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Growth of Mn2+, Co2+ and Ni2+-doped near-stoichiometric LiNbO3 by Bridgman method using K2O flux
Journal of Alloys and Compounds 463 (2008) 446–452
Growth of Mn2+, Co2+ and Ni2+-doped near-stoichiometric LiNbO3 by Bridgman method using K2O flux Sichun Zhang, Haiping Xia ∗ , Jinhao Wang, Yuepin Zhang Key Laboratory of Photo-electronic Materials, Ningbo University, Ningbo 315211, China Received 22 July 2007; received in revised form 2 September 2007; accepted 9 September 2007 Available online 18 September 2007
Abstract The near-stoichiometric LiNbO3 (SLN) single crystals doped Mn2+ , Co2+ and Ni2+ in 0.5 mol% concentration in the raw compositions were grown by the Bridgman method under the conditions of taking K2 O as flux, a high temperature gradient (90–100 ◦ C/cm) for solid–liquid interface. The XRD, absorption spectra, excitation spectra and emission spectra have been carried out. From the absorption edges of Mn2+ , Co2+ and Ni2+ -doped SLN crystals, the molar ratio of [Li+ ]/[Nb5+ ] are estimated to be about 0.977. The absorption spectra of Mn2+ :SLN have shown a broad absorption band centered at ∼571 nm (6 A1g (6 S) → 4 T1g (4 G)), three absorption peaks at 520, 549 and 612 nm (overlapping of the 4 T1 (F)–4 A2 (F), 4 T1 (F)–4 T1 (P)), and a wide absorption band at 1400 nm (4 T1 (F) → 4 T2 (F)) of Co2+ :SLN, Ni2+ :SLN, and five absorption peaks at 381 nm (3 A2g (F) → 3 T1g (P)), 733 nm (3 A2g (F) → 3 T1g (F)), 1280 nm (3 A2g (F) → 3 T2g (F)), 430 nm (3 A2g (F) → 1 T2g (D)), and 840 nm (3 A2g (F) → 1 E(D)) of Ni2+ :SLN were observed. A red emission at 612 nm (4 T1g (4 G) → 6 A1g (6 S)) for Mn2+ :SLN, a red emission at 775 nm (4 T1 (P) → 4 T1 (F)) for Co2+ :SLN, and a green emission at 577 nm (1 T2g (D) → 3 A2g (F)) and a red emission at 820 nm (1 T2g (D) → 3 T2g (F)) for Ni2+ :SLN were observed under excited by 416, 520 and 550 nm lights, respectively. The concentration distribution of Mn2+ , Co2+ and Ni2+ ion in SLN crystals was investigated primarily from the absorption and emission spectra for various parts. The effective distribution coefficient for Mn2+ was less than 1. While, for Co2+ and Ni2+ were more than 1. © 2007 Elsevier B.V. All rights reserved. Keywords: The flux Bridgman method; Mn2+ ; Co2+ ; Ni2+ ; The near-stoichiometric LiNbO3 single crystal; Spectrum
1. Introduction Lithium nibate (LiNbO3 , LN) single crystals are well known as an important technological material in opto-electronic applications. Un-doped LN single crystals have been extensively applied for planar waveguide substrates, Q-switches, phase modulators and surface acoustic wave wafers because of its large acoustic-optic and electro-optic coefficient [1–3]. LN crystals doped with transition-metal (TM) ions, e.g., Ti3+ , Cr3+ and Fe2+ [4,5], or rare-metal (RE) ions, e.g., Eu3+ and Er3+ [6,7] have also attracted much attention because of their potential applications in electro-optic, waveguide and laser technologies. LiNbO3 is a peculiar lattice in which four similar sites are available for cationic impurities, and without distorting the main C3 local symmetry: Li+ , Nb5+ , NbLi 4+ and structural vacancy
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Corresponding author. Tel.: +86 574 87600753; fax: +86 574 87600753. E-mail address: [email protected] (H. Xia).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.09.052
sites. The sites consist of oxygen octahedral distorted along one of the 1 1 1 axes. Since the sites of the octahedra are about the same in the four cases, the charge of impurity is expected to play a major role in determining preference among these sites. TM and RE are incorporated into LN single crystals in order to characterize their optical behaviors. Among the transition metal ions, Mn2+ is a typical luminescent ion [3–5]. It has been studied in various inorganic hosts covering a wide range of emissions from blue to red. Fluorescence mechanism usually involves both parity and forbidden transitions, therefore, the intensity of the emission is relatively weak [8]. Co2+ ion has been a strong colorant, which produces an intensive blue color in crystals and its color shade changes with the transformation of its tetrahedral co-ordination to octahedral with a change in host composition [6,7]. Ni2+ has become another important ion with rich optical properties in different transparent materials. Advantage of Ni2+ ion is its sensitivity towards their ligand field environment. It was reported [9] that host materials doped with Ni2+ ions in octahedral sites showed green light, red light and a broad emis-
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sion band in the near infrared which is used for tunable solid state lasers in this important spectral region. However, LiNbO3 is a typical non-stoichiometric material, and is usually grown by Czochralski method from a congruent melt with a Li content, χ = [Li]/([Li] + [Nb]) = 48.6 mol% [2,3]. The Li deficiency induces many intrinsic defects. These defects lead to the optical damage and reduce the applicability of LN, RE:LN and TM:LN crystals. So, it is vital to increase the Li content in LiNbO3 crystals to enhance the quality of LN crystal. Usually, some special techniques were used to enhance the ratio of [Li+ ]/[Nb5+ ] in LN crystal by the continuous filing double crucible method, the vapor transport equilibration method and the flux growth technique [10–12]. The improvement of the above methods and the development of new growth techniques to grown the SLN crystals are very important and necessary, because they become possible to extent beyond the limitations of present crystal growth defects to obtain the demanded single crystals. In this paper, the near-stoichiometric LiNbO3 (SLN) single crystals doped Mn2+ , Co2+ and Ni2+ in 0.5 mol% concentration in the raw compositions were grown firstly by the Bridgman method under the conditions of taking K2 O as flux, and taking high temperature gradient of about 90–100 ◦ C/cm of the interface as seeding and growing of the crystal. The details of the growth setup were described and the optical properties of Mn2+ , Co2+ and Ni2+ -doped SLN crystals were discussed. 2. Experimental The feed materials for Mn2+ , Co2+ and Ni2+ -doped SLN crystals growth were synthesized from the high purity Li2 CO3 (99.99%), Nb2 O5 (99.99%) and K2 CO3 (99.99%) with the molar proportion of 1:1:0.25. The mixture were ground for 2 h in a mortar and sintered at 1050 ◦ C. MnO, CoO and NiO (99.99%) were the dopants. The composition doping concentration for single crystal was
