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Structure and luminescence spectra of lutetium and yttrium borates synthesized from ammonium nitrate melt
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 537 (2005) 144–148 www.elsevier.com/locate/nima
Structure and luminescence spectra of lutetium and yttrium borates synthesized from ammonium nitrate melt Nikolay V. Klassena,, Semion Z. Shmuraka, Ivan M. Shmyt’koa, Galina K. Strukovaa, Stephen E. Derenzob, Marvin J. Weberb a
Institute of Solid State Physics, Russian Academy of Sciences, 142 432 Chernogolovka, Russian Federation b Lawrence Berkeley National Laboratory, University of California, CA 94720, USA Available online 25 August 2004
Abstract Lutetium and yttrium borates doped with europium, terbium, gadolinium, etc. have been synthesized by dissolving initial oxides and nitrates in ammonium nitrate melt and thermal decomposition of the solvent. Annealings in the range of 500–11001C modified the dimensions of the grains from 2 to 3 nm to more than 100 nm. Significant dependence of the structure of lutetium borate on slight doping with rare earth ions has been found: terbium makes high-temperature vaterite phase preferential at room temperature, whereas europium stabilizes low-temperature calcite phase. Influence of the structure of the borates on the pattern of the luminescence spectra of europium dopant was observed. Possibilities for manufacturing of scintillating lutetium borate ceramics by means of this method of synthesis are discussed. r 2004 Elsevier B.V. All rights reserved. Keywords: Scintillation detectors; Powder processing; Chemical synthesis; Photoluminescence
1. Introduction Borates of rare earth metals doped with cerium demonstrate promising scintillation parameters (high density, rather high light yield and fast decay time of scintillations). Lutetium borate (LuBO3) doped with cerium has particularly interesting characteristics for applications as Corresponding author. Tel.: +7-096-524-9702;
fax: +7-096-524-0701. E-mail address: [email protected] (N.V. Klassen).
radiation detector: density 7.4 g/cm3 for vaterite phase and 6.9 g/cm3 for calcite phase, the light yield and decay time of scintillations about 27,000 photons/MeV and 30 ns, correspondingly. These data were obtained by the measurements of lutetium borate powders [1–4]. But several attempts to obtain lutetium borate in optically transparent single crystalline form were not successful: due to the structural transformation from high-temperature vaterite phase to lowtemperature calcite phase taking place at 13001C, the crystals at room temperature contain multiple
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.254
ARTICLE IN PRESS N.V. Klassen et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 144–148
microcracks which disturb their optical transparency severely. This paper is concerned with the studies of the structure and the luminescence excitation and emission spectra of lutetium and yttrium borates, synthesized from the solutions of the initial compounds in ammonium nitrate melt. The results of these studies demonstrate the possibilities of manufacturing of optically transparent lutetium borate.
2. Methods and results of experiments The basic procedure of the synthesis method consists in dissolving of initial raw materials (oxides or nitrates of constituent metals) in ammonium nitrate melt, which turned out to be a rather effective solvent (its melting point is about 2001C). Subsequent thermal decomposition of the solution in the temperature range 270–3001C leaves a set of combinations of the initial metals with oxygen, which are arranged into the final composition of the necessary complex oxides during further low-temperature annealing (in the temperature range from 500 to 12001C). This method has been developed earlier for the synthesis of high-temperature superconductors [5]. Besides the essential energy profits (due to comparatively low temperatures of all the procedures) it has several qualitative advantages: the synthesis proceeds at the molecular level, which guarantees homogeneous distribution of the components; a wide set of dopants can be introduced into the basic composition; the dimensions of the single crystalline grains can be regulated from several nanometers to tens of micrometers by a proper annealing procedure. Our first experiments on the synthesis of lutetium and yttrium borates by means of this procedure gave rather encouraging results. Two of them seem to be the most important: (a) we have found that slight doping of additional rare earth ions can stabilize one of the lutetium borate phases (either vaterite or calcite) at room temperature giving the possibility to grow the crystals with homogeneous phase distribution; (b) the grains with nanoscopic dimensions can be obtained
145
