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Growth and characterization of single crystal fiber with controlled concentration gradient in GdTaO4–ErTaO4 system
ARTICLE IN PRESS
Journal of Crystal Growth 294 (2006) 447–451 www.elsevier.com/locate/jcrysgro
Growth and characterization of single crystal fiber with controlled concentration gradient in GdTaO4–ErTaO4 system R.A. Silvaa,, G. Tiraob,1, C. Cusatisb, J.P. Andreetaa a
Instituto de Fı´sica de Sa˜o Carlos, Depto de Fı´sica e Cieˆncia dos Materiais, Universidade de Sa˜o Paulo, CP 369, CEP 13560-590, Sa˜o Carlos, SP, Brazil Laborato´rio de O´ptica de Raios X e Instrumentac- a˜o, Depto de Fı´sica, Universidade Federal do Parana´, CP 19091, CEP 81531-990, Curitiba, PR, Brazil
b
Received 13 March 2006; received in revised form 20 June 2006; accepted 23 June 2006 Communicated by R.S. Feigelson
Abstract The LHPG technique has been successfully applied to the growth of single crystals in fiber shape with a controlled composition and lattice parameter gradient along the fiber axis. The GdTaO4–ErTaO4 system was chosen for having a full solubility range. The obtained fibers had a constant gradient of lattice parameters along the axis with a value of 1.24%/cm for the (4 4¯ 4) reflection, hence of concentration gradient. Structural characterization results showed that the fibers had a good crystalline quality essential for the optimization of others physical properties. Luminescent characterization showed the behavior of the intensity of green and red luminescence (by 488 nm pumping) as a function of concentration of Er3+ and three processes populating the levels (2H11/2, 4S3/2) and 4 F9/2 by cross-relaxation energy transfer has been proposed. The results presented show that the compositional gradient and the LHPG technique can be used not only for the growth of crystals with controlled lattice parameter gradients, for potential X-ray diffraction applications, but also as a combinatorial method for the rapid study of material properties as a function of concentration using in situ measurements. r 2006 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 61.72.Ff; 78.30.j; 77.84.Dy; 78.55.m; 42.70.Hj Keywords: A1. Combinatorial methods; A1. Gradient crystals; A1. X-ray diffraction; A2. Laser heated pedestal growth; B1. Rare-earth tantalates
1. Introduction The use of crystals with controlled lattice parameters gradient has been proposed as suitable devices for X-ray applications [1–3]. Besides, the gradient of composition of these crystals makes possible the study of physical properties as a function of composition along the gradient region. So the technique can be considered as a combinatorial method [4–8]. Using the fact that the composition changes continuously between the ends of the crystal, each point of crystal can be considered locally as a single crystal where composition and physical properties are correlated. In fact Corresponding author. Tel.: +55 11 273 9828; fax: +55 11 273 9824.
E-mail address: [email protected] (R.A. Silva). Present address: Facultad de Matema´tica Astronomı´ a y Fı´ sica, Universidad Nacional de Co´rdoba, 5000 Co´rdoba, Argentina. 1
0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.06.041
this idea has already been reported in literature (for instance Ref. [9]) and recently renewed interest has surged with the works of Cohen-Adad et al. [10], Laversenne et al. [11] and Boulon et al. [12]. As part of our search for suitable compounds to obtain crystal fibers with controlled variation of composition and consequently lattice spacing along one dimension, we present the results of the growth and characterization of gradient crystals in GdTaO4–ErTaO4 system, taking advantage of our experience in growth of single crystal fiber of rare-earth tantalates [13,14]. As rare-earth tantalates RTaO4 are isomorphic for RQNd to Yb [15], these compounds are expected to form complete solid solutions. Therefore, they have a good potential for producing crystals with concentration and lattice spacing gradients. The used approach is described in earlier works [14,16] where the LHPG method was used for the growth of these
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types of gradient crystals. Structural and luminescent characterization was made to evaluate the potentialities of the Gd1xErxTaO4 fibers for X-ray optic applications and to confirm the use of this technique as a combinatorial preparation method.