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0.5 mol%. The K+ ion content of the starting mixture kept same for Mn2+ , Co2+ and Ni2+ -doped SLN crystals. The congruent LiNbO3 (CLN) seed with c-axis direction and the feed material with ∼130 g were filled in a quasi-sealed Pt crucible with cylinder shape. The length of the CLN seed put under the bottom was 3–4 cm. The crucible was put in an Al2 O3 refractory tube filled with Al2 O3 powder to fix the crucible and isolate it from external temperature fluctuations. The refractory tube together with the crucible was moved into the furnace chamber. The furnace chamber was gradually heated to 1295–1300 ◦ C, then held the temperature. The Al2 O3 refractory tube was adjusted to a proper position, where the upper part of seed was melted, so the liquid–solid interface was established between upper part of seed and melt. Because the K2 O was added to the starting components and as flux of LN crystal, the melting temperature of congruent crystals reduces by more than 100 ◦ C, which causes difficult in seeding process. For the successful seeding and growth of good quality crystals a steep thermal gradient of the solid–liquid interface was also required. The seeding temperature and temperature gradient of the solid–liquid interface were controlled at ∼1160 ◦ C and 90–100 ◦ C/cm in this experiment. After keeping such a position and a temperature for 12 h to stabilize the melts originating from the feed materials and the upper part of seed, a growth process was carried out by lowering the crucible at a rate of 1–3 mm h−1 . The furnace was cooled slowly to room temperature after the growth had finished. The crucibles were stripped after taking them out of the refractory tube and as-grown crystals were obtained. Fig. 1(a), (b), and (c) shows the photographs of Mn2+ , Co2+ and Ni2+ -doped SLN as-grown crystals, respectively. The Mn2+ , Co2+ and Ni2+ ion doped SLN crystals present purple, pink and brown color, respectively. Some opaque region at the top of the above crystals, which were attributed to the serious deviation of the composition of LN, K2 O and other metal oxide dopants, were observed. The color of Mn2+ :SLN crystal changed gradually more heavy from lower to upper parts, while, for Co2+ :SLN and Ni2+ :SLN crystals, the color became lighter from lower to upper parts. It was suggested that the effective distribution coefficient of Mn2+ ion in SLN was less than 1 and Co2+ and Ni2+ more than 1. The obtained crystals were cut into small pieces, and well polished to 1.5 mm in thickness for optical measurements. The X-ray diffraction (XRD) spectra were obtained on a XD-98X diffractometer with a Cu K␣ (0.15406 nm). The optical absorption spectra were measured using a Perkin-Elmer Lambda 950 spectrophotometer, in the region of 300–2000 nm, at room temperature. The excitation and emission spectra were recorded on an F-4500 FL Spectrophotometer.
Fig. 1. The photos of (a) Mn2+ -, (b) Co2+ - and (c) Ni2+ -doped SLN. (For interpretation of the references to color in the text, the reader is referred to the web version of the article.)
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Fig. 2. The XRD of (a) Mn2+ -, (b) Co2+ - and (c) Ni2+ -doped SLN.
Fig. 3. The UV absorption edge of Mn2+ -doped SLN.
3. Results and discussion 3.1. The characterization of SLN crystals The X-ray diffraction patterns of the above crystals were recorded, Fig. 2 lists the XRD pattern of Mn2+ , Co2+ and Ni2+ doped SLN. The three curves were similar, and all of diffraction peaks of the samples can be assigned to those of LN crystal [12]. The variations of the peak intensity between the curves indicated the compositional inhomogeneity of the crystals. Amorphous features were not seen in the low-angle region. The study of UV absorption edge is a useful approach to characterize the LN crystal composition [13]. In general, the absorption edge is defined as the wavelength corresponding to the absorption coefficient of 15 or 20 cm−1 . The following equation describes the relationship between absorption edge and the Li content in LN crystal [14]: λ20 = 320.4 − 1.829χ − 5.485χ2
(1)
λ15 = 321.9 − 1.579χ − 5.745χ2
(2)
in which λ20 and λ15 are the wavelengths corresponding to the absorption coefficient of 20 and 15 cm−1 , respectively. The χ value represents the deviation of Li concentration from the congruent composition, i.e. χ% = [Li]% − 48.38%. In this work, we use λ20 (cm−1 ) as the reference point of the absorption edge. Fig. 3 shows the absorption spectra of Mn2+ :SLN and CLN crystals. From Fig. 3, we can see that the absorption edge of CLN crystal is about 320 nm, while the absorption edge of Mn2+ :SLN crystal is 312.4 nm. It means that the optical transparency region of Mn2+ :SLN crystal is wider than that of CLN crystal. For the description’s convenience, we define the absorption edge of CLN crystal as the fundamental absorption edge. Thus, the blue shift of absorption edge is 7.6 nm, compared with the fundamental absorption edge. The similar blue shift of absorption edge, about ∼7.6 nm, also existed in Co2+ and Ni2+ -doped SLN crystals. Using Eq. (1), the Li contents at Mn2+ , Co2+ , and Ni2+ -doped SLN crystals were calculated to be 49.43 mol%. Compared with that, ∼48.6 mol%, in CLN, the composition of Li in the obtained SLN crystals was enhanced obviously. The similar blue shift of the absorption edge, about 13 nm, existed in the other SLN crys-
tals grown by Czochralski technique using K2 O as flux [15]. The phenomena were explained that the K+ changed the incorporation mechanism of the Li ions into the solid phase by modifying the physico-chemical properties of the melt [13], resulting into the enhanced contents of Li in the LN crystals. 3.2. Optical spectra of SLN crystals 3.2.1. Mn2+ :SLN crystal Due to the LN crystal can only provide the octahedral sites for cations, the Mn2+ , Co2+ and Ni2+ ions were located in the octahedral sites. The visible absorption spectra of the upper and the lower parts of Mn2+ :SLN crystal are shown in Fig. 4, which exhibit broad absorption bands centered at ∼571 nm. The spectra are very similar to those of Mn2+ located in octahedral co-ordination sites of other crystals [16,17]. It can be concluded that a majority of Mn ion in the SLN appear +2 valence in octahedral co-ordination. The energy levels for Mn2+ ion in octahedral environment (CN = 6) are 6 A1g (6 S), 4 T1g (4 G), 4 T (4 G), 4 E –4 A (4 G), 4 T (4 D) and 4 E (4 D). 4 E –4 A (4 G) 2g g 1g 2g g g 1g and 4 Eg (4 D) levels have relatively less influence compared to the other levels by crystal field. It means that the relative sharp lines can be expected in the absorption or excitation spectrum,
Fig. 4. The UV/vis absorption spectra of the upper and lower part of Mn2+ :SLN crystal.
