 and Ba2 positions in a-–BBO structure and thus concentrations of these two centers are approximately equal. For the Yb3 þ ion in Ba1 positions (point symmetry – C3v) and Yb3 þ in Ba2 position (D3) the ground 2 F7/2 and excited 2F5/2 multiplets are split into three levels and two levels, respectively. Therefore, for both types of positions the absorption and emission spectra associated with transitions 2 F5/222F7/2 in Yb3 þ ion consist of two and three individual components, respectively. Experimentally observable number of bands of the absorption, equal to six, specifies that Yb3 þ replacement positions of Ba1 and Ba2 lead to decrease of symmetry in an nearest oxygen surrounding of ions of Yb31 þ and Yb32 þ . In a-BBO it was difficult to separate the bands associated with f-d transitions for Tb3 þ at the Ba1 and Ba2 sites because of the relatively large width of absorption/luminescence bands. We suppose that Tb3 þ ions occupy both Ba1 and Ba2 sites with comparable probability as in the case of Yb3 þ . Spectra of the X-ray excited luminescence (RL) were recorded using a X-ray source URS55 table with a W-anticathode tube. The operating parameters were 40 kV voltage and 20 mA current. Spectra were recorded with a MDR2 monochromator and two photomultipliers (FEU100 and cooled FEU83) which together cover a range from UV (200 nm) to 1200 nm in a near IR. The RL spectrum for Tb, Yb:BBO is given in Fig. 7. One can see here the same broad band at 340 nm (see also Fig. 4) and two sets of lines in the 400–700 nm and 900–1100 nm ranges related to Tb3 þ and Yb3 þ , respectively. The line at 378 nm on the background of the broad 340 nm band may be associated with the Yb2 þ emission (Fig. 7).
b-BBO crystals grown by a flux technique. Na þ ions can occupy substitutional positions replacing Ba2 þ (Na þ -Ba2 þ ) or be located inside the channels with charge compensation by Ba and O vacancies [22,26–28]. In BBO crystals doped with RE3 þ ions (Yb3 þ and Tb3 þ ) charge compensation is carried out as formation of vacancies of barium (2Re3 þ -2Ba2 þ þVBa), and Na þ ions replacing Ba2 þ (Re3 þ -Ba2 þ , Na þ -Ba2 þ ). The presence of such defects leads to a change in the shape of PL curve in the region 300–450 nm with a maximum near 360 nm at UV excitation. The absorption bands appearing on cooling could be explained by the formation of excitons under UV illumination which influences neutral donors. We assume that these absorption bands may be attributed to the transitions in the indirect excitons localized on some neutral impurity present in examined samples. On the other hand, in this region the luminescence associated with 4f135d-4f14 transitions of Yb2 þ ion was expected. Similar luminescence with lmax ¼362 nm was observed in SrB4O7 [32,33]: there Yb2 þ replaces Sr2 þ in ninefold oxygen surrounding. Therefore, the sharp peak at 378 nm observed in the RL spectrum can be obviously attributed to the 4f135d-4f14 transitions of Yb2 þ ion. In SrB4O7 such transitions in Yb2 þ ion result in the excitation (absorption) band with a maximum at 327 nm and emission band at 362 nm [32,33]. The rhombohedral cell of b-BBO contains six molecules of BaB2O4 corresponding to four (B3O6)3 anion groups of C3 position symmetry and six Ba2 þ cations of C1 position symmetry. The lowest electron configuration of Yb2 þ ion is 4f14, and therefore, probable optical transitions were the transitions between 4f14 and 4f13 5d configurations. Piper et al. [35] showed that in the cubic coordination the lowest excited levels of 4f13 5d configuration were T2u þEu. Transitions from these levels to the ground state are forbidden. In b-BaB2O4 Yb2 þ occurs in an irregular coordination (C1 symmetry of position), as eight oxygen atoms form a distorted cube. This results in the splitting of the T2u þ Eu level into the levels with A symmetry. Therefore all electron-vibrational A2A transitions are allowed, ruling out the states with different multiplicities. The structure of a-BaB2O4 contains two crystallographically nonequivalent barium atoms located at the sites of 32(D3) and 3(C3v) point symmetries. Around the barium (Ba2) in D3 point symmetry position the oxygens are arranged in a trigonal prism. Around the barium (Ba1) which lies at the C3v point symmetry position the oxygen coordination is ninefold. This is expected to split the T2u þEu levels into levels of symmetry A2 þ 2E or A1 þ2E, respectively. We suggest that the lowest excited level has symmetry A1 or A2. The component with E symmetry is assumed to correspond to the excitation band at about 335–340 nm. Piper et al. [35] indicate a higher level (about 2000 cm 1 above the T2u þ Eu levels) with symmetry T1u. Under D3 (C3v) symmetry this level splits into A2 þ E or A1 þE. Transition between these levels and the ground state are formally allowed. Therefore,