2. Experimental procedure After a thermal treatment at 800 1C for 12 h, the starting materials Gd2O3 (REacton 99.99%), Er2O3 (REacton 99.9%) and Ta2O5 (Puratronics 99.993%) were weighed in a stoichiometric ratio and mixed by ball milling for 24 h. Separate pedestals of Gd2O3+Ta2O5 and Er2O3+Ta2O5 (1.2 mm in diameter) were prepared by a cold extrusion process mixing the oxide powders with an organic binder (polyvinyl alcohol) [13,17]. Composite pedestals were made and the growth experiments were performed as schematized in Fig. 1(a). The LHPG apparatus used for the growth experiments is described elsewhere [18]. The experiments were realized in air atmosphere. Fibers were pulled using polycrystalline Gd2O3–Ta2O5 pedestals as seed. Pulling rates of 0.8–1.0 mm/min were used without rotation of the seed or nutrient. Axial composition profiles along the length of the fibers were determined by energy-dispersive X-ray spectroscopy (EDXS) using an electron microprobe coupled to a scanning electron microscope (Zeiss-Leica 440). The crystalline orientation of the growth direction (fiber axis) and the unit cell parameters were determined by using a four-circle diffractometer. A four-bounce monochromator of Ge(1 1 1) in the dispersive set-up (+, +) selecting the Cu-Ka1 radiation with a divergence of FWHM 40 arcsec, was used for measurements of X-ray rocking curve (XRC) profiles of some reflections to determine the variation of interplanar spacing along the fiber. X-ray diffraction topographies (XRDT) were also obtained using Cu-Ka1 radiation (in Bragg case only because of the high absorption of the sample) to show the global quality of the crystal. The X-ray beam from a conventional tube in the line focus configuration was collimated to an angular divergence of 180 arcsec in the diffraction plane. This beam illuminated
the 11 mm height of the fiber and the image was collected with a CCD-camera (1128 1256 pixels of 25 25 mm2 each) located at 280 mm of the sample. The crystal was continually rocked in order to allow the diffraction condition to be satisfied along the whole fiber. Unpolarized photoluminescence spectra in backscattering mode were recorded at various positions along the fiber at room temperature with an Ar+ laser at 488 nm as excitation source.
3. Results The prepared pedestals had a high density and mechanical strength. This high density was very important to stabilize the growth process, as cited in earlier work [13]. During the growth process a continuous increase of the laser power was required due to the different fusion points of the compounds GdTaO4 and ErTaO4. The obtained fibers presented excellent transparency and cracks were not observed by optical microscopy in magnifications up to 100 . The diameter varied from 0.7 to 0.9 mm depending of the feeding/pulling ratio. A GdTaO4–ErTaO4 fiber is shown in Fig. 1(b). The diameter fluctuations mainly in the gradient region are attributed to poor laser power control and the continuous variation of composition during the growth experiment. The concentration gradient is easily verified by the change of coloration along the fiber, from colorless at GdTaO4 end to dark pink at the ErTaO4 end. In the several grown fibers, the length of the gradient regions varied from 8 to 12 mm depending of the diameter of the fibers. The axial composition gradient profile of a fiber with length of the gradient region of approximately 9 mm is shown in Fig. 2. The concentration of Gd and Er varied linearly from pure GdTaO4 to pure ErTaO4. The axial Gd–Er composition gradient was of approximately 12 at%/mm. The fibers crystallized in monoclinic structure (I2/a space group), with measured lattice parameters of a ¼ (5.4070.01) A˚, b ¼ (11.0770.03) A˚, c ¼ (5.087 0.01) A˚, b ¼ (95.870.1)1 and a ¼ (5.3070.01) A˚, b ¼ (10.9070.01) A˚, c ¼ (5.0470.01) A˚, b ¼ (95.570.2)1 for GdTaO4 and ErTaO4 respectively, in accordance to reported values in literature [15].
Fig. 1. (a) Schematic of composite pedestal of Gd2O3+Ta2O5 and Er2O3+Ta2O5 and (b) photo of a GdTaO4–ErTaO4 gradient fiber. The distance between two adjacent traces is 1 mm.
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Fig. 2. Axial relative composition profiles of Gd, Er and Ta in GdTaO4–ErTaO4 single crystal fiber.
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seen, mainly at the ends where only the pure compounds are present. In the middle region some of profile presented subsctructures evidencing the poorer crystalline quality in this region. It can clearly be seen how the Bragg-angle changes along the crystal, corresponding to the lattice parameters changes as expected. The results of XRCprofiles as a function of position the z-position (position z ¼ 0 mm indicates the end with GdTaO4) allowed a plot and calculation of the relative variation Dd/dav as function of z-position where Dd is the variation of the interplanar spacing and dav the average interplanar distance. The results obtained for the (4 4¯ 4) reflection are shown in Fig. 4. The linear fitting in the gradient region gave an interplanar spacing gradient Dd/dav/z ¼ 1.24%/cm. This value is intermediate to those reported in literature for others gradient crystals [19,21,22]. In Fig. 5 the XRDT image of the (4 4¯ 4) reflection to characterize the global quality of the fiber is shown. The image was obtained by rocking the fiber enabling all the crystal carried out the Bragg diffraction condition. The diffractions of both Cu-Ka1 (more intensity image) and CuKa2 (less intensity image) lines can be observed. The regions without lattice spacing variation (vertical bright lines in both extremes) and the region presenting lattice spacing gradient (tilted long line in middle region) are easily observable. The extreme regions show a higher and more uniform intensity evidencing the higher structural quality where there are only the pure compounds. The observed intensity variations, more notably in the middle region are evidence of a lower structural quality in this region. This occurred probably because of continuous
Fig. 3. Normalized XRC profiles of the (4 4¯ 4) reflection at different zpositions along the fiber axis using a four-bounce monochromator of Ge(1 1 1) in the dispersive set-up (+, +) selecting the Cu-Ka1 radiation. The growth direction of the fiber was (1¯ 1 1).