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which is the criterion for assignments of levels for Mn2+ ion [7,16]. The absorption peak at 571 nm can be attributed to 6 A (6 S) → 4 T (4 G)) of Mn2+ ions in octahedral symmetry 1g 1g [7,16]. The absorption intensity of the upper part is obviously stronger than the lower one’s. Therefore the Mn2+ ions content in the crystal increased along the growth direction, and it can be concluded that the effective distribution coefficient of Mn2+ in crystal was less than of 1. Other transitions are spin and parity forbidden for electric dipole radiation in an octahedral environment, hence the absorption bands are weak and cannot be found in Fig. 4. Fig. 5 shows the excitation spectra and emission spectra of Mn2+ :SLN crystal. Excitation spectra were monitored at an emission wavelength of λem = 620 nm. A sharp excitation band at 416 nm was measured, which was assigned to the 6 A (6 S) → 4 E –4 A (4 G) of Mn2+ ion in octahedral environ1g g 1g ment [7,16]. Under an UV source Mn2+ :SLN crystal is usually expected to emit a green or a red color. The emission color is strongly dependent on the co-ordination environment of Mn2+ in the host matrix, and it emits a green light when it is tetrahedrally co-ordination (CN = 4), whereas it emits red in octahedral co-ordination (CN = 6). In the emission spectra (Fig. 5(b)), with excitation wavelength at 416 nm the samples exhibit a broad red band at ∼612 nm, and is assigned to the spin forbidden 4 T (4 G) → 6 A (6 S) transition of isolated Mn2+ ions in octahe1g 1g dral symmetry [7,16]. The similar mechanism was proposed to that involved in the generation of a red emission from Mn2+ :SLN thin film [8,16]. The energy level of Mn2+ in SLN crystal was shown schematically in Fig. 10(a). The electron from the Mn2+ ground state 6 A1g (6 S) is excited to the conduction band (CB) by photons. From Fig. 5(a), we can estimate that the cut-off wavelength of the photo-excitation is about 430 nm (2.89 eV). The free electrons in the CB can relax to the 4 T1g (4 G) excited state though non-radiative processes, it is then followed by radiative transitions from the 4 T1g (4 G) excited state to the A1g (6 S) ground state giving red emission at ∼612 nm. Our results show that the CB and the 4 T1g (4 G) excited state are 2.89 and 2.03 eV, respectively, above the A1g (6 S) ground state.
Fig. 5. The excitation (detected by 620 nm light) and emission spectra of Mn2+ :SLN when excited by 416 nm LD.
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Fig. 6. The visible absorption spectra of the upper and the lower parts of Co2+ :SLN crystal.
3.2.2. Co2+ :SLN crystal Fig. 6 is the visible absorption spectra of the upper and the lower parts of Co2+ :SLN crystal. The absorption intensity of two curves is extremely similar. Three absorption peaks, 520, 549 and 612 nm are observed in the visible light wave band. The absorption band with a central wavelength of 1400 nm and approximately 800 nm in width in the infrared wave band is also observed. In LN crystal, because the Co ion has two valences, +2 and +3, there is the possibility to from two kinds of Co2+ and Co3+ octahedrons, and the octahedrons will be distorted. It may be predicted from the crystal-field theory that the Co3+ octahedrons has two absorption bands of comparable intensities in some crystals near 714 nm (1 A1 –1 T1 ) and 435 nm (1 A1 –1 T2 ) [18], but it is clear that these two absorption bands have not been observed in the absorption spectra for the crystals. Therefore, the Co ion may be judged to provide +2 valences in the crystal. In octahedral co-ordination (Co2+ ) free ion ground state 4 F splits into the 4 T1 , 4 T2 and 4 A2 states, with the 4 T1 state as the lowest. Co2+ in this co-ordination has three bands which correspond to the spin allowed transitions 4 T1 (F) → 4 T2 (F), 4 T1 (F) → 4 A2 (F) and 4 T1 (F) → 4 T1 (P) [7,18]. The absorption peaks, 520, 549 and 612 nm in Fig. 6 can be attributed to the overlapping of the 4 T (F)–4 A (F) and 4 T (F)–4 T (P) of Co2+ octahedrons located 1 2 1 1 at the Li and Nb sites [7,18], although the detailed mechanism is not extremely clear. The board absorption centered at 1400 nm is assigned to the spin allowed transitions 4 T1 (F) → 4 T2 (F) of isolated Co2+ ions in octahedral symmetry [7,18]. Comparing the intensities of the absorption bands in Fig. 6, the lower part’s intensity is obviously stronger than that of the upper part. Therefore the Co2+ ions content in the crystal decreased along with the growth direction, and it can be concluded that the effective distribution coefficient of Co2+ in crystal was more than 1. The excitation spectra of the upper and the lower parts of Co2+ :SLN crystal are shown in Fig. 7(a), with an emission at λem = 780 nm. A sharp excitation peak at 510 nm (4 T1 (F) → 4 T1 (P)) has been obtained. The emission spectra for the upper and the lower parts of Co2+ :SLN crystal are presented in Fig. 7(b) (λex = 520 nm). It reveals a sharp emission peak
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Fig. 8. The UV/vis absorption spectra of Ni2+ :SLN crystal. 3A
2g (F) →
3T
3A
2g (F) →
of Ni2+ in the octahedral sites, respectively. An absorption hump around 430 nm and a weak absorption peak at 840 nn can be assigned to the spin-forbidden d–d transitions of 3 A2g (F) → 1 T2g (D) and 3 A (F) → 1 E(D) of the octahedral Ni2+ , respectively [7,8]. The 2g crystal field parameter Dq and the Racah parameters B and C obtained from the absorption peak centers of the absorption spectra using energy matrices for the d8 ion in an octahedral field 1g (F),
3T
2g (F)
Fig. 7. The excitation spectra (detected by 780 nm light) and emission spectra of Co2+ :SLN when excited by 520 nm LD.
