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Exciton luminescence of scheelite- and raspite-structured PbWO4 crystals
Journal of Luminescence 87}89 (2000) 1243}1245
Exciton luminescence of scheelite- and raspite-structured PbWO crystals Minoru Itoh *, Dmitri L. Alov, Masami Fujita
Department of Electrical and Electronic Engineering, Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow 142432, Russia Maritime Safety Academy, Wakaba, Kure 737-8512, Japan
Abstract The re#ection and luminescence spectra of scheelite- and raspite-structured PbWO crystals have been studied at 6 K with use of synchrotron radiation as a light source. An intense re#ection band due to excitons is found at 4.25 eV for the former and at 3.72 eV for the latter. The scheelite and raspite samples emit a single luminescence peaking at 2.80 eV (`bluea band) and at 2.25 eV (`greena band), respectively, when they are excited with photons in the region of the fundamental absorption including the lowest exciton band. The 2.80-eV luminescence is likely due to radiative decay of the excitons localized on tetrahedral WO groups, while the 2.25-eV luminescence is ascribed to the excitons localized on octahedral WO groups. The present result will resolve a long-standing dispute about the origin of the `bluea and `greena emission bands in PbWO . 2000 Elsevier Science B.V. All rights reserved. Keywords: Exciton luminescence; Lead tungstate; Scintillator
1. Introduction Despite long history of the studies on tungstate phosphors since the 1940s [1], there remain some open problems with the luminescence properties of these materials. Because lead tungstate (PbWO ) was recently chosen as a scintillating substance for electromagnetic calorimeter of Large Hadron Collider at CERN, the solution of these problems now becomes particularly important. One of the problems on PbWO crystals is the fact that the luminescence characteristics are very sensitive to the speci"c conditions of their synthesis. In general, two emission bands, called `bluea and `greena bands, are detected, depending on the photon energy of exciting light or the temperature of sample. The nature of these emission bands is still in dispute (see, for example, Ref. [2]), mainly because of the complex luminescence data. The lead tungstate is a quite unique material because it has two di!erent structural modi"cations under normal * Corresponding author. Fax: #81-26-269-5572. E-mail address: [email protected] (M. Itoh)
conditions; scheelite- and raspite-type. In the present study, the re#ection and luminescence spectra due to excitons in these two types of crystals are examined at low temperatures. The obtained results provide clear evidence for the origin of the `bluea and `greena emission bands in PbWO . 2. Experiment A large ingot of scheelite crystal was grown by the Czochralski method. By referring to their luminescence characteristics, the scheelite samples (&3;3;2 mm in size) used in the present study were carefully selected from a lot of single crystals cleaved from the ingot. Raspite crystals have not yet been successfully prepared in the laboratory. The studied sample was a natural crystal (&1;0.2;0.2 mm in size) found in Broken Hill, Australia. Each structure of the scheelite and raspite samples was con"rmed by the X-ray analysis. The experiments were performed at beam line 1B of the storage ring UVSOR in Okazaki. This beam line
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 2 8 - 1
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M. Itoh et al. / Journal of Luminescence 87}89 (2000) 1243}1245
provided us with vacuum ultraviolet light having a spot size of about 1 mm in diameter. Photoluminescence emitted from the sample was analyzed by means of a 0.32-m grating monochromator equipped with a CCD camera or a PM tube. The luminescence spectra reported here were corrected for the dispersion of the analyzing monochromator and for the spectral response of the detection system.
3. Results A solid curve of Fig. 1 represents the re#ection spectrum of a scheelite crystal measured at 6 K. This spectrum is similar to the recent result in Ref. [3], although our resolution is better. The solid curve in the inset of Fig. 1 shows the re#ection spectrum in the exciton-band region on an expanded scale. One can see a sharp exciton peak at 4.25 eV and a high-energy hump at 4.38 eV. The doublet structure appearing around 22 eV is attributed to the Pb 5dP6p transitions. We observed, for the "rst time, the re#ection spectrum of a raspite crystal. The result obtained at 6 K is presented by a solid curve in Fig. 2. As shown in the inset of Fig. 2, there exist a broad band at 4.18 eV and a weak low-energy peak at 3.72 eV. The 3.72-eV peak is probably due to the exciton transition of this material. The scheelite and raspite samples emit a single luminescence peaking at 2.80 eV (`bluea band) and at
Fig. 2. Re#ection spectrum (solid curve) of a raspite PbWO crystal measured at 6 K. Excitation spectrum for the 2.25-eV luminescence is also shown by a dotted curve. The inset shows these two spectra in the exciton-band region on an expanded scale. The peak intensities of both curves in the exciton-band region have been normalized to unity.