In our evaluation of the crystalline quality of the fibers, a fiber with the (1¯ 1 1) direction as crystalline orientation of the growth direction was used. The (2 4¯ 2), (4 0 4) and (4 4¯ 4) reflections were used to study the interplanar spacing variation, as well as the structural quality along the fiber. These reflections were selected because diffraction Braggplane is approximately parallel (5.81-off, 61-off and 0.11off, respectively) to the fiber axis. The XRC profiles of these reflections presented well-defined peaks with a mean value of FWHM (excluding the instrumental contribution and the contribution of the gradient itself) of 80, 135 and 105 arcsec for the (2 4¯ 2), (4 0 4) and (4 4¯ 4) reflections, respectively. These values are reasonable when compared to other good quality gradient crystals [19–22]. The XRCprofiles of the (4 4¯ 4) reflection at some positions along the fiber axis are shown in Fig. 3. Well-defined peaks can be
Fig. 4. Relative variation of interplanar distance for the (4 4¯ 4) reflection as a function of the z-position on the fiber. Linear fitting from z ¼ 3 to 11 mm.
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Fig. 5. XRDT of the (4 4¯ 4) reflection with Cu-Ka radiation and with 60 min of exposure time. The fiber was continuously rocked enabling the whole fiber satisfy the Bragg diffraction condition. It can be see the diffractions of both Cu-Ka1 (more intensity image) and Cu-Ka2 (less intensity image) lines showing the global crystalline fiber quality.
Fig. 7. (a) Schematic representation of energy levels of Er3+ in a Gd1xErxTaO4 single crystal and (b) proposed cross-relaxation processes.
Fig. 6. Unpolarized luminescence spectra of Gd1xErxTaO4 single crystal at different positions/concentrations along the fiber axis.
change of the melt composition so altering the conditions of growth as cited above. The samples enabled us to estimate the influence of the Er3+ concentration in Gd1xErxTaO4 (0pxp0) on the luminescence intensity obtained by 488 nm Ar+ laser excitation. In Fig. 6 the luminescence spectra are shown in 500–700 nm range, for several positions along the fiber. The uncertainty in the concentration is because of the laser illuminated an area with a considerable variation of composition due to the high concentration gradient of the samples. The analysis of the spectra can be accom-
plished using the energy diagram of Er3+ ion in crystals (Fig. 7). The 488 nm Ar+ laser excites Er3+ into the 4F7/2 level. Then excited Er3+ decays nonradiatively to the (2H11/ 4 4 F9/2 levels, and undergo the radiative 2, S3/2) and 2 transitions ( H11/2, 4S3/2)-4I15/2 and 4F9/2-4I5/2 originating the green and red luminescence, respectively (Fig. 7(a)). The following features can be noted: the intensity of green luminescence shows a maximum at low concentrations of Er3+ and decreases until concentrations around x ¼ 0.5, remaining approximately constant up to x ¼ 1. Oppositely, the intensity of red luminescence remains approximately constant until x0.5 and then increases reaching its maximum at x ¼ 1. The intensities of the emissions corresponding to the two transitions reflect the relative populations of these two levels. It is proposed that in this system two possible processes populate these two levels (Fig. 7(b)). The (2H11/2, 4S3/2) and the 4F9/2 levels are fed by non-radiative relaxation from these upper levels. As the green and red emissions exceed each other in different range of concentration, there must be some cross-relaxation energy transfer populating these levels. In these processes, energy migrates from a donor to an acceptor in an isolated ion pair, raising the latter ion into a higher energy state. Energy transfer can take place via the two transitions 4I11/2-4I15/2 and 4I11/2-4F7/2 and subsequent relaxation feeds the 4S3/2 level. The 4F9/2 level energy transfer can take place via the transitions 4I11/2-4I15/2 and 4 I13/2-4F9/2. Another cross-relaxation energy transfer process is also possible via the two transitions 2H11/ 4 4 4 2- F9/2 and I11/2- F9/2. These two latter energy transfer processes include non-resonant transition. So these processes have to be accomplished by multi-phonon absorp-