at 775 nm. This emission transition of Co2+ ions in the visible region is assigned to 4 T1 (P) → 4 T1 (F) of Co2+ ions in octahedral co-ordination [7]. 3.2.3. Ni2+ :SLN crystal The absorption spectrum of Ni2+ -doped SLN is shown in Fig. 8. The spectrum is very similar to those of Ni2+ located in octahedral co-ordination sites of other crystals [19]. It can be decided that Ni ions in the SLN appear +2 valence in octahedral co-ordination. The Ni2+ ion in octahedral co-ordination has the electronic configuration (A) 3d8 where (A) represents the closed organic shell. The energy levels of Ni2+ ion in octahedral symmetry with a ground state of 3 A2g (F) are 3 T2g (F), 1 E(D), 3 T (F), 1 T (D), 3 T (P), etc. Three spin-allowed transitions 1g 2g 1g 3 A (F) → 3 T (P), 3 A (F) → 3 T (F) and 3 A (F) → 3 T (F) 2g 1g 2g 1g 2g 2g ordered by increasing energy are expected. In addition to these three spin-allowed transitions, two spin-forbidden transitions 3 A2g (F) → 1 T2g (D) and 3 A2g (F) → 1 E(D), two weak absorption bands also exist [7,8]. The main three peaks are seen in the absorption spectrum. Intensive absorption peaks at 381, 733 and 1280 nm can be assigned to the spin-allowed upward d–d transitions of 3 A2g (F) → 3 T1g (P),
Fig. 9. The excitation spectra the excitation spectra (detected by 820 nm light) and emission spectra of Ni2+ :SLN when excited by 550 nm LD of the upper and the lower parts of Ni2+ :SLN crystal.
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[20] are calculated as 781, 1096 and 4353 cm−1 , respectively. The Dq in Ni:SLN is smaller than it in another crystals (Ni2+ :BeAl2 O4 , Dq = 937 cm−1 ; Ni2+ :MgGa2 O4 , Dq = 980 cm−1 ; Ni2+ :LiGa5 O6 , Dq = 977 cm−1 [8,21]). It indicates that the effect of host LN on Ni2+ is lower. According to ligand field theory, the lowest multiplet term 3 F of the free Ni2+ ions splits into different levels through the anisotropic hybridization. Host materials can affect the level splitting of Ni2+ ion. In Ni-doped crystal, the luminescence of the doped crystal is associated with the d–d optical transitions 3 A (F) → 3 T (F), 1 E (D) and 3 T (P), 1 T (D) → 3 T (F) 2g 1g g 1g 2g 1g and 3 T2g (F). Therefore, the energy levels of excitation states of Ni2+ are different as the matrices change [7,8]. Usually, two kinds of environments around Ni2+ are proposed as octahedral and tetrahedral sites. In this system, the site symmetry around Ni2+ can be ascribed to the former case. The excitation spec-
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tra of the upper and the lower parts of Ni2+ :SLN are shown in Fig. 9(a) with an emission of λem = 820 nm. A sharp excitation peak at 550 nm has been obtained from the Ni2+ samples. This excitation peak coincides with the absorption transition 3 A (F) → 1 T (D) of Ni2+ in octahedral sites. Fig. 9(b) presents 2g 2g the emission spectra of Ni2+ samples at an excitation 550 nm. It reveals luminescence properties in two regions, varying from green (577 nm) to red (820 nm). According to the energy levels of Ni2+ ions transitions in octahedral sites, the emission in green and red regions are assigned to the 1 T2g (D) → 3 A2g (F) and 1 T (D) → 3 T (F) transitions, respectively [8]. Fig. 10 shows 2g 2g the emission processes in (a) Mn2+ , (b) Co2+ and (c) Ni2+ ions doped SLN crystals. Given the similar circumstance for Mn2+ , Co2+ , and Ni2+ ions in the SLN crystals, the change of excitation and emission intensity—i.e., stronger excitation and emission intensity of Mn2+ , and weaker excitation and emission intensity of Co2+ and Ni2+ in the lower part of the crystal, implied that the content of Mn2+ in the upper part grown at the later period was higher than that of lower part grown at the initial period, and the content of Co2+ and Ni2+ in the upper part were lower than that of lower part. The result could be deduced from their color and absorption intensity change, further suggesting that the effective distribution coefficient of Mn2+ in SLN was less than 1, while the effective distribution coefficient of Co2+ and Ni2+ were more than that of 1. 4. Conclusions The near-stoichiometric LiNbO3 single crystals doped Mn2+ , and Ni2+ were grown by the Bridgman method under the conditions of taking K2 O as flux and a big temperature gradient for solid–liquid interface. The molar ratio of [Li+ ]/[Nb5+ ] are estimated from the absorption edge of the crystals to be ∼0.977. The Mn, Co and Ni ions located in the distorted octahedrons and have +2 valent states. The Mn2+ content in the SLN single crystal increased along growth direction, and the effective distribution coefficient of Mn2+ in SLN was less than 1. On the contrary, the Co2+ and Ni2+ were greater than 1. From Mn2+ ions doped SLN related emission spectra, it has been found out that at λex = 410 nm, a red emission (612 nm) (4 T1g (4 G) → 6 A1g (6 S)) was observed. The Co2+ :SLN has revealed a red emission (775 nm) (4 T1 (P) → 4 T1 (F)) at λex = 520 nm. The Ni2+ ions doped SLN has shown a green (577 nm) (1 T2g (D) → 3 A2g (F)) and also a red (820 nm) (1 T2g (D) → 3 T2g (F)) emission, at λex = 550 nm. These Mn2+ , Co2+ and Ni2+ ions doped SLN crystals have demonstrated their potential as novel luminescent optical materials of technological importance. Co2+
Acknowledgements
Fig. 10. The emission processes in (a) Mn2+ , (b) Co2+ and (c) Ni2+ ions doped SLN crystals.
The authors greatly appreciate the support of this research by the Project of Science and Technology of Zhejiang Province under Grant No. 2007C21121 and the Project of Science and Technology of Ningbo City under Grant No. 2005B100026.