Fig. 3. Luminescence spectra of scheelite- and raspite-structured PbWO crystals, excited at 6 K with 4.35- and 4.10-eV photons, respectively. Each curve has been normalized at the maximum.
Fig. 1. Re#ection spectrum (solid curve) of a scheelite PbWO crystal measured at 6 K. Excitation spectrum for the 2.80-eV luminescence is also shown by a dotted curve. The inset shows these two spectra in the exciton-band region on an expanded scale. The peak intensities of both curves in the exciton-band region have been normalized to unity.
2.25 eV (`greena band), respectively, when they are excited at 6 K with photons in the region of the exciton band. The emission bands of both crystals are shown in Fig. 3. Their line shapes are approximated by a Gaussian curve, and do not depend on the photon energy of the exciting light used in this experiment. The emission intensities of the 2.25- and 2.80-eV bands were comparable with each other under photo-excitation in the exciton-band region. It should be noted that our scheelite sample is a quite rare one because it demonstrates only the `bluea luminescence in the whole temperature range up to 290 K. The excitation spectra for the 2.80- and 2.25-eV
M. Itoh et al. / Journal of Luminescence 87}89 (2000) 1243}1245
bands are shown by dotted curves in Figs. 1 and 2, respectively. 4. Discussion The monoclinic form of PbWO (raspite-type) can be treated as a distortion of the tetragonal form of PbWO (scheelite-type). In the scheelite structure, W atoms are in tetrahedral O-atom cages and isolated from each other, whereas in raspite, two W atoms share two O atoms to form a chain of edge-shared octahedra [4]. The volumes of the unit cells of these two structures are very close; the di!erence is less than 1%. In spite of the di!erence in the cation coordination, the packing of the O atoms can be regarded as essentially the same in both structures. A recent electronic band structure calculation of scheelite PbWO [5] has indicated that the valence and conduction bands are predominantly built up of molecular orbitals from the O 2p and W 5d states in the WO\ clusters, respectively. The Pb 6s and 6p states also hybridize with the valence and conduction bands, respectively. It is worth noting that the re#ection spectrum between 5 and 12 eV in Fig. 1 is satisfactorily consistent with the total density-of-states calculated in Ref. [5]. Because of the close similarity of the crystal structures, the electronic band structure of raspite PbWO is ex pected not to be so di!erent from that of scheelite crystals. The lowest exciton peaks in Figs. 1 and 2 would be ascribed to the transitions from the O 2pp (and Pb 6s) states to the W 5d states. The complexity of the electronic band structure near the band edge in PbWO may ex plain the appearance of the sub-structures in the exciton-band regions of Figs. 1 and 2. Now let us discuss the origin of the 2.80- and 2.25-eV emission bands. From their excitation spectra in Figs. 1 and 2, it is obvious that both bands are stimulated with photons above the onset of the lowest exciton re#ection bands. This indicates that the 2.80- and 2.25-eV bands are an intrinsic feature of the scheelite and raspite crystals, respectively. These two bands are strongly excited in the region of the exciton band. According to Ref. [5], the exciton is composed of a hole in the O 2pp orbitals with small contribution of the Pb 6s state and an electron in the W 5d orbitals. Therefore, we suggest that the 2.80-eV (`bluea) luminescence in scheelite crystals is ascribed to radiative annihilation of the excitons localized (self-trapped) on regular WO groups in which four oxygen ions form the tetrahedron with a tungsten ion at
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the center. On the other hand, the 2.25-eV (`greena) luminescence in raspite crystals is likely due to the excitons localized on octahedral WO groups in which a tungsten ion is surrounded by six oxygen ions. It is reasonable to suppose that the di!erence in molecular structure between WO and WO groups results in the di!erent relaxed con"gurations of the excitons generated in scheelite- and raspite-structured PbWO crystals [6], leading to the di!erence in magnitude of a Stokes shift. From the above results, it is strongly suggested that the raspite-type inclusions exist in synthetic lead tungstate crystals, which usually crystallize in a scheelite-type structure. These inclusions probably result from the thermal stress that is unavoidably introduced in large-volume crystals grown by using the melting technique. This causes the complex luminescence properties due to the coexistence of the `bluea and `greena bands in usual PbWO crystals prepared synthetically. It is well known [3] that the luminescence e$ciency of PbWO is con siderably low. We suppose that the local deformation due to octahedral inclusions is partly responsible for such low e$ciency, but not a main reason for that.