V.P. Solntsev et al. / Optics Communications 285 (2012) 5205–5209
we assign the weak absorption band at about 335–340 nm to transition to these levels. On the other hand, it is difficult to explain the large width and intensity of excitation bands near 350 nm only by the transitions between 4f14 and 4f135d configurations of Yb2 þ ions. BBO crystals are to wide band crystals: the width of forbidden band is Eg ¼6.2 eV at 300 K. Transparency range estimated at 0.5 transmission level is 198–2600 nm for 8 mm thick crystal. The bluish-white PL is typical of both undoped BBO crystals and of samples doped with RE3 þ . Thus it likely refers to the luminescence of native defects, probably anionic vacancies and different type complexes. The sharp peak near 375 nm in RL spectrum in a crystal a-BBO can be explained by the electron transfer reaction mechanism: Yb3 þ and Tb3 þ are a pair of conjugate electronic configuration rare earth ions. If they are codoped in the BaB2O4 matrix, an electron transfer may take place between Yb3 þ and Tb3 þ to provide a more stable electronic configuration of 4f7 and 4f14, corresponding to Tb4 þ and Yb2 þ ions, respectively. Tb4 þ ions have been proven to exist in a fluoride system [36].
5. Conclusions We showed that rare-earth ions RE3 þ enter the structure of BaB2O4, replacing Ba2 þ ions. Thus compensation of a superfluous charge can be carried out both by Na þ ions in the closest positions of Ba2 þ ions and by structural defects or by uncontrollable impurities which are responsible for additional absorption bands in the UV region. The nature of these bands is a subject of special research. We found that the incorporation of the Tb3 þ ion does not influence the wavelength position of Yb3 þ , Yb2 þ ions in the BaB2O4:Yb, Tb phosphor. It has been established that the replacement of Ba by Yb3 þ at Ba1 positions of C3v symmetry and Ba2 positions of D3 symmetry leads to decrease in the symmetry of the oxygen surrounding of Yb3 þ ions. References [1] D. Eimerl, L. Davis, S. Velsko, E.K. Graham, A. Zalkin, Journal of Applied Physics 62 (1987) 1968. [2] D. Pang, R. Zhang, J. Sun, Q. Wang, Optics and Laser Technology 33 (2001) 249. [3] D.N. Nikogosyan, Nonlinear Optical Crystals: A Complete Survey, Springer, NY, 2005, pp. 419. [4] A.D. Mighel, A. Perloff, S. Block, Acta Crystallographica 20 (1966) 819.
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Growth and optical properties of Yb3 þ and Tb3 þ codoped BaB2O4 crystals V.P. Solntsev a,n, A.P. Yelisseyev a, T.B. Bekker a, A.E. Kokh a, S. Yu. Stonoga a, A.V. Davydov a, A. Maillard b a b
Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences, 630090 Novosibirsk, Russia Lorrain University, Metz, France
a r t i c l e i n f o
abstract
Article history: Received 13 June 2012 Received in revised form 13 July 2012 Accepted 13 July 2012 Available online 28 August 2012
Optical absorption and luminescence spectra of ytterbium and terbium codoped BaB2O4 (b-BBO and aBBO) crystals grown in different conditions have been studied. Low-temperature absorption peaks were observed in all samples. Features related to rare earth ions were observed in absorption and luminescence spectra. Absorption and emission in the range 860–1000 nm are caused by 2F5/222F7/2 transitions in Yb3 þ ions. Emission peaks at 500, 550, 590 and 630 nm correspond to 5D4-7F6, 7F5, 7F4, and 7F3 transitions of Tb3 þ ions, respectively. The probable reasons of variations in spectroscopic features related to Yb in BBO host are discussed. It has been shown that the replacement of Ba2 þ by Yb3 þ in the lattice of BaB2O4 results in the decrease in the symmetry of oxygen surrounding of Yb3 þ . & 2012 Elsevier B.V. All rights reserved.