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Growth of Mn2+, Co2+ and Ni2+-doped near-stoichiometric LiNbO3 by Bridgman method using K2O flux Sichun Zhang, Haiping Xia ∗ , Jinhao Wang, Yuepin Zhang Key Laboratory of Photo-electronic Materials, Ningbo University, Ningbo 315211, China Received 22 July 2007; received in revised form 2 September 2007; accepted 9 September 2007 Available online 18 September 2007
Abstract The near-stoichiometric LiNbO3 (SLN) single crystals doped Mn2+ , Co2+ and Ni2+ in 0.5 mol% concentration in the raw compositions were grown by the Bridgman method under the conditions of taking K2 O as flux, a high temperature gradient (90–100 ◦ C/cm) for solid–liquid interface. The XRD, absorption spectra, excitation spectra and emission spectra have been carried out. From the absorption edges of Mn2+ , Co2+ and Ni2+ -doped SLN crystals, the molar ratio of [Li+ ]/[Nb5+ ] are estimated to be about 0.977. The absorption spectra of Mn2+ :SLN have shown a broad absorption band centered at ∼571 nm (6 A1g (6 S) → 4 T1g (4 G)), three absorption peaks at 520, 549 and 612 nm (overlapping of the 4 T1 (F)–4 A2 (F), 4 T1 (F)–4 T1 (P)), and a wide absorption band at 1400 nm (4 T1 (F) → 4 T2 (F)) of Co2+ :SLN, Ni2+ :SLN, and five absorption peaks at 381 nm (3 A2g (F) → 3 T1g (P)), 733 nm (3 A2g (F) → 3 T1g (F)), 1280 nm (3 A2g (F) → 3 T2g (F)), 430 nm (3 A2g (F) → 1 T2g (D)), and 840 nm (3 A2g (F) → 1 E(D)) of Ni2+ :SLN were observed. A red emission at 612 nm (4 T1g (4 G) → 6 A1g (6 S)) for Mn2+ :SLN, a red emission at 775 nm (4 T1 (P) → 4 T1 (F)) for Co2+ :SLN, and a green emission at 577 nm (1 T2g (D) → 3 A2g (F)) and a red emission at 820 nm (1 T2g (D) → 3 T2g (F)) for Ni2+ :SLN were observed under excited by 416, 520 and 550 nm lights, respectively. The concentration distribution of Mn2+ , Co2+ and Ni2+ ion in SLN crystals was investigated primarily from the absorption and emission spectra for various parts. The effective distribution coefficient for Mn2+ was less than 1. While, for Co2+ and Ni2+ were more than 1. © 2007 Elsevier B.V. All rights reserved. Keywords: The flux Bridgman method; Mn2+ ; Co2+ ; Ni2+ ; The near-stoichiometric LiNbO3 single crystal; Spectrum
1. Introduction Lithium nibate (LiNbO3 , LN) single crystals are well known as an important technological material in opto-electronic applications. Un-doped LN single crystals have been extensively applied for planar waveguide substrates, Q-switches, phase modulators and surface acoustic wave wafers because of its large acoustic-optic and electro-optic coefficient [1–3]. LN crystals doped with transition-metal (TM) ions, e.g., Ti3+ , Cr3+ and Fe2+ [4,5], or rare-metal (RE) ions, e.g., Eu3+ and Er3+ [6,7] have also attracted much attention because of their potential applications in electro-optic, waveguide and laser technologies. LiNbO3 is a peculiar lattice in which four similar sites are available for cationic impurities, and without distorting the main C3 local symmetry: Li+ , Nb5+ , NbLi 4+ and structural vacancy
∗
Corresponding author. Tel.: +86 574 87600753; fax: +86 574 87600753. E-mail address: [email protected] (H. Xia).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.09.052
sites. The sites consist of oxygen octahedral distorted along one of the 1 1 1 axes. Since the sites of the octahedra are about the same in the four cases, the charge of impurity is expected to play a major role in determining preference among these sites. TM and RE are incorporated into LN single crystals in order to characterize their optical behaviors. Among the transition metal ions, Mn2+ is a typical luminescent ion [3–5]. It has been studied in various inorganic hosts covering a wide range of emissions from blue to red. Fluorescence mechanism usually involves both parity and forbidden transitions, therefore, the intensity of the emission is relatively weak [8]. Co2+ ion has been a strong colorant, which produces an intensive blue color in crystals and its color shade changes with the transformation of its tetrahedral co-ordination to octahedral with a change in host composition [6,7]. Ni2+ has become another important ion with rich optical properties in different transparent materials. Advantage of Ni2+ ion is its sensitivity towards their ligand field environment. It was reported [9] that host materials doped with Ni2+ ions in octahedral sites showed green light, red light and a broad emis-
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sion band in the near infrared which is used for tunable solid state lasers in this important spectral region. However, LiNbO3 is a typical non-stoichiometric material, and is usually grown by Czochralski method from a congruent melt with a Li content, χ = [Li]/([Li] + [Nb]) = 48.6 mol% [2,3]. The Li deficiency induces many intrinsic defects. These defects lead to the optical damage and reduce the applicability of LN, RE:LN and TM:LN crystals. So, it is vital to increase the Li content in LiNbO3 crystals to enhance the quality of LN crystal. Usually, some special techniques were used to enhance the ratio of [Li+ ]/[Nb5+ ] in LN crystal by the continuous filing double crucible method, the vapor transport equilibration method and the flux growth technique [10–12]. The improvement of the above methods and the development of new growth techniques to grown the SLN crystals are very important and necessary, because they become possible to extent beyond the limitations of present crystal growth defects to obtain the demanded single crystals. In this paper, the near-stoichiometric LiNbO3 (SLN) single crystals doped Mn2+ , Co2+ and Ni2+ in 0.5 mol% concentration in the raw compositions were grown firstly by the Bridgman method under the conditions of taking K2 O as flux, and taking high temperature gradient of about 90–100 ◦ C/cm of the interface as seeding and growing of the crystal. The details of the growth setup were described and the optical properties of Mn2+ , Co2+ and Ni2+ -doped SLN crystals were discussed. 2. Experimental The feed materials for Mn2+ , Co2+ and Ni2+ -doped SLN crystals growth were synthesized from the high purity Li2 CO3 (99.99%), Nb2 O5 (99.99%) and K2 CO3 (99.99%) with the molar proportion of 1:1:0.25. The mixture were ground for 2 h in a mortar and sintered at 1050 ◦ C. MnO, CoO and NiO (99.99%) were the dopants. The composition doping concentration for single crystal was