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tion and/or emission to conserve energy. As the energy mismatch between the two transitions is on the order to the phonon energy in oxide lattices, these processes are reasonable. Considering just these three cross-relaxation processes that result in populating of the two levels, it is clearly seen that they depend in a different manner from the Er3+ concentration. This may be due to the fact that the energy transfer processes involved in the two mechanisms are of different nature (i.e. dipole–dipole, dipole–quadrupole, etc.). Additional measurements based on (1) lifetime of the emissions, (2) a study of the kinetics of the processes, and (3) the dependence of temperature are required for verifying the phonon assisted cross-relaxation processes and a complete understanding of the system. Such studies are currently in progress. 4. Conclusion Single crystal fibers with controlled gradient of concentration and lattice parameters of GdTaO4–ErTaO4 system have been successfully obtained using the LHPG technique using an approach which does not require thermal treatment of the reagents or pedestals. The approach is based in previous works [14,16]. It is a fast and inexpensive method and can be applied to a variety of oxides systems. The fibers obtained have good structural quality and the value of the interplanar spacing gradient of 1.24%/cm is a high value compared to others concentration gradient fibers reported in literature (around 1.1%/cm in (K,Rb)C8H5O4 gradient crystals [19], 0.8%/cm in Bi–Sb crystals [21] and 1.65%/cm in CaMoO4–SrMoO4 crystals [16]). Luminescent characterization showed the behavior of the intensity of green and red luminescence (by 488 nm pumping) as a function of concentration of Er3+ and three processes populating the levels (2H11/2, 4S3/2) and 4F9/2 by cross-relaxation energy transfer has been proposed. The results show that the GdTaO4–ErTaO4 system is a promising system for X-ray optics applications. In addition, the use of the controlled gradient of concentration shows that Gd1xErxTaO4 can also be a potential candidate for devices emitting in green for low concentrations of Er3+ and devices emitting in red for high concentrations of Er3+. The verification of this potentiality is in progress with additional measurements for studying the dynamics of the system. This technique can be successfully applied, not only as method for obtaining materials for X-ray applications, like monochromators [21,23,24] and collimators [25], but also as combinatorial method for rapid investigation of properties as a function of concentration by in situ measurements.
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Acknowledgments The authors are grateful to Dr. A.R. Zanatta for luminescence measurements and the Brazilian agencies FAPESP, CAPES and CNPq and PRONEX/F. Araucaria/ CPNq for financial support.
References [1] R.K. Smither, Rev. Sci. Instrum. 53 (1982) 131. [2] H. Bradaczek, G. Hildebrandt, J. Optoelectron. Adv. Mater. 1 (1999) 3. [3] G. Hildebrandt, H. Bradaczek, Rigaku J. 17 (2000) 13. [4] E.J. Amis, S.-D. Xiang, J.-C. Zhao, Mater. Res. Soc. Bull. 27 (2002) 295. [5] R.B. van Dover, L.F. Schneemeyer, R.M. Fleming, Nature 392 (1998) 162. [6] G. Bricen˜o, H. Chang, X. Sun, P.G. Schultz, X.-D. Xiang, Science 270 (1995) 273. [7] X.-D. Xiang, X. Sun, G. Bricen˜o, Y. Lou, K.-A. Wang, H. Chang, W.G. Wallace-Freedman, S.-W. Chen, P.G. Schultz, Science 268 (1995) 1738. [8] E. Danielson, J.H. Golden, E.W. McFarland, C.M. Reaves, W.H. Weinberg, X.D. Wu, Nature 389 (1997) 944. [9] R. Triboulet, G. Neu, J. Crystal Growth 65 (1983) 262. [10] M.T. Cohen-Adad, L. Laversenne, M. Gharbi, C. Goutaudier, G. Boulon, R. Cohen-Adad, J. Phase Equilibr. 22 (2001) 379. [11] L. Laversenne, C. Goutaudier, Y. Guyot, M.Th. Cohen-Adad, G. Boulon, J. Alloys Compounds 341 (2002) 214. [12] G. Boulon, L. Laversenne, C. Goutaudier, Y. Guyot, M.T. CohenAdad, J. Lumin. 