giving good chances to manufacture high-quality scintillating ceramics as it has been done in the case of yttrium garnet ceramics for the laser light generation [6]. The nanoscopic dimensions of the initial powders for manufacturing of optically transparent ceramics are very important, because in this case the structural non-homogeneities should be less essential than the wavelength of the light and the light scattering by them will be negligible. Lutetium and yttrium borates were obtained either non-doped or doped with europium, gadolinium and terbium. The initial lutetium and boron were used in the form of oxides, yttrium, gadolinium, terbium and europium were used in the form of nitrates. All the initial constituents were characterized by 3N chemical purity. The initial powder constituents were mixed with ammonium nitrate powder (the total concentration of the metal components did not exceed 10%). Then the mixture was melted at 2001C and subjected to the thermolysis by means of heating up to 250–3001C. The ammonium nitrate was decomposed and a complex oxide phase including the basic metals and rare earth dopants was formed. The production of the final homogeneous compound was achieved by annealing in the range of 800–11001C (depending on the kind of the material and the desirable size of its grains). In all the cases described below, the concentrations of the rare earth dopants (Tb, Gd or Eu ) were about 0.5 wt% (0.55, 0.56 and 0.58 mol%, correspondingly), being determined by mixing of the initial powders. The crystalline structures of the materials obtained by this way were studied by means of X-ray diffractometry. The existence of definite phases was determined by the presence of a corresponding set of X-ray angular reflections [7], the relative compositions of the phases were estimated by the integral intensities of these reflections. The accuracy of the values of the concentrations of the phases determined by this way was about 5%. The dimensions of the grains were determined by the analysis of the widths of corresponding reflections. The widening of the reflections was observed when the dimensions of the grains were less than 0.1 mm. We have found
ARTICLE IN PRESS 146
N.V. Klassen et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 144–148
that the dimensions of the grains were increasing gradually from several nanometers to 0.1 mm in the temperature range of the annealings between 500 and 11001C. For characterization of the light emission properties, the spectroscopic studies of the photoluminescence were applied. The spectra of the photoexcited light emission were studied in combination with the spectra of excitation of the photoluminescence. For this purpose, the optical spectrometer supplied with two monochromators (for the light flow exciting the luminescence and for the emitted light) was used. We consider that the most important results of this work are connected with the influence of low concentrations of rare earth dopants (0.5–0.6 mol%) on the phase composition of lutetium borate. The X-ray diffraction diagrams of undoped LuBO3 and that doped with Eu, Gd and Tb after the final annealings at about 11001C and measured at room temperature are shown in Fig. 1. It turned out that lutetium borate doped with 0.6 mol% of Eu contained the calcite phase only (the second diagram from the bottom), whereas the traces of the high-temperature vaterite phase (at the level of 5%) are seen in the undoped material. The second diagram can be used as the etalon one, because it contains the reflections of the calcite phase only. All the additional reflections, seen in the other three diagrams, correspond to the vaterite phase. The analysis of the diffractograms shows that 0.55 mol% of gadolinium result in approximately equal concentrations of the calcite and vaterite phases, whereas 0.5 mol% of terbium make the vaterite phase prevailing at room temperature (80% of the vaterite and 20% only of the calcite) in spite of the fact that the vaterite phase of lutetium borate is known as equilibrium at much higher temperatures (above 13301C). The luminescence spectra of lutetium and yttrium borates doped with europium are presented in Fig. 2. The excitation spectra of the same materials are shown in Fig. 3. All the spectra correspond to the f–f optical transitions in Eu ions and reveal their variations induced by the differences in the structures of these materials. In the
Fig. 1. X-ray diffractograms of lutetium borate powders (nondoped and doped with Eu, Gd, Tb).
luminescence spectrum of yttrium borate, more developed fine structure is observed. This can be explained by bigger amount of permitted optical transitions in this material. The high-energy part (seen at the left-hand side) of the luminescence excitation spectrum in yttrium borate has at least two bands, whereas the left part of the analogous spectrum of lutetium borate has one wide band. More developed structure of the emission and excitation spectra of yttrium borate can be attributed to the differences in the phase structures: lutetium borate has the calcite structure, whereas yttrium borate has the vaterite structure. The assumption that the differences in the optical spectra of f–f transitions in Eu ions are induced by the variations of the structure are confirmed by the observations of gradual modifications of the optical spectra as the function of the annealing temperature with europium. In the frames the wavelengths of the registration of the luminescence are shown.
ARTICLE IN PRESS N.V. Klassen et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 144–148
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3. Conclusions
Fig. 2. Photoluminescence spectra of lutetium and yttrium borates doped with europium (in the frames the wavelengths of the excitation light are shown).