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0.5 mol%. The K+ ion content of the starting mixture kept same for Mn2+ , Co2+ and Ni2+ -doped SLN crystals. The congruent LiNbO3 (CLN) seed with c-axis direction and the feed material with ∼130 g were filled in a quasi-sealed Pt crucible with cylinder shape. The length of the CLN seed put under the bottom was 3–4 cm. The crucible was put in an Al2 O3 refractory tube filled with Al2 O3 powder to fix the crucible and isolate it from external temperature fluctuations. The refractory tube together with the crucible was moved into the furnace chamber. The furnace chamber was gradually heated to 1295–1300 ◦ C, then held the temperature. The Al2 O3 refractory tube was adjusted to a proper position, where the upper part of seed was melted, so the liquid–solid interface was established between upper part of seed and melt. Because the K2 O was added to the starting components and as flux of LN crystal, the melting temperature of congruent crystals reduces by more than 100 ◦ C, which causes difficult in seeding process. For the successful seeding and growth of good quality crystals a steep thermal gradient of the solid–liquid interface was also required. The seeding temperature and temperature gradient of the solid–liquid interface were controlled at ∼1160 ◦ C and 90–100 ◦ C/cm in this experiment. After keeping such a position and a temperature for 12 h to stabilize the melts originating from the feed materials and the upper part of seed, a growth process was carried out by lowering the crucible at a rate of 1–3 mm h−1 . The furnace was cooled slowly to room temperature after the growth had finished. The crucibles were stripped after taking them out of the refractory tube and as-grown crystals were obtained. Fig. 1(a), (b), and (c) shows the photographs of Mn2+ , Co2+ and Ni2+ -doped SLN as-grown crystals, respectively. The Mn2+ , Co2+ and Ni2+ ion doped SLN crystals present purple, pink and brown color, respectively. Some opaque region at the top of the above crystals, which were attributed to the serious deviation of the composition of LN, K2 O and other metal oxide dopants, were observed. The color of Mn2+ :SLN crystal changed gradually more heavy from lower to upper parts, while, for Co2+ :SLN and Ni2+ :SLN crystals, the color became lighter from lower to upper parts. It was suggested that the effective distribution coefficient of Mn2+ ion in SLN was less than 1 and Co2+ and Ni2+ more than 1. The obtained crystals were cut into small pieces, and well polished to 1.5 mm in thickness for optical measurements. The X-ray diffraction (XRD) spectra were obtained on a XD-98X diffractometer with a Cu K␣ (0.15406 nm). The optical absorption spectra were measured using a Perkin-Elmer Lambda 950 spectrophotometer, in the region of 300–2000 nm, at room temperature. The excitation and emission spectra were recorded on an F-4500 FL Spectrophotometer.
Fig. 1. The photos of (a) Mn2+ -, (b) Co2+ - and (c) Ni2+ -doped SLN. (For interpretation of the references to color in the text, the reader is referred to the web version of the article.)
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Fig. 2. The XRD of (a) Mn2+ -, (b) Co2+ - and (c) Ni2+ -doped SLN.
Fig. 3. The UV absorption edge of Mn2+ -doped SLN.
3. Results and discussion 3.1. The characterization of SLN crystals The X-ray diffraction patterns of the above crystals were recorded, Fig. 2 lists the XRD pattern of Mn2+ , Co2+ and Ni2+ doped SLN. The three curves were similar, and all of diffraction peaks of the samples can be assigned to those of LN crystal [12]. The variations of the peak intensity between the curves indicated the compositional inhomogeneity of the crystals. Amorphous features were not seen in the low-angle region. The study of UV absorption edge is a useful approach to characterize the LN crystal composition [13]. In general, the absorption edge is defined as the wavelength corresponding to the absorption coefficient of 15 or 20 cm−1 . The following equation describes the relationship between absorption edge and the Li content in LN crystal [14]: λ20 = 320.4 − 1.829χ − 5.485χ2
(1)
λ15 = 321.9 − 1.579χ − 5.745χ2
(2)
in which λ20 and λ15 are the wavelengths corresponding to the absorption coefficient of 20 and 15 cm−1 , respectively. The χ value represents the deviation of Li concentration from the congruent composition, i.e. χ% = [Li]% − 48.38%. In this work, we use λ20 (cm−1 ) as the reference point of the absorption edge. Fig. 3 shows the absorption spectra of Mn2+ :SLN and CLN crystals. From Fig. 3, we can see that the absorption edge of CLN crystal is about 320 nm, while the absorption edge of Mn2+ :SLN crystal is 312.4 nm. It means that the optical transparency region of Mn2+ :SLN crystal is wider than that of CLN crystal. For the description’s convenience, we define the absorption edge of CLN crystal as the fundamental absorption edge. Thus, the blue shift of absorption edge is 7.6 nm, compared with the fundamental absorption edge. The similar blue shift of absorption edge, about ∼7.6 nm, also existed in Co2+ and Ni2+ -doped SLN crystals. Using Eq. (1), the Li contents at Mn2+ , Co2+ , and Ni2+ -doped SLN crystals were calculated to be 49.43 mol%. Compared with that, ∼48.6 mol%, in CLN, the composition of Li in the obtained SLN crystals was enhanced obviously. The similar blue shift of the absorption edge, about 13 nm, existed in the other SLN crys-
tals grown by Czochralski technique using K2 O as flux [15]. The phenomena were explained that the K+ changed the incorporation mechanism of the Li ions into the solid phase by modifying the physico-chemical properties of the melt [13], resulting into the enhanced contents of Li in the LN crystals. 3.2. Optical spectra of SLN crystals 3.2.1. Mn2+ :SLN crystal Due to the LN crystal can only provide the octahedral sites for cations, the Mn2+ , Co2+ and Ni2+ ions were located in the octahedral sites. The visible absorption spectra of the upper and the lower parts of Mn2+ :SLN crystal are shown in Fig. 4, which exhibit broad absorption bands centered at ∼571 nm. The spectra are very similar to those of Mn2+ located in octahedral co-ordination sites of other crystals [16,17]. It can be concluded that a majority of Mn ion in the SLN appear +2 valence in octahedral co-ordination. The energy levels for Mn2+ ion in octahedral environment (CN = 6) are 6 A1g (6 S), 4 T1g (4 G), 4 T (4 G), 4 E –4 A (4 G), 4 T (4 D) and 4 E (4 D). 4 E –4 A (4 G) 2g g 1g 2g g g 1g and 4 Eg (4 D) levels have relatively less influence compared to the other levels by crystal field. It means that the relative sharp lines can be expected in the absorption or excitation spectrum,
Fig. 4. The UV/vis absorption spectra of the upper and lower part of Mn2+ :SLN crystal.