102 (2003) 417. [13] R. Almeida Silva, G. Tirao, C. Cusatis, J.P. Andreeta, J. Crystal Growth 274 (2005) 512. [14] R. Almeida Silva, G. Tirao, C. Cusatis, J.P. Andreeta, J. Crystal Growth 277 (2005) 308. [15] L.H. Brixner, H.-Y. Chen, J. Eletrochem. Soc. 130 (1983) 2435. [16] L.B. Barbosa, D. Reyes Ardila, E.M. Kakuno, R.H. Camparin, C. Cusatis, J.P. Andreeta, J. Crystal Growth 250 (2003) 67. [17] R. Almeida Silva, A.S.S. de Camargo, C. Cusatis, L.A.O. Nunes, J.P. Andreeta, J. Crystal Growth 262 (2004) 246. [18] D. Reyes Ardila, J.P. Andreeta, C.T.M. Ribeiro, M. Siu Li, Rev. Sci. Instrum. 70 (1999) 4606. [19] S.V. Moshkin, O.M. Boldyreva, T.I. Ivanova, M.A. Kuz’mina, I.P. Shakhverdova, P.V. Petrashen, R.N. Kyutt, H. Bradaczek, J. Crystal Growth 171 (1997) 226. [20] A. Erko, F. Scha¨fers, W. Gudat, N.V. Abrosimov, S.N. Rossolenko, V. Alex, S. Groth, W. Schro¨der, Nucl. Instrum. Methods A 374 (1996) 408. [21] S. Penzel, W. Neumann, J. Crystal Growth 198/199 (1999) 811. [22] H.J. Koh, N. Scha¨fer, K. Shimamure, T. Fukuda, J. Crystal Growth 167 (1996) 38. [23] E.V. Shulakov, V.Sh. Shekhtman, S.S. Khasanov, I.A. Smirnova, Nucl. Instrum. Methods A 448 (2000) 152. [24] A. Erko, N.V. Abrosimov, V. Alex, Cryst. Res. Technol. 37 (7) (2002) 685. [25] P. Petrashen, A. Erko, Nucl. Instrum. Methods A 467–468 (2001) 358.
Journal of Crystal Growth 294 (2006) 447–451 www.elsevier.com/locate/jcrysgro
Growth and characterization of single crystal fiber with controlled concentration gradient in GdTaO4–ErTaO4 system R.A. Silvaa,, G. Tiraob,1, C. Cusatisb, J.P. Andreetaa a
Instituto de Fı´sica de Sa˜o Carlos, Depto de Fı´sica e Cieˆncia dos Materiais, Universidade de Sa˜o Paulo, CP 369, CEP 13560-590, Sa˜o Carlos, SP, Brazil Laborato´rio de O´ptica de Raios X e Instrumentac- a˜o, Depto de Fı´sica, Universidade Federal do Parana´, CP 19091, CEP 81531-990, Curitiba, PR, Brazil
b
Received 13 March 2006; received in revised form 20 June 2006; accepted 23 June 2006 Communicated by R.S. Feigelson
Abstract The LHPG technique has been successfully applied to the growth of single crystals in fiber shape with a controlled composition and lattice parameter gradient along the fiber axis. The GdTaO4–ErTaO4 system was chosen for having a full solubility range. The obtained fibers had a constant gradient of lattice parameters along the axis with a value of 1.24%/cm for the (4 4¯ 4) reflection, hence of concentration gradient. Structural characterization results showed that the fibers had a good crystalline quality essential for the optimization of others physical properties. Luminescent characterization showed the behavior of the intensity of green and red luminescence (by 488 nm pumping) as a function of concentration of Er3+ and three processes populating the levels (2H11/2, 4S3/2) and 4 F9/2 by cross-relaxation energy transfer has been proposed. The results presented show that the compositional gradient and the LHPG technique can be used not only for the growth of crystals with controlled lattice parameter gradients, for potential X-ray diffraction applications, but also as a combinatorial method for the rapid study of material properties as a function of concentration using in situ measurements. r 2006 Elsevier B.V. All rights reserved. PACS: 61.10.Nz; 61.72.Ff; 78.30.j; 77.84.Dy; 78.55.m; 42.70.Hj Keywords: A1. Combinatorial methods; A1. Gradient crystals; A1. X-ray diffraction; A2. Laser heated pedestal growth; B1. Rare-earth tantalates
1. Introduction The use of crystals with controlled lattice parameters gradient has been proposed as suitable devices for X-ray applications [1–3]. Besides, the gradient of composition of these crystals makes possible the study of physical properties as a function of composition along the gradient region. So the technique can be considered as a combinatorial method [4–8]. Using the fact that the composition changes continuously between the ends of the crystal, each point of crystal can be considered locally as a single crystal where composition and physical properties are correlated. In fact Corresponding author. Tel.: +55 11 273 9828; fax: +55 11 273 9824.