The natural assumption which can be made from the data presented above – strong perturbations of the phase diagrams of lutetium borate by comparatively low concentrations of rare earth ions (gadolinium and terbium especially which induce the strong shift of the phase equilibrium at the room temperature towards vaterite). The results described above have the essential practical importance. On the one hand, stabilization of the vaterite phase at low temperatures can promote growth of optically transparent crystals from the melt. On the other hand, it is possible to make the structural state of the crystal close to the phase transformation by means of an adequate level of doping. The local transformation from the vaterite to calcite phase can be induced in this situation by the absorption of ionizing quanta, resulting in local variation of the optical density. Thus, the trajectories of gamma–quanta can be registered by optical means and solid state analogy of Wilson chamber will be created. One more possibility to obtain optically homogeneous lutetium borate scintillators is connected with the preparation of nanoscopic powders of this material, which can be used for vacuum compacting of scintillation ceramics.
Acknowledgments This work has been supported by CRDF Grant RE-2368-CG-02, RFBR Grant 02-02-96001 and Grant of President of Russia for support of scientific schools NSh—2169.2003.2. References
Fig. 3. Excitation spectra of the photoluminescence of lutetium (bottom) and yttrium (top) borates doped with europium. The wavelengths of the spectral maxima of exicitation are shown in the frames.
[1] M.J. Weber, S.E. Derenzo, C. Dujardin, W.W. Moses, in: P. Dorenbos, C. van Eijk (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications, SCINT 95, Delft University Press, Delft, 1996, p. 325. [2] W.W. Moses, et al., in: Yin Zhiwen, et al. (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications SCINT 97, Shanghai, 1997, pp. 358–3661.
ARTICLE IN PRESS 148
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[3] L. Zhang, et al., in: Yin Zhiwen, et al. (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications SCINT 97, Shanghai, 1997, pp. 303–306. [4] C.W.E. van Eijk, in: Yin Zhiwen, et al. (Eds.), Proceedings of the International Conference on Inorganic Scintilla-
tors and their Applications SCINT 97, Shanghai, 1997, pp. 3–12. [5] G.K. Strukova, et al., Physica C 387 (2003) 359. [6] J. Lu, et al., Appl. Phys. Lett. 77 (2000) 3707. [7] Tables of crystalline structures, 1997 JCPDS—International Centre for Diffraction Data.
Nuclear Instruments and Methods in Physics Research A 537 (2005) 144–148 www.elsevier.com/locate/nima
Structure and luminescence spectra of lutetium and yttrium borates synthesized from ammonium nitrate melt Nikolay V. Klassena,, Semion Z. Shmuraka, Ivan M. Shmyt’koa, Galina K. Strukovaa, Stephen E. Derenzob, Marvin J. Weberb a
Institute of Solid State Physics, Russian Academy of Sciences, 142 432 Chernogolovka, Russian Federation b Lawrence Berkeley National Laboratory, University of California, CA 94720, USA Available online 25 August 2004
Abstract Lutetium and yttrium borates doped with europium, terbium, gadolinium, etc. have been synthesized by dissolving initial oxides and nitrates in ammonium nitrate melt and thermal decomposition of the solvent. Annealings in the range of 500–11001C modified the dimensions of the grains from 2 to 3 nm to more than 100 nm. Significant dependence of the structure of lutetium borate on slight doping with rare earth ions has been found: terbium makes high-temperature vaterite phase preferential at room temperature, whereas europium stabilizes low-temperature calcite phase. Influence of the structure of the borates on the pattern of the luminescence spectra of europium dopant was observed. Possibilities for manufacturing of scintillating lutetium borate ceramics by means of this method of synthesis are discussed. r 2004 Elsevier B.V. All rights reserved. Keywords: Scintillation detectors; Powder processing; Chemical synthesis; Photoluminescence
1. Introduction Borates of rare earth metals doped with cerium demonstrate promising scintillation parameters (high density, rather high light yield and fast decay time of scintillations). Lutetium borate (LuBO3) doped with cerium has particularly interesting characteristics for applications as Corresponding author. Tel.: +7-096-524-9702;
fax: +7-096-524-0701. E-mail address: [email protected] (N.V. Klassen).
radiation detector: density 7.4 g/cm3 for vaterite phase and 6.9 g/cm3 for calcite phase, the light yield and decay time of scintillations about 27,000 photons/MeV and 30 ns, correspondingly. These data were obtained by the measurements of lutetium borate powders [1–4]. But several attempts to obtain lutetium borate in optically transparent single crystalline form were not successful: due to the structural transformation from high-temperature vaterite phase to lowtemperature calcite phase taking place at 13001C, the crystals at room temperature contain multiple
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.07.254
ARTICLE IN PRESS N.V. Klassen et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 144–148
microcracks which disturb their optical transparency severely. This paper is concerned with the studies of the structure and the luminescence excitation and emission spectra of lutetium and yttrium borates, synthesized from the solutions of the initial compounds in ammonium nitrate melt. The results of these studies demonstrate the possibilities of manufacturing of optically transparent lutetium borate.