Keywords: Yb3 þ , Tb3 þ :BaB2O4 crystal Crystal growth Optical properties Optical absorption Luminescence
1. Introduction BaB2O4 (b-BBO) crystals of low-temperature modification are widely used in nonlinear-optical devices. These efficient crystals have large nonlinear optical coefficients, high laser damage thresholds, wide optical transparency range, high birefringence, good chemical stability and appropriate hardness. b-BBO crystals have a number of unique properties and are the only nonlinear crystals that enable the transformation of Nd:YAG laser emission into the fifth harmonic (5HG) at 213 nm [1–4]. Powerful infra-red lasers are used to generate coherent UV radiation in powerful solid-state laser systems and the requirements on optical quality of nonlinear crystals are very high. Doping of BaB2O4 matrix by rare-earth cations is a widespread approach to achieve new optical properties and to improve physical properties of undoped matrix. The introduction of 15.5 mol% Nd2O3 into BBO melt leads to the crystallization of b-BaB2O4 at 995 1C [5]. This result shows that it is possible to grow Nd3 þ :b-BBO crystals without any solvent. However, no data on the features of the crystals and their nonlinear properties are reported. In [6] introduction of 2 mol% Yb2O3 and 0.5 mol% SrO relative to BaO in the BBO melt is shown to lead to crystallization of a-BaB2O4. It is known that the most common state of rare-earth (RE) ions is trivalent though some RE ions tend to exist in an unusual valence state such as, for example, Sm2 þ , Eu2 þ , Yb2 þ , Ce4 þ , Pr4 þ ,
n
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0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.102
and Tb4 þ , because all of them have more stable electronic configurations. So in [7] it is shown that two rare-earth ions with the number of 4e electrons equal to seven or fourteen have conjugate electronic configurations and these two conjugate electronic configurations of RE ions tend to transfer an electron to each other to produce a more stable electronic configuration. The introduction of Tb3 þ and Yb3 þ codopants to the BaB2O4 matrix provides the conditions for RE to exist in a more favorable charge state Yb2 þ as follows: Tb3 þ (4e8)þYb3 þ (4e13)Tb4 þ (4e7) þYb2 þ (4e14). Also, it has been shown that the relative intensity of Eu2 þ luminescence is increased when Eu3 þ and Tb3 þ are incorporated in BaB4O7 [7]. By analogy, in this work we expect that simultaneous introduction of Yb3 þ and Tb3 þ will lead to the change of a valency condition of Yb3 þ , that is, to charge transfer between Yb3 þ and Tb3 þ . Charge transfer luminescence of Yb3 þ became of interest due to possible applications of Yb-containing materials for scintillation detectors in neutrino physics. The main technique of growing b-BBO crystals is crystallization from high temperature solution. The most common solvents for growing these crystals nowadays are Na2O and compounds in the BaO–B2O3–Na2O ternary system [8], a common problem of which is the high viscosity of the solution. Growth of b-BBO crystals in fluoride systems makes it possible to decrease the viscosity of the melts. It is reported that the viscosity of the BaB2O4–NaF system is by 15% lower than that of BaB2O4–Na2O in the corresponding temperature range [9]. Investigations showed that the BaB2O4–NaF section is not quasi-binary and includes the primary crystallization field of a new compound Ba2Na3[B3O6]2F with symmetry P63/m and lattice parameters a ¼7.346(1), ˚ This is a result of chemical interaction between c¼12.636(2) A.
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BaB2O4 and NaF, another product of this interaction being BaF2. The compound belongs to the ternary reciprocal system Na, Ba// BO2, F [10,11]. The BaB2O4–Ba2Na3[B3O6]2F section is quasi-binary with the eutectic coordinates 810751C, 85 mol% Ba2Na3[B3O6]2F, 15 mol% BaB2O4. This system was reported to be appropriate for growing bulk b-BaB2O4 crystals [12]. The aim of our work is to study the influence of growth conditions of BBO crystals doped with Tb and Yb RE ions on the RE charge state, using optical absorption and luminescence spectroscopy.