Acknowledgements The authors would like to thank Mr. J. Murakami and Mr. S. Matsumoto for their assistance in the experiment. This work was performed under the Joint Studies Program of the Institute for Molecular Science.
References [1] F.A. KroK ger, in: Some Aspects of the Luminescence in Solids, Elsevier, Amsterdam, 1948. [2] P. Dorenbos, C.W.E. van Eijk (Eds.), Proceedings of the International Conference on Inorganic Scintillators and Their Applications, Delft University, Delft, 1996, Part IV. [3] A.N. Belsky, V.V. Mikhailin, A.N. Vasil'ev, I. Da"nei, P. Lecoq, C. Pedrini, P. Chevallier, P. Dhez, P. Martin, Chem. Phys. Lett. 243 (1995) 552. [4] T. Fujita, I. Kawada, K. Kato, Acta Crystallogr. B 33 (1977) 162. [5] Y. Zhang, N.A.W. Holzwarth, R.T. Williams, Phys. Rev. B 57 (1998) 12 738. [6] D.L. Alov, S.I. Rybchenko, Mater. Sci. Forum 239}241 (1997) 279.
Exciton luminescence of scheelite- and raspite-structured PbWO crystals Minoru Itoh *, Dmitri L. Alov, Masami Fujita
Department of Electrical and Electronic Engineering, Faculty of Engineering, Shinshu University, Wakasato 4-17-1, Nagano 380-8553, Japan Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow 142432, Russia Maritime Safety Academy, Wakaba, Kure 737-8512, Japan
Abstract The re#ection and luminescence spectra of scheelite- and raspite-structured PbWO crystals have been studied at 6 K with use of synchrotron radiation as a light source. An intense re#ection band due to excitons is found at 4.25 eV for the former and at 3.72 eV for the latter. The scheelite and raspite samples emit a single luminescence peaking at 2.80 eV (`bluea band) and at 2.25 eV (`greena band), respectively, when they are excited with photons in the region of the fundamental absorption including the lowest exciton band. The 2.80-eV luminescence is likely due to radiative decay of the excitons localized on tetrahedral WO groups, while the 2.25-eV luminescence is ascribed to the excitons localized on octahedral WO groups. The present result will resolve a long-standing dispute about the origin of the `bluea and `greena emission bands in PbWO . 2000 Elsevier Science B.V. All rights reserved. Keywords: Exciton luminescence; Lead tungstate; Scintillator
1. Introduction Despite long history of the studies on tungstate phosphors since the 1940s [1], there remain some open problems with the luminescence properties of these materials. Because lead tungstate (PbWO ) was recently chosen as a scintillating substance for electromagnetic calorimeter of Large Hadron Collider at CERN, the solution of these problems now becomes particularly important. One of the problems on PbWO crystals is the fact that the luminescence characteristics are very sensitive to the speci"c conditions of their synthesis. In general, two emission bands, called `bluea and `greena bands, are detected, depending on the photon energy of exciting light or the temperature of sample. The nature of these emission bands is still in dispute (see, for example, Ref. [2]), mainly because of the complex luminescence data. The lead tungstate is a quite unique material because it has two di!erent structural modi"cations under normal * Corresponding author. Fax: #81-26-269-5572. E-mail address: [email protected] (M. Itoh)