2. Methods and results of experiments The basic procedure of the synthesis method consists in dissolving of initial raw materials (oxides or nitrates of constituent metals) in ammonium nitrate melt, which turned out to be a rather effective solvent (its melting point is about 2001C). Subsequent thermal decomposition of the solution in the temperature range 270–3001C leaves a set of combinations of the initial metals with oxygen, which are arranged into the final composition of the necessary complex oxides during further low-temperature annealing (in the temperature range from 500 to 12001C). This method has been developed earlier for the synthesis of high-temperature superconductors [5]. Besides the essential energy profits (due to comparatively low temperatures of all the procedures) it has several qualitative advantages: the synthesis proceeds at the molecular level, which guarantees homogeneous distribution of the components; a wide set of dopants can be introduced into the basic composition; the dimensions of the single crystalline grains can be regulated from several nanometers to tens of micrometers by a proper annealing procedure. Our first experiments on the synthesis of lutetium and yttrium borates by means of this procedure gave rather encouraging results. Two of them seem to be the most important: (a) we have found that slight doping of additional rare earth ions can stabilize one of the lutetium borate phases (either vaterite or calcite) at room temperature giving the possibility to grow the crystals with homogeneous phase distribution; (b) the grains with nanoscopic dimensions can be obtained
145
giving good chances to manufacture high-quality scintillating ceramics as it has been done in the case of yttrium garnet ceramics for the laser light generation [6]. The nanoscopic dimensions of the initial powders for manufacturing of optically transparent ceramics are very important, because in this case the structural non-homogeneities should be less essential than the wavelength of the light and the light scattering by them will be negligible. Lutetium and yttrium borates were obtained either non-doped or doped with europium, gadolinium and terbium. The initial lutetium and boron were used in the form of oxides, yttrium, gadolinium, terbium and europium were used in the form of nitrates. All the initial constituents were characterized by 3N chemical purity. The initial powder constituents were mixed with ammonium nitrate powder (the total concentration of the metal components did not exceed 10%). Then the mixture was melted at 2001C and subjected to the thermolysis by means of heating up to 250–3001C. The ammonium nitrate was decomposed and a complex oxide phase including the basic metals and rare earth dopants was formed. The production of the final homogeneous compound was achieved by annealing in the range of 800–11001C (depending on the kind of the material and the desirable size of its grains). In all the cases described below, the concentrations of the rare earth dopants (Tb, Gd or Eu ) were about 0.5 wt% (0.55, 0.56 and 0.58 mol%, correspondingly), being determined by mixing of the initial powders. The crystalline structures of the materials obtained by this way were studied by means of X-ray diffractometry. The existence of definite phases was determined by the presence of a corresponding set of X-ray angular reflections [7], the relative compositions of the phases were estimated by the integral intensities of these reflections. The accuracy of the values of the concentrations of the phases determined by this way was about 5%. The dimensions of the grains were determined by the analysis of the widths of corresponding reflections. The widening of the reflections was observed when the dimensions of the grains were less than 0.1 mm. We have found
ARTICLE IN PRESS 146
N.V. Klassen et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 144–148
that the dimensions of the grains were increasing gradually from several nanometers to 0.1 mm in the temperature range of the annealings between 500 and 11001C. For characterization of the light emission properties, the spectroscopic studies of the photoluminescence were applied. The spectra of the photoexcited light emission were studied in combination with the spectra of excitation of the photoluminescence. For this purpose, the optical spectrometer supplied with two monochromators (for the light flow exciting the luminescence and for the emitted light) was used. We consider that the most important results of this work are connected with the influence of low concentrations of rare earth dopants (0.5–0.6 mol%) on the phase composition of lutetium borate. The X-ray diffraction diagrams of undoped LuBO3 and that doped with Eu, Gd and Tb after the final annealings at about 11001C and measured at room temperature are shown in Fig. 1. It turned out that lutetium borate doped with 0.6 mol% of Eu contained the calcite phase only (the second diagram from the bottom), whereas the traces of the high-temperature vaterite phase (at the level of 5%) are seen in the undoped material. The second diagram can be used as the etalon one, because it contains the reflections of the calcite phase only. All the additional reflections, seen in the other three diagrams, correspond to the vaterite phase. The analysis of the diffractograms shows that 0.55 mol% of gadolinium result in approximately equal concentrations of the calcite and vaterite phases, whereas 0.5 mol% of terbium make the vaterite phase prevailing at room temperature (80% of the vaterite and 20% only of the calcite) in spite of the fact that the vaterite phase of lutetium borate is known as equilibrium at much higher temperatures (above 13301C). The luminescence spectra of lutetium and yttrium borates doped with europium are presented in Fig. 2. The excitation spectra of the same materials are shown in Fig. 3. All the spectra correspond to the f–f optical transitions in Eu ions and reveal their variations induced by the differences in the structures of these materials. In the
Fig. 1. X-ray diffractograms of lutetium borate powders (nondoped and doped with Eu, Gd, Tb).