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which is the criterion for assignments of levels for Mn2+ ion [7,16]. The absorption peak at 571 nm can be attributed to 6 A (6 S) → 4 T (4 G)) of Mn2+ ions in octahedral symmetry 1g 1g [7,16]. The absorption intensity of the upper part is obviously stronger than the lower one’s. Therefore the Mn2+ ions content in the crystal increased along the growth direction, and it can be concluded that the effective distribution coefficient of Mn2+ in crystal was less than of 1. Other transitions are spin and parity forbidden for electric dipole radiation in an octahedral environment, hence the absorption bands are weak and cannot be found in Fig. 4. Fig. 5 shows the excitation spectra and emission spectra of Mn2+ :SLN crystal. Excitation spectra were monitored at an emission wavelength of λem = 620 nm. A sharp excitation band at 416 nm was measured, which was assigned to the 6 A (6 S) → 4 E –4 A (4 G) of Mn2+ ion in octahedral environ1g g 1g ment [7,16]. Under an UV source Mn2+ :SLN crystal is usually expected to emit a green or a red color. The emission color is strongly dependent on the co-ordination environment of Mn2+ in the host matrix, and it emits a green light when it is tetrahedrally co-ordination (CN = 4), whereas it emits red in octahedral co-ordination (CN = 6). In the emission spectra (Fig. 5(b)), with excitation wavelength at 416 nm the samples exhibit a broad red band at ∼612 nm, and is assigned to the spin forbidden 4 T (4 G) → 6 A (6 S) transition of isolated Mn2+ ions in octahe1g 1g dral symmetry [7,16]. The similar mechanism was proposed to that involved in the generation of a red emission from Mn2+ :SLN thin film [8,16]. The energy level of Mn2+ in SLN crystal was shown schematically in Fig. 10(a). The electron from the Mn2+ ground state 6 A1g (6 S) is excited to the conduction band (CB) by photons. From Fig. 5(a), we can estimate that the cut-off wavelength of the photo-excitation is about 430 nm (2.89 eV). The free electrons in the CB can relax to the 4 T1g (4 G) excited state though non-radiative processes, it is then followed by radiative transitions from the 4 T1g (4 G) excited state to the A1g (6 S) ground state giving red emission at ∼612 nm. Our results show that the CB and the 4 T1g (4 G) excited state are 2.89 and 2.03 eV, respectively, above the A1g (6 S) ground state.
Fig. 5. The excitation (detected by 620 nm light) and emission spectra of Mn2+ :SLN when excited by 416 nm LD.
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Fig. 6. The visible absorption spectra of the upper and the lower parts of Co2+ :SLN crystal.
3.2.2. Co2+ :SLN crystal Fig. 6 is the visible absorption spectra of the upper and the lower parts of Co2+ :SLN crystal. The absorption intensity of two curves is extremely similar. Three absorption peaks, 520, 549 and 612 nm are observed in the visible light wave band. The absorption band with a central wavelength of 1400 nm and approximately 800 nm in width in the infrared wave band is also observed. In LN crystal, because the Co ion has two valences, +2 and +3, there is the possibility to from two kinds of Co2+ and Co3+ octahedrons, and the octahedrons will be distorted. It may be predicted from the crystal-field theory that the Co3+ octahedrons has two absorption bands of comparable intensities in some crystals near 714 nm (1 A1 –1 T1 ) and 435 nm (1 A1 –1 T2 ) [18], but it is clear that these two absorption bands have not been observed in the absorption spectra for the crystals. Therefore, the Co ion may be judged to provide +2 valences in the crystal. In octahedral co-ordination (Co2+ ) free ion ground state 4 F splits into the 4 T1 , 4 T2 and 4 A2 states, with the 4 T1 state as the lowest. Co2+ in this co-ordination has three bands which correspond to the spin allowed transitions 4 T1 (F) → 4 T2 (F), 4 T1 (F) → 4 A2 (F) and 4 T1 (F) → 4 T1 (P) [7,18]. The absorption peaks, 520, 549 and 612 nm in Fig. 6 can be attributed to the overlapping of the 4 T (F)–4 A (F) and 4 T (F)–4 T (P) of Co2+ octahedrons located 1 2 1 1 at the Li and Nb sites [7,18], although the detailed mechanism is not extremely clear. The board absorption centered at 1400 nm is assigned to the spin allowed transitions 4 T1 (F) → 4 T2 (F) of isolated Co2+ ions in octahedral symmetry [7,18]. Comparing the intensities of the absorption bands in Fig. 6, the lower part’s intensity is obviously stronger than that of the upper part. Therefore the Co2+ ions content in the crystal decreased along with the growth direction, and it can be concluded that the effective distribution coefficient of Co2+ in crystal was more than 1. The excitation spectra of the upper and the lower parts of Co2+ :SLN crystal are shown in Fig. 7(a), with an emission at λem = 780 nm. A sharp excitation peak at 510 nm (4 T1 (F) → 4 T1 (P)) has been obtained. The emission spectra for the upper and the lower parts of Co2+ :SLN crystal are presented in Fig. 7(b) (λex = 520 nm). It reveals a sharp emission peak
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Fig. 8. The UV/vis absorption spectra of Ni2+ :SLN crystal. 3A
2g (F) →
3T
3A
2g (F) →
of Ni2+ in the octahedral sites, respectively. An absorption hump around 430 nm and a weak absorption peak at 840 nn can be assigned to the spin-forbidden d–d transitions of 3 A2g (F) → 1 T2g (D) and 3 A (F) → 1 E(D) of the octahedral Ni2+ , respectively [7,8]. The 2g crystal field parameter Dq and the Racah parameters B and C obtained from the absorption peak centers of the absorption spectra using energy matrices for the d8 ion in an octahedral field 1g (F),
3T
2g (F)
Fig. 7. The excitation spectra (detected by 780 nm light) and emission spectra of Co2+ :SLN when excited by 520 nm LD.