conditions; scheelite- and raspite-type. In the present study, the re#ection and luminescence spectra due to excitons in these two types of crystals are examined at low temperatures. The obtained results provide clear evidence for the origin of the `bluea and `greena emission bands in PbWO . 2. Experiment A large ingot of scheelite crystal was grown by the Czochralski method. By referring to their luminescence characteristics, the scheelite samples (&3;3;2 mm in size) used in the present study were carefully selected from a lot of single crystals cleaved from the ingot. Raspite crystals have not yet been successfully prepared in the laboratory. The studied sample was a natural crystal (&1;0.2;0.2 mm in size) found in Broken Hill, Australia. Each structure of the scheelite and raspite samples was con"rmed by the X-ray analysis. The experiments were performed at beam line 1B of the storage ring UVSOR in Okazaki. This beam line
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 5 2 8 - 1
1244
M. Itoh et al. / Journal of Luminescence 87}89 (2000) 1243}1245
provided us with vacuum ultraviolet light having a spot size of about 1 mm in diameter. Photoluminescence emitted from the sample was analyzed by means of a 0.32-m grating monochromator equipped with a CCD camera or a PM tube. The luminescence spectra reported here were corrected for the dispersion of the analyzing monochromator and for the spectral response of the detection system.
3. Results A solid curve of Fig. 1 represents the re#ection spectrum of a scheelite crystal measured at 6 K. This spectrum is similar to the recent result in Ref. [3], although our resolution is better. The solid curve in the inset of Fig. 1 shows the re#ection spectrum in the exciton-band region on an expanded scale. One can see a sharp exciton peak at 4.25 eV and a high-energy hump at 4.38 eV. The doublet structure appearing around 22 eV is attributed to the Pb 5dP6p transitions. We observed, for the "rst time, the re#ection spectrum of a raspite crystal. The result obtained at 6 K is presented by a solid curve in Fig. 2. As shown in the inset of Fig. 2, there exist a broad band at 4.18 eV and a weak low-energy peak at 3.72 eV. The 3.72-eV peak is probably due to the exciton transition of this material. The scheelite and raspite samples emit a single luminescence peaking at 2.80 eV (`bluea band) and at
Fig. 2. Re#ection spectrum (solid curve) of a raspite PbWO crystal measured at 6 K. Excitation spectrum for the 2.25-eV luminescence is also shown by a dotted curve. The inset shows these two spectra in the exciton-band region on an expanded scale. The peak intensities of both curves in the exciton-band region have been normalized to unity.
Fig. 3. Luminescence spectra of scheelite- and raspite-structured PbWO crystals, excited at 6 K with 4.35- and 4.10-eV photons, respectively. Each curve has been normalized at the maximum.
Fig. 1. Re#ection spectrum (solid curve) of a scheelite PbWO crystal measured at 6 K. Excitation spectrum for the 2.80-eV luminescence is also shown by a dotted curve. The inset shows these two spectra in the exciton-band region on an expanded scale. The peak intensities of both curves in the exciton-band region have been normalized to unity.