2. Crystal growth The compositions used for growing barium borate crystals BaB2O4 are given in Table 1. All growth experiments were carried out in air. a-BBO crystals were grown from a stoichiometric melt containing 6 wt% Yb2O3 and 0.8 wt% Tb2O3 (sample A). b-BaB2O4 crystals were grown in the system BaB2O4–NaF from the composition 65 mol% BaB2O4, 35 mol% NaF, by adding 6 wt% Yb2O3 and 0.8 wt% Tb2O3 into the melt (B) and in the system BaB2O4–Ba2Na3[B3O6]2F from the composition 30 mol% BaB2O4, 70 mol% Ba2Na3 [B3O6]2F, by adding 2 wt% Yb2O3, 0.8 wt% Tb2O3 into the melt (C). Growth experiments were carried out in a precisely controlled furnace with a high symmetry and stability of thermal field. High temperature solution in the amount of 40 g was prepared in a platinum crucible with a diameter of 40 mm through the intermediate stage of solid-phase synthesis. BaCO3, H3BO3, Na2CO3, NaF, Tb2O3, and Yb2O3 of high purity grade were used as the starting materials. The crystals were grown on a single crystal seed of 5 5 mm2 cross section oriented along the optical axis. After determining the equilibrium temperature the seed was allowed to grow at the rate of temperature decrease of 1 1C/day.
Fig. 1. Raman spectra of the BBO samples. A: a-BaB2O4, B, C: b-BaB2O4. A–C correspond to the compositions shown in Table 1.
3. Experimental details and results Fig. 2. Absorption spectra of a- (A) and b-BBO (B, C) crystals at T ¼80 K. A–C correspond to the compositions shown in Table 1.
3.1. Raman spectra The Raman scattering spectra of a- and b-BBO cryatals were recorded using a Horiba Dilor Jobin Yvon spectrometer. Spectral resolution was 2 cm 1 (FWHM). No polarization selection was applied in the Raman experiment. The Raman spectrum A of a-BBO is typical of R3c space group (Fig. 1). The Raman spectra B, C (Fig. 1) of b-BBO crystals grown in NaF and Ba2Na3[B3O6]2F fluxes, respectively, suggest that the crystals belong to the space group R3c. The spectra can be divided into two well-resolved parts. Low-frequency lines (0–300 cm 1) are related to outer external modes, whereas the high-frequency peaks (300–1000 cm 1) are due to the internal vibrational modes of the metaborate rings [13,14].The samples were also tested using X-ray diffraction (DRON-3 diffractometer, radiation CuKa). 3.2. Absorption spectra The samples with dimensions of 4 4 1.5 mm3 were cut from grown crystals and polished for spectral measurements. The Table 1 Compositions used for the growth of BaB2O4 crystals. Solution composition (mol%)
A a-BaB2O4 B b-BaB2O4 C b-BaB2O4
Dopants (wt%)
BaO
Na2O
B2O3
NaF
Yb2O3
Tb2O3
50.0 39.4 30
– – 13
50.0 39.4 44
21.2 13
6 6 2
0.8 0.8 0.8
absorption spectra were recorded in unpolarized light at 80 K and 300 K using a SF-20 (LOMO) spectrophotometer. The absorption spectra in the UV spectral range at 80 K are presented in Fig. 2. When the temperature decreased to 80 K, aand b-BBO crystals showed three additional absorption bands at 190–215, 220–250 and 250–295 nm, which were not observed at room temperature. Weak dependences of position of these bands on crystal phase and type of the flux used for growth suggest that this absorption is connected, mainly, with native defects [15]. These defects are supposed to be ionized at room temperature and therefore, are invisible in absorption. At low temperature they capture electrons from the conduction band and become neutral donors. The latter are able to bind excited electron–hole pairs to form bound excitons at UV-excitation of electrons of the valence band [16]. The levels of the bound exciton are located close to the fundamental absorption edge. Because the intrinsic absorption in BBO crystal is at the indirect edge, the bound excitons in the crystal may be excited by photons with the participation of phonons [16]. Zhang et al. [17] suggest that these absorption bands are due to the transitions in the indirect excitons bound to neutral impurities which are present in the BBO samples. It is worth noting that in Pb:BBO crystals the absorption band at 240 nm is related to Pb2 þ [18–20]. This agrees well with the absorption edge of PbB4O7 crystal (l ¼235 nm) [21]. In addition to the above-mentioned absorption bands typical of undoped BBO crystals, a new weak band at 335–340 nm (Fig. 2) was observed in the doped crystals at 80 K. This band may be due
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Fig. 3. Fragment of absorption spectra of a-BBO (1) (crystal was grown from composition A, Table 1) and b-BBO (2) (crystal was grown from composition B, Table 1), doped with Yb and Tb, T¼ 80 K. Spectrum (1) is shifted upward for clarity.