luminescence spectrum of yttrium borate, more developed fine structure is observed. This can be explained by bigger amount of permitted optical transitions in this material. The high-energy part (seen at the left-hand side) of the luminescence excitation spectrum in yttrium borate has at least two bands, whereas the left part of the analogous spectrum of lutetium borate has one wide band. More developed structure of the emission and excitation spectra of yttrium borate can be attributed to the differences in the phase structures: lutetium borate has the calcite structure, whereas yttrium borate has the vaterite structure. The assumption that the differences in the optical spectra of f–f transitions in Eu ions are induced by the variations of the structure are confirmed by the observations of gradual modifications of the optical spectra as the function of the annealing temperature with europium. In the frames the wavelengths of the registration of the luminescence are shown.
ARTICLE IN PRESS N.V. Klassen et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 144–148
147
3. Conclusions
Fig. 2. Photoluminescence spectra of lutetium and yttrium borates doped with europium (in the frames the wavelengths of the excitation light are shown).
The natural assumption which can be made from the data presented above – strong perturbations of the phase diagrams of lutetium borate by comparatively low concentrations of rare earth ions (gadolinium and terbium especially which induce the strong shift of the phase equilibrium at the room temperature towards vaterite). The results described above have the essential practical importance. On the one hand, stabilization of the vaterite phase at low temperatures can promote growth of optically transparent crystals from the melt. On the other hand, it is possible to make the structural state of the crystal close to the phase transformation by means of an adequate level of doping. The local transformation from the vaterite to calcite phase can be induced in this situation by the absorption of ionizing quanta, resulting in local variation of the optical density. Thus, the trajectories of gamma–quanta can be registered by optical means and solid state analogy of Wilson chamber will be created. One more possibility to obtain optically homogeneous lutetium borate scintillators is connected with the preparation of nanoscopic powders of this material, which can be used for vacuum compacting of scintillation ceramics.
Acknowledgments This work has been supported by CRDF Grant RE-2368-CG-02, RFBR Grant 02-02-96001 and Grant of President of Russia for support of scientific schools NSh—2169.2003.2. References
Fig. 3. Excitation spectra of the photoluminescence of lutetium (bottom) and yttrium (top) borates doped with europium. The wavelengths of the spectral maxima of exicitation are shown in the frames.
[1] M.J. Weber, S.E. Derenzo, C. Dujardin, W.W. Moses, in: P. Dorenbos, C. van Eijk (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications, SCINT 95, Delft University Press, Delft, 1996, p. 325. [2] W.W. Moses, et al., in: Yin Zhiwen, et al. (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications SCINT 97, Shanghai, 1997, pp. 358–3661.
ARTICLE IN PRESS 148
N.V. Klassen et al. / Nuclear Instruments and Methods in Physics Research A 537 (2005) 144–148
[3] L. Zhang, et al., in: Yin Zhiwen, et al. (Eds.), Proceedings of the International Conference on Inorganic Scintillators and their Applications SCINT 97, Shanghai, 1997, pp. 303–306. [4] C.W.E. van Eijk, in: Yin Zhiwen, et al. (Eds.), Proceedings of the International Conference on Inorganic Scintilla-
tors and their Applications SCINT 97, Shanghai, 1997, pp. 3–12. [5] G.K. Strukova, et al., Physica C 387 (2003) 359. [6] J. Lu, et al., Appl. Phys. Lett. 77 (2000) 3707. [7] Tables of crystalline structures, 1997 JCPDS—International Centre for Diffraction Data.