E-mail address: [email protected] (R.A. Silva). Present address: Facultad de Matema´tica Astronomı´ a y Fı´ sica, Universidad Nacional de Co´rdoba, 5000 Co´rdoba, Argentina. 1
0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.06.041
this idea has already been reported in literature (for instance Ref. [9]) and recently renewed interest has surged with the works of Cohen-Adad et al. [10], Laversenne et al. [11] and Boulon et al. [12]. As part of our search for suitable compounds to obtain crystal fibers with controlled variation of composition and consequently lattice spacing along one dimension, we present the results of the growth and characterization of gradient crystals in GdTaO4–ErTaO4 system, taking advantage of our experience in growth of single crystal fiber of rare-earth tantalates [13,14]. As rare-earth tantalates RTaO4 are isomorphic for RQNd to Yb [15], these compounds are expected to form complete solid solutions. Therefore, they have a good potential for producing crystals with concentration and lattice spacing gradients. The used approach is described in earlier works [14,16] where the LHPG method was used for the growth of these
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types of gradient crystals. Structural and luminescent characterization was made to evaluate the potentialities of the Gd1xErxTaO4 fibers for X-ray optic applications and to confirm the use of this technique as a combinatorial preparation method.
2. Experimental procedure After a thermal treatment at 800 1C for 12 h, the starting materials Gd2O3 (REacton 99.99%), Er2O3 (REacton 99.9%) and Ta2O5 (Puratronics 99.993%) were weighed in a stoichiometric ratio and mixed by ball milling for 24 h. Separate pedestals of Gd2O3+Ta2O5 and Er2O3+Ta2O5 (1.2 mm in diameter) were prepared by a cold extrusion process mixing the oxide powders with an organic binder (polyvinyl alcohol) [13,17]. Composite pedestals were made and the growth experiments were performed as schematized in Fig. 1(a). The LHPG apparatus used for the growth experiments is described elsewhere [18]. The experiments were realized in air atmosphere. Fibers were pulled using polycrystalline Gd2O3–Ta2O5 pedestals as seed. Pulling rates of 0.8–1.0 mm/min were used without rotation of the seed or nutrient. Axial composition profiles along the length of the fibers were determined by energy-dispersive X-ray spectroscopy (EDXS) using an electron microprobe coupled to a scanning electron microscope (Zeiss-Leica 440). The crystalline orientation of the growth direction (fiber axis) and the unit cell parameters were determined by using a four-circle diffractometer. A four-bounce monochromator of Ge(1 1 1) in the dispersive set-up (+, +) selecting the Cu-Ka1 radiation with a divergence of FWHM 40 arcsec, was used for measurements of X-ray rocking curve (XRC) profiles of some reflections to determine the variation of interplanar spacing along the fiber. X-ray diffraction topographies (XRDT) were also obtained using Cu-Ka1 radiation (in Bragg case only because of the high absorption of the sample) to show the global quality of the crystal. The X-ray beam from a conventional tube in the line focus configuration was collimated to an angular divergence of 180 arcsec in the diffraction plane. This beam illuminated
the 11 mm height of the fiber and the image was collected with a CCD-camera (1128 1256 pixels of 25 25 mm2 each) located at 280 mm of the sample. The crystal was continually rocked in order to allow the diffraction condition to be satisfied along the whole fiber. Unpolarized photoluminescence spectra in backscattering mode were recorded at various positions along the fiber at room temperature with an Ar+ laser at 488 nm as excitation source.
3. Results The prepared pedestals had a high density and mechanical strength. This high density was very important to stabilize the growth process, as cited in earlier work [13]. During the growth process a continuous increase of the laser power was required due to the different fusion points of the compounds GdTaO4 and ErTaO4. The obtained fibers presented excellent transparency and cracks were not observed by optical microscopy in magnifications up to 100 . The diameter varied from 0.7 to 0.9 mm depending of the feeding/pulling ratio. A GdTaO4–ErTaO4 fiber is shown in Fig. 1(b). The diameter fluctuations mainly in the gradient region are attributed to poor laser power control and the continuous variation of composition during the growth experiment. The concentration gradient is easily verified by the change of coloration along the fiber, from colorless at GdTaO4 end to dark pink at the ErTaO4 end. In the several grown fibers, the length of the gradient regions varied from 8 to 12 mm depending of the diameter of the fibers. The axial composition gradient profile of a fiber with length of the gradient region of approximately 9 mm is shown in Fig. 2. The concentration of Gd and Er varied linearly from pure GdTaO4 to pure ErTaO4. The axial Gd–Er composition gradient was of approximately 12 at%/mm. The fibers crystallized in monoclinic structure (I2/a space group), with measured lattice parameters of a ¼ (5.4070.01) A˚, b ¼ (11.0770.03) A˚, c ¼ (5.087 0.01) A˚, b ¼ (95.870.1)1 and a ¼ (5.3070.01) A˚, b ¼ (10.9070.01) A˚, c ¼ (5.0470.01) A˚, b ¼ (95.570.2)1 for GdTaO4 and ErTaO4 respectively, in accordance to reported values in literature [15].