to structural defects or f–d transitions in Yb2 þ located at the Ba2 þ site. In BBO crystals doped with Yb3 þ ions the dominating features are absorption bands in the 860–1000 nm spectral range which are associated with 2F7/2-2F5/2 in the Yb3 þ ion [22,23]. In Fig. 3 one can see six bands at 904, 920, 936, 942, 957 and 968 nm for a-BBO crystals and three bands at 901, 958 and 964 nm for b-BBO. Yu et al. [6] report a complex two-peak band in the absorption and luminescence spectra of Yb:a-BBO crystals at 300 K with maximums at 940 and 979 nm. The peak at about 940 nm was attributed to the transition 2F7/2-2F5/2 of Yb3 þ . And the peak at about 979 nm should be involved with Yb and/or native point defects [6]. We do not think that this interpretation of absorption bands is correct because the ionic radius of Yb3 þ (0.86 10 10 m) is considerably smaller that the ionic radius of Ba2 þ (1.34 10 10 m) and, hence, Yb3 þ can easily occupy both the position of Ba1 (coordination number 9) and the position of Ba2 (coordination number 6).
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Fig. 4. PL spectra for crystals a-BBO (1) (crystal was grown from composition A, Table 1) and b-BBO (2) (crystal was grown from composition B, Table 1), doped with Yb and Tb at excitations 250 nm and 300 K. Arrow shows the excitation wavelengths.
Fig. 5. PL spectra at 365 nm excitation for crystals a-BBO (1) (crystal was grown from composition A, Table 1) and b-BBO (2) (crystal was grown from composition C, Table 1), doped with Yb and Tb. T¼ 80 K. Fine structure is associated with Re3 þ . Spectral resolution is 1.5 nm. Spectrum (1) is shifted upward for clarity.
3.3. Luminescence spectra The photoluminescence (PL) spectra of BBO crystals codoped with Tb and Yb were recorded at 80 and 300 K with a luminescence spectrometer SDL1 and a 1 kW Xe lamp used as the excitation light source. Necessary excitation wavelength was separated using a diffraction MDR2 monochromator combined with an appropriate color glass filter. A short wave excitation near 250 nm produces PL emission in a broad band centered at 360 nm (Fig. 4). This emission is observed for both undoped and RE3 þ –doped BBO samples and is likely associated with native defects in BBO. These defects may be oxygen vacancies V0 [20,24–26], which generate deep levels of electron type in the forbidden zone and the vacancies of barium (VBa) forming holetype centers. The peaks of thermostimulated luminescence (TSL) at 123, 200 and 240 1C in the samples with sodium content of 0.02 wt% and more were observed in [27], whereas other TSL peaks in the 80–90 1C range of TL were present when the sodium content was less than 0.002 wt% [28,29]. The authors found that TSL emission takes place in the wide 400 nm band in the first case and in the 360 nm band in the other one. This implies that bands at 190–215 nm, 220–250 nm, and 250–295 nm in the luminescence excitation spectra connected with PL emission in the 300– 450 nm range are due to different luminescence centers in BBO [15,18–20,30–32]. Indeed we see that the shape of a wide PL band
in the 270–400 nm range is rather complicated (Fig. 4) and several individual components could be separated. The emission of BBO crystals codoped with Tb and Yb have a greenish tint and a set of narrow lines associated with Tb3 þ luminescence are observed in the 400–700 nm range (Fig. 5). The main group near 550 nm is due to 5D4-7F5 transitions whereas weaker groups at 500, 590 and 630 nm correspond to 5D4-7F6, 7 F4, 7F3 transitions, respectively (Fig. 5). A group of lines in the 960–1100 nm range are associated with the 2F5/2-2F7/2 transitions in the Yb3 þ ion (Fig. 6). Such transitions are typical of the Yb3 þ ion in low-symmetric eightfold oxygen surrounding [18,33]. For the Yb3 þ ion in low symmetric (point symmetry–C1) eightfold oxygen surrounding the ground 2F7/2 and excited 2F5/2 multiplets are split into four levels and three levels, respectively. In this case the spectra are expected to consist of three and four individual components in absorption and emission, respectively. The analysis of PL results for BBO samples with Yb3 þ þTb3 þ and with Yb3 þ shows that eight peaks of emission (at 976, 983, 992, 998, 1006, 1014, 1026 and 1042 nm) in a–-BBO crystals and four peaks of emission (at 969, 978, 991 and 1009 nm) in b–BBO are observed on the background of a wide band at 80 K (Fig. 6). As follows from Fig. 3 six and three bands related to Yb3 þ in a-BBO and b-BBO crystals are observed at absorption respectively.