at 775 nm. This emission transition of Co2+ ions in the visible region is assigned to 4 T1 (P) → 4 T1 (F) of Co2+ ions in octahedral co-ordination [7]. 3.2.3. Ni2+ :SLN crystal The absorption spectrum of Ni2+ -doped SLN is shown in Fig. 8. The spectrum is very similar to those of Ni2+ located in octahedral co-ordination sites of other crystals [19]. It can be decided that Ni ions in the SLN appear +2 valence in octahedral co-ordination. The Ni2+ ion in octahedral co-ordination has the electronic configuration (A) 3d8 where (A) represents the closed organic shell. The energy levels of Ni2+ ion in octahedral symmetry with a ground state of 3 A2g (F) are 3 T2g (F), 1 E(D), 3 T (F), 1 T (D), 3 T (P), etc. Three spin-allowed transitions 1g 2g 1g 3 A (F) → 3 T (P), 3 A (F) → 3 T (F) and 3 A (F) → 3 T (F) 2g 1g 2g 1g 2g 2g ordered by increasing energy are expected. In addition to these three spin-allowed transitions, two spin-forbidden transitions 3 A2g (F) → 1 T2g (D) and 3 A2g (F) → 1 E(D), two weak absorption bands also exist [7,8]. The main three peaks are seen in the absorption spectrum. Intensive absorption peaks at 381, 733 and 1280 nm can be assigned to the spin-allowed upward d–d transitions of 3 A2g (F) → 3 T1g (P),
Fig. 9. The excitation spectra the excitation spectra (detected by 820 nm light) and emission spectra of Ni2+ :SLN when excited by 550 nm LD of the upper and the lower parts of Ni2+ :SLN crystal.
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[20] are calculated as 781, 1096 and 4353 cm−1 , respectively. The Dq in Ni:SLN is smaller than it in another crystals (Ni2+ :BeAl2 O4 , Dq = 937 cm−1 ; Ni2+ :MgGa2 O4 , Dq = 980 cm−1 ; Ni2+ :LiGa5 O6 , Dq = 977 cm−1 [8,21]). It indicates that the effect of host LN on Ni2+ is lower. According to ligand field theory, the lowest multiplet term 3 F of the free Ni2+ ions splits into different levels through the anisotropic hybridization. Host materials can affect the level splitting of Ni2+ ion. In Ni-doped crystal, the luminescence of the doped crystal is associated with the d–d optical transitions 3 A (F) → 3 T (F), 1 E (D) and 3 T (P), 1 T (D) → 3 T (F) 2g 1g g 1g 2g 1g and 3 T2g (F). Therefore, the energy levels of excitation states of Ni2+ are different as the matrices change [7,8]. Usually, two kinds of environments around Ni2+ are proposed as octahedral and tetrahedral sites. In this system, the site symmetry around Ni2+ can be ascribed to the former case. The excitation spec-
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tra of the upper and the lower parts of Ni2+ :SLN are shown in Fig. 9(a) with an emission of λem = 820 nm. A sharp excitation peak at 550 nm has been obtained from the Ni2+ samples. This excitation peak coincides with the absorption transition 3 A (F) → 1 T (D) of Ni2+ in octahedral sites. Fig. 9(b) presents 2g 2g the emission spectra of Ni2+ samples at an excitation 550 nm. It reveals luminescence properties in two regions, varying from green (577 nm) to red (820 nm). According to the energy levels of Ni2+ ions transitions in octahedral sites, the emission in green and red regions are assigned to the 1 T2g (D) → 3 A2g (F) and 1 T (D) → 3 T (F) transitions, respectively [8]. Fig. 10 shows 2g 2g the emission processes in (a) Mn2+ , (b) Co2+ and (c) Ni2+ ions doped SLN crystals. Given the similar circumstance for Mn2+ , Co2+ , and Ni2+ ions in the SLN crystals, the change of excitation and emission intensity—i.e., stronger excitation and emission intensity of Mn2+ , and weaker excitation and emission intensity of Co2+ and Ni2+ in the lower part of the crystal, implied that the content of Mn2+ in the upper part grown at the later period was higher than that of lower part grown at the initial period, and the content of Co2+ and Ni2+ in the upper part were lower than that of lower part. The result could be deduced from their color and absorption intensity change, further suggesting that the effective distribution coefficient of Mn2+ in SLN was less than 1, while the effective distribution coefficient of Co2+ and Ni2+ were more than that of 1. 4. Conclusions The near-stoichiometric LiNbO3 single crystals doped Mn2+ , and Ni2+ were grown by the Bridgman method under the conditions of taking K2 O as flux and a big temperature gradient for solid–liquid interface. The molar ratio of [Li+ ]/[Nb5+ ] are estimated from the absorption edge of the crystals to be ∼0.977. The Mn, Co and Ni ions located in the distorted octahedrons and have +2 valent states. The Mn2+ content in the SLN single crystal increased along growth direction, and the effective distribution coefficient of Mn2+ in SLN was less than 1. On the contrary, the Co2+ and Ni2+ were greater than 1. From Mn2+ ions doped SLN related emission spectra, it has been found out that at λex = 410 nm, a red emission (612 nm) (4 T1g (4 G) → 6 A1g (6 S)) was observed. The Co2+ :SLN has revealed a red emission (775 nm) (4 T1 (P) → 4 T1 (F)) at λex = 520 nm. The Ni2+ ions doped SLN has shown a green (577 nm) (1 T2g (D) → 3 A2g (F)) and also a red (820 nm) (1 T2g (D) → 3 T2g (F)) emission, at λex = 550 nm. These Mn2+ , Co2+ and Ni2+ ions doped SLN crystals have demonstrated their potential as novel luminescent optical materials of technological importance. Co2+
Acknowledgements
Fig. 10. The emission processes in (a) Mn2+ , (b) Co2+ and (c) Ni2+ ions doped SLN crystals.
The authors greatly appreciate the support of this research by the Project of Science and Technology of Zhejiang Province under Grant No. 2007C21121 and the Project of Science and Technology of Ningbo City under Grant No. 2005B100026.
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