Fig. 1. (a) Schematic of composite pedestal of Gd2O3+Ta2O5 and Er2O3+Ta2O5 and (b) photo of a GdTaO4–ErTaO4 gradient fiber. The distance between two adjacent traces is 1 mm.
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Fig. 2. Axial relative composition profiles of Gd, Er and Ta in GdTaO4–ErTaO4 single crystal fiber.
449
seen, mainly at the ends where only the pure compounds are present. In the middle region some of profile presented subsctructures evidencing the poorer crystalline quality in this region. It can clearly be seen how the Bragg-angle changes along the crystal, corresponding to the lattice parameters changes as expected. The results of XRCprofiles as a function of position the z-position (position z ¼ 0 mm indicates the end with GdTaO4) allowed a plot and calculation of the relative variation Dd/dav as function of z-position where Dd is the variation of the interplanar spacing and dav the average interplanar distance. The results obtained for the (4 4¯ 4) reflection are shown in Fig. 4. The linear fitting in the gradient region gave an interplanar spacing gradient Dd/dav/z ¼ 1.24%/cm. This value is intermediate to those reported in literature for others gradient crystals [19,21,22]. In Fig. 5 the XRDT image of the (4 4¯ 4) reflection to characterize the global quality of the fiber is shown. The image was obtained by rocking the fiber enabling all the crystal carried out the Bragg diffraction condition. The diffractions of both Cu-Ka1 (more intensity image) and CuKa2 (less intensity image) lines can be observed. The regions without lattice spacing variation (vertical bright lines in both extremes) and the region presenting lattice spacing gradient (tilted long line in middle region) are easily observable. The extreme regions show a higher and more uniform intensity evidencing the higher structural quality where there are only the pure compounds. The observed intensity variations, more notably in the middle region are evidence of a lower structural quality in this region. This occurred probably because of continuous
Fig. 3. Normalized XRC profiles of the (4 4¯ 4) reflection at different zpositions along the fiber axis using a four-bounce monochromator of Ge(1 1 1) in the dispersive set-up (+, +) selecting the Cu-Ka1 radiation. The growth direction of the fiber was (1¯ 1 1).
In our evaluation of the crystalline quality of the fibers, a fiber with the (1¯ 1 1) direction as crystalline orientation of the growth direction was used. The (2 4¯ 2), (4 0 4) and (4 4¯ 4) reflections were used to study the interplanar spacing variation, as well as the structural quality along the fiber. These reflections were selected because diffraction Braggplane is approximately parallel (5.81-off, 61-off and 0.11off, respectively) to the fiber axis. The XRC profiles of these reflections presented well-defined peaks with a mean value of FWHM (excluding the instrumental contribution and the contribution of the gradient itself) of 80, 135 and 105 arcsec for the (2 4¯ 2), (4 0 4) and (4 4¯ 4) reflections, respectively. These values are reasonable when compared to other good quality gradient crystals [19–22]. The XRCprofiles of the (4 4¯ 4) reflection at some positions along the fiber axis are shown in Fig. 3. Well-defined peaks can be
Fig. 4. Relative variation of interplanar distance for the (4 4¯ 4) reflection as a function of the z-position on the fiber. Linear fitting from z ¼ 3 to 11 mm.
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Fig. 5. XRDT of the (4 4¯ 4) reflection with Cu-Ka radiation and with 60 min of exposure time. The fiber was continuously rocked enabling the whole fiber satisfy the Bragg diffraction condition. It can be see the diffractions of both Cu-Ka1 (more intensity image) and Cu-Ka2 (less intensity image) lines showing the global crystalline fiber quality.
Fig. 7. (a) Schematic representation of energy levels of Er3+ in a Gd1xErxTaO4 single crystal and (b) proposed cross-relaxation processes.