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To confirm our assumption, we would like to refer to the work of Yoo et al. [34], where the authors observed four absorption bands at 237, 272, 317, and 379 nm for Yb2 þ in the PL spectrum with l ¼400 nm excitation for the Ba5(PO4)3Cl:Yb crystal. The peak at 379 nm almost coincides with those observed in our research. The remaining peaks could not be distinguished on the background of the wide band associated with the native defects due to the weakness of the spectrum of Yb2 þ (Figs. 2 and 7). Thus one can conclude that electronic excitation is transfered effectively from the BBO matrix to Re3 þ ions.
4. Discussion It is well known that sodium ions are the main impurity in Fig. 6. PL spectra at 365 nm excitation for crystals a- (1) (A, Table 1) and b-BBO (2) (B, Table 1), doped with Tb and Yb. T¼ 80 K. Fine structure is associated with Yb3 þ . Spectral resolution is 1.5 nm. Spectrum (1) is shifted upwards for clarity.
Fig. 7. X-ray excited luminescence spectrum of Tb, Yb:a-BBO sample at 300 K.
These data evidence that Yb3 þ ions replace Ba1 and Ba2 positions in a-–BBO structure and thus concentrations of these two centers are approximately equal. For the Yb3 þ ion in Ba1 positions (point symmetry – C3v) and Yb3 þ in Ba2 position (D3) the ground 2 F7/2 and excited 2F5/2 multiplets are split into three levels and two levels, respectively. Therefore, for both types of positions the absorption and emission spectra associated with transitions 2 F5/222F7/2 in Yb3 þ ion consist of two and three individual components, respectively. Experimentally observable number of bands of the absorption, equal to six, specifies that Yb3 þ replacement positions of Ba1 and Ba2 lead to decrease of symmetry in an nearest oxygen surrounding of ions of Yb31 þ and Yb32 þ . In a-BBO it was difficult to separate the bands associated with f-d transitions for Tb3 þ at the Ba1 and Ba2 sites because of the relatively large width of absorption/luminescence bands. We suppose that Tb3 þ ions occupy both Ba1 and Ba2 sites with comparable probability as in the case of Yb3 þ . Spectra of the X-ray excited luminescence (RL) were recorded using a X-ray source URS55 table with a W-anticathode tube. The operating parameters were 40 kV voltage and 20 mA current. Spectra were recorded with a MDR2 monochromator and two photomultipliers (FEU100 and cooled FEU83) which together cover a range from UV (200 nm) to 1200 nm in a near IR. The RL spectrum for Tb, Yb:BBO is given in Fig. 7. One can see here the same broad band at 340 nm (see also Fig. 4) and two sets of lines in the 400–700 nm and 900–1100 nm ranges related to Tb3 þ and Yb3 þ , respectively. The line at 378 nm on the background of the broad 340 nm band may be associated with the Yb2 þ emission (Fig. 7).