2.25 eV (`greena band), respectively, when they are excited at 6 K with photons in the region of the exciton band. The emission bands of both crystals are shown in Fig. 3. Their line shapes are approximated by a Gaussian curve, and do not depend on the photon energy of the exciting light used in this experiment. The emission intensities of the 2.25- and 2.80-eV bands were comparable with each other under photo-excitation in the exciton-band region. It should be noted that our scheelite sample is a quite rare one because it demonstrates only the `bluea luminescence in the whole temperature range up to 290 K. The excitation spectra for the 2.80- and 2.25-eV
M. Itoh et al. / Journal of Luminescence 87}89 (2000) 1243}1245
bands are shown by dotted curves in Figs. 1 and 2, respectively. 4. Discussion The monoclinic form of PbWO (raspite-type) can be treated as a distortion of the tetragonal form of PbWO (scheelite-type). In the scheelite structure, W atoms are in tetrahedral O-atom cages and isolated from each other, whereas in raspite, two W atoms share two O atoms to form a chain of edge-shared octahedra [4]. The volumes of the unit cells of these two structures are very close; the di!erence is less than 1%. In spite of the di!erence in the cation coordination, the packing of the O atoms can be regarded as essentially the same in both structures. A recent electronic band structure calculation of scheelite PbWO [5] has indicated that the valence and conduction bands are predominantly built up of molecular orbitals from the O 2p and W 5d states in the WO\ clusters, respectively. The Pb 6s and 6p states also hybridize with the valence and conduction bands, respectively. It is worth noting that the re#ection spectrum between 5 and 12 eV in Fig. 1 is satisfactorily consistent with the total density-of-states calculated in Ref. [5]. Because of the close similarity of the crystal structures, the electronic band structure of raspite PbWO is ex pected not to be so di!erent from that of scheelite crystals. The lowest exciton peaks in Figs. 1 and 2 would be ascribed to the transitions from the O 2pp (and Pb 6s) states to the W 5d states. The complexity of the electronic band structure near the band edge in PbWO may ex plain the appearance of the sub-structures in the exciton-band regions of Figs. 1 and 2. Now let us discuss the origin of the 2.80- and 2.25-eV emission bands. From their excitation spectra in Figs. 1 and 2, it is obvious that both bands are stimulated with photons above the onset of the lowest exciton re#ection bands. This indicates that the 2.80- and 2.25-eV bands are an intrinsic feature of the scheelite and raspite crystals, respectively. These two bands are strongly excited in the region of the exciton band. According to Ref. [5], the exciton is composed of a hole in the O 2pp orbitals with small contribution of the Pb 6s state and an electron in the W 5d orbitals. Therefore, we suggest that the 2.80-eV (`bluea) luminescence in scheelite crystals is ascribed to radiative annihilation of the excitons localized (self-trapped) on regular WO groups in which four oxygen ions form the tetrahedron with a tungsten ion at
1245
the center. On the other hand, the 2.25-eV (`greena) luminescence in raspite crystals is likely due to the excitons localized on octahedral WO groups in which a tungsten ion is surrounded by six oxygen ions. It is reasonable to suppose that the di!erence in molecular structure between WO and WO groups results in the di!erent relaxed con"gurations of the excitons generated in scheelite- and raspite-structured PbWO crystals [6], leading to the di!erence in magnitude of a Stokes shift. From the above results, it is strongly suggested that the raspite-type inclusions exist in synthetic lead tungstate crystals, which usually crystallize in a scheelite-type structure. These inclusions probably result from the thermal stress that is unavoidably introduced in large-volume crystals grown by using the melting technique. This causes the complex luminescence properties due to the coexistence of the `bluea and `greena bands in usual PbWO crystals prepared synthetically. It is well known [3] that the luminescence e$ciency of PbWO is con siderably low. We suppose that the local deformation due to octahedral inclusions is partly responsible for such low e$ciency, but not a main reason for that.
Acknowledgements The authors would like to thank Mr. J. Murakami and Mr. S. Matsumoto for their assistance in the experiment. This work was performed under the Joint Studies Program of the Institute for Molecular Science.
References [1] F.A. KroK ger, in: Some Aspects of the Luminescence in Solids, Elsevier, Amsterdam, 1948. [2] P. Dorenbos, C.W.E. van Eijk (Eds.), Proceedings of the International Conference on Inorganic Scintillators and Their Applications, Delft University, Delft, 1996, Part IV. [3] A.N. Belsky, V.V. Mikhailin, A.N. Vasil'ev, I. Da"nei, P. Lecoq, C. Pedrini, P. Chevallier, P. Dhez, P. Martin, Chem. Phys. Lett. 243 (1995) 552. [4] T. Fujita, I. Kawada, K. Kato, Acta Crystallogr. B 33 (1977) 162. [5] Y. Zhang, N.A.W. Holzwarth, R.T. Williams, Phys. Rev. B 57 (1998) 12 738. [6] D.L. Alov, S.I. Rybchenko, Mater. Sci. Forum 239}241 (1997) 279.