Fig. 6. Unpolarized luminescence spectra of Gd1xErxTaO4 single crystal at different positions/concentrations along the fiber axis.
change of the melt composition so altering the conditions of growth as cited above. The samples enabled us to estimate the influence of the Er3+ concentration in Gd1xErxTaO4 (0pxp0) on the luminescence intensity obtained by 488 nm Ar+ laser excitation. In Fig. 6 the luminescence spectra are shown in 500–700 nm range, for several positions along the fiber. The uncertainty in the concentration is because of the laser illuminated an area with a considerable variation of composition due to the high concentration gradient of the samples. The analysis of the spectra can be accom-
plished using the energy diagram of Er3+ ion in crystals (Fig. 7). The 488 nm Ar+ laser excites Er3+ into the 4F7/2 level. Then excited Er3+ decays nonradiatively to the (2H11/ 4 4 F9/2 levels, and undergo the radiative 2, S3/2) and 2 transitions ( H11/2, 4S3/2)-4I15/2 and 4F9/2-4I5/2 originating the green and red luminescence, respectively (Fig. 7(a)). The following features can be noted: the intensity of green luminescence shows a maximum at low concentrations of Er3+ and decreases until concentrations around x ¼ 0.5, remaining approximately constant up to x ¼ 1. Oppositely, the intensity of red luminescence remains approximately constant until x0.5 and then increases reaching its maximum at x ¼ 1. The intensities of the emissions corresponding to the two transitions reflect the relative populations of these two levels. It is proposed that in this system two possible processes populate these two levels (Fig. 7(b)). The (2H11/2, 4S3/2) and the 4F9/2 levels are fed by non-radiative relaxation from these upper levels. As the green and red emissions exceed each other in different range of concentration, there must be some cross-relaxation energy transfer populating these levels. In these processes, energy migrates from a donor to an acceptor in an isolated ion pair, raising the latter ion into a higher energy state. Energy transfer can take place via the two transitions 4I11/2-4I15/2 and 4I11/2-4F7/2 and subsequent relaxation feeds the 4S3/2 level. The 4F9/2 level energy transfer can take place via the transitions 4I11/2-4I15/2 and 4 I13/2-4F9/2. Another cross-relaxation energy transfer process is also possible via the two transitions 2H11/ 4 4 4 2- F9/2 and I11/2- F9/2. These two latter energy transfer processes include non-resonant transition. So these processes have to be accomplished by multi-phonon absorp-
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tion and/or emission to conserve energy. As the energy mismatch between the two transitions is on the order to the phonon energy in oxide lattices, these processes are reasonable. Considering just these three cross-relaxation processes that result in populating of the two levels, it is clearly seen that they depend in a different manner from the Er3+ concentration. This may be due to the fact that the energy transfer processes involved in the two mechanisms are of different nature (i.e. dipole–dipole, dipole–quadrupole, etc.). Additional measurements based on (1) lifetime of the emissions, (2) a study of the kinetics of the processes, and (3) the dependence of temperature are required for verifying the phonon assisted cross-relaxation processes and a complete understanding of the system. Such studies are currently in progress. 4. Conclusion Single crystal fibers with controlled gradient of concentration and lattice parameters of GdTaO4–ErTaO4 system have been successfully obtained using the LHPG technique using an approach which does not require thermal treatment of the reagents or pedestals. The approach is based in previous works [14,16]. It is a fast and inexpensive method and can be applied to a variety of oxides systems. The fibers obtained have good structural quality and the value of the interplanar spacing gradient of 1.24%/cm is a high value compared to others concentration gradient fibers reported in literature (around 1.1%/cm in (K,Rb)C8H5O4 gradient crystals [19], 0.8%/cm in Bi–Sb crystals [21] and 1.65%/cm in CaMoO4–SrMoO4 crystals [16]). Luminescent characterization showed the behavior of the intensity of green and red luminescence (by 488 nm pumping) as a function of concentration of Er3+ and three processes populating the levels (2H11/2, 4S3/2) and 4F9/2 by cross-relaxation energy transfer has been proposed. The results show that the GdTaO4–ErTaO4 system is a promising system for X-ray optics applications. In addition, the use of the controlled gradient of concentration shows that Gd1xErxTaO4 can also be a potential candidate for devices emitting in green for low concentrations of Er3+ and devices emitting in red for high concentrations of Er3+. The verification of this potentiality is in progress with additional measurements for studying the dynamics of the system. This technique can be successfully applied, not only as method for obtaining materials for X-ray applications, like monochromators [21,23,24] and collimators [25], but also as combinatorial method for rapid investigation of properties as a function of concentration by in situ measurements.
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Acknowledgments The authors are grateful to Dr. A.R. Zanatta for luminescence measurements and the Brazilian agencies FAPESP, CAPES and CNPq and PRONEX/F. Araucaria/ CPNq for financial support.
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