b-BBO crystals grown by a flux technique. Na þ ions can occupy substitutional positions replacing Ba2 þ (Na þ -Ba2 þ ) or be located inside the channels with charge compensation by Ba and O vacancies [22,26–28]. In BBO crystals doped with RE3 þ ions (Yb3 þ and Tb3 þ ) charge compensation is carried out as formation of vacancies of barium (2Re3 þ -2Ba2 þ þVBa), and Na þ ions replacing Ba2 þ (Re3 þ -Ba2 þ , Na þ -Ba2 þ ). The presence of such defects leads to a change in the shape of PL curve in the region 300–450 nm with a maximum near 360 nm at UV excitation. The absorption bands appearing on cooling could be explained by the formation of excitons under UV illumination which influences neutral donors. We assume that these absorption bands may be attributed to the transitions in the indirect excitons localized on some neutral impurity present in examined samples. On the other hand, in this region the luminescence associated with 4f135d-4f14 transitions of Yb2 þ ion was expected. Similar luminescence with lmax ¼362 nm was observed in SrB4O7 [32,33]: there Yb2 þ replaces Sr2 þ in ninefold oxygen surrounding. Therefore, the sharp peak at 378 nm observed in the RL spectrum can be obviously attributed to the 4f135d-4f14 transitions of Yb2 þ ion. In SrB4O7 such transitions in Yb2 þ ion result in the excitation (absorption) band with a maximum at 327 nm and emission band at 362 nm [32,33]. The rhombohedral cell of b-BBO contains six molecules of BaB2O4 corresponding to four (B3O6)3 anion groups of C3 position symmetry and six Ba2 þ cations of C1 position symmetry. The lowest electron configuration of Yb2 þ ion is 4f14, and therefore, probable optical transitions were the transitions between 4f14 and 4f13 5d configurations. Piper et al. [35] showed that in the cubic coordination the lowest excited levels of 4f13 5d configuration were T2u þEu. Transitions from these levels to the ground state are forbidden. In b-BaB2O4 Yb2 þ occurs in an irregular coordination (C1 symmetry of position), as eight oxygen atoms form a distorted cube. This results in the splitting of the T2u þ Eu level into the levels with A symmetry. Therefore all electron-vibrational A2A transitions are allowed, ruling out the states with different multiplicities. The structure of a-BaB2O4 contains two crystallographically nonequivalent barium atoms located at the sites of 32(D3) and 3(C3v) point symmetries. Around the barium (Ba2) in D3 point symmetry position the oxygens are arranged in a trigonal prism. Around the barium (Ba1) which lies at the C3v point symmetry position the oxygen coordination is ninefold. This is expected to split the T2u þEu levels into levels of symmetry A2 þ 2E or A1 þ2E, respectively. We suggest that the lowest excited level has symmetry A1 or A2. The component with E symmetry is assumed to correspond to the excitation band at about 335–340 nm. Piper et al. [35] indicate a higher level (about 2000 cm 1 above the T2u þ Eu levels) with symmetry T1u. Under D3 (C3v) symmetry this level splits into A2 þ E or A1 þE. Transition between these levels and the ground state are formally allowed. Therefore,
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we assign the weak absorption band at about 335–340 nm to transition to these levels. On the other hand, it is difficult to explain the large width and intensity of excitation bands near 350 nm only by the transitions between 4f14 and 4f135d configurations of Yb2 þ ions. BBO crystals are to wide band crystals: the width of forbidden band is Eg ¼6.2 eV at 300 K. Transparency range estimated at 0.5 transmission level is 198–2600 nm for 8 mm thick crystal. The bluish-white PL is typical of both undoped BBO crystals and of samples doped with RE3 þ . Thus it likely refers to the luminescence of native defects, probably anionic vacancies and different type complexes. The sharp peak near 375 nm in RL spectrum in a crystal a-BBO can be explained by the electron transfer reaction mechanism: Yb3 þ and Tb3 þ are a pair of conjugate electronic configuration rare earth ions. If they are codoped in the BaB2O4 matrix, an electron transfer may take place between Yb3 þ and Tb3 þ to provide a more stable electronic configuration of 4f7 and 4f14, corresponding to Tb4 þ and Yb2 þ ions, respectively. Tb4 þ ions have been proven to exist in a fluoride system [36].
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