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Electron and hole trapping in PbCl2 and PbCl2:Tl crystals
Nuclear Instruments and Methods in Physics Research B 141 (1998) 538±541
Electron and hole trapping in PbCl2 and PbCl2 :Tl crystals S.V. Nistor
a,b,*
, E. Goovaerts a, M. Stefan b, D. Schoemaker
a
a
b
Physics Department, University of Antwerp (U.I.A.), B-2610 Antwerpen-Wilrijk, Belgium Institute of Atomic Physics (N.I.M.P.), P.O. Box MG 7 Magurele, Bucuresti 76900, Romania
Abstract Formation of primary paramagnetic point defects under low temperature X-ray irradiation have been studied by ESR and optical absorption in pure and thallium doped PbCl2 single crystals. Besides Pb3 self-trapped electron 2 (STEL) centers the PbCl2 :Tl crystals exhibit trapped-electron (PbTl) -type centers. Based on production properties of paramagnetic centers it is suggested that besides forming Tl2 centers the holes are self trapped at pairs of neighbouring Clÿ anions resulting in Vk type centers with various orientation and length of the Cl±Cl axis. Ó 1998 Published by Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 76.30.Mi; 65.70.+y Keywords: ESR; Paramagnetic; Centers; Irradiation; Electron; Hole; Self-trapping
1. Introduction Single crystals of PbCl2 exhibit under UV excitation at room temperatures photochemical changes resulting in formation of colloidal lead and desorption of halogen [1]. The primary defect involved in the photolytic formation of lead colloids has not yet been clearly determined, although it has been suggested that the ®rst step in this process involves the trapping of electrons at lead cations [1,2]. Early Electron Spin Resonance (ESR) studies on low temperature UV, X-ray or c-ray irradiated PbCl2 single crystals resulted in the observation of the so-called A, B and C paramagnetic centers with g 2. The identi®cation of the *
Corresponding author.
A spectrum, as being due to trapped electron Pb and Pb3 2 centers [3], disagrees with the results of more recent, extensive studies [4,5] on such centers in alkali halides. The identi®cation of the socalled C spectrum as some sort of anion trappedhole center does seem very unlikely as well. Recently we have shown [6] that conduction electrons produced by illumination above the band gap in pure PbCl2 crystals are trapped at pairs of nearest-neighbor Pb2 cations along the a axis, resulting in paramagnetic self-trapped-electron (STEL) centers. The STEL center represents a paramagnetic Pb3 2 molecular ion with a strongly bent molecular bond and electron con®guration complementary to that of the Xÿ 2 (X halogen) self-trapped hole (STH) center in alkali halides [7]. Its observation suggests that electron self-trapping may represent the initial step in the formation
0168-583X/98/$19.00 Ó 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 0 5 1 - 2
S.V. Nistor et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 538±541
539
of lead clusters by photochemical decomposition of lead halides at temperatures where diusion of anion vacancies occurs. Here we present the results of an ESR and optical study concerning the electron and hole trapping properties of low temperature irradiated thallium doped PbCl2 crystals. It reports the simultaneous formation, besides the already known [8] trapped-hole Tl2 centers of a new heteronuclear dimer type (PbTl)2 trapped electron center with bent structure. Moreover, it suggests that the Atype ESR spectrum is in fact due to the presence of several Vk type hole self-trapped centers with slightly dierent spin-Hamiltonian parameters.
ment and procedures employed in the previous studies [5,6]. As in the case of pure PbCl2 crystals strong ESR spectra are produced after relatively short (typically 20 min) X-ray irradiation at T 80 K. Besides the lines attributed to STEL centers [6] and to Tl2 centers [8], several additional strongly anisotropic lines with identical production/bleaching properties were observed. A careful analysis [9] of the ESR transitions recorded with the magnetic ®eld rotated in the three main crystal planes shows that the EPR spectrum is described by the following spin-Hamiltonian:
2. Trapped-electron centers in PbCl2 :Tl
with S I1 I2 1=2. The spin-Hamiltonian Eq. (1) fully describes the ESR spectrum of a dimer type paramagnetic center (PbTl)2 containing pairs of Tl and Pb2 ions, called (PbTl)2 which have trapped an electron in a bonding molecular orbital. The examination of (PbTl)2 center's ESR parameters (Tables 1 and 2) strongly suggests that a strong bending of its molecular bond is also pres-
1 1 H H g S S A Tl I1 SA Pb I2 g0 lB g0
The samples employed in the present study were cut from oxygen-free PbCl2 single crystals doped in the melt with either 0.1 or 0.2 mol% TlCl, prepared by earlier described procedures [8]. The irradiation as well as the optical and ESR measurements were performed with the same equip-
1
Table 1 Principal values and orientation (in degrees) of the g tensors, as the Euler angles for a Rzy0 z00 a; b; c rotation from the cba crystal axes, for the dimer PbTl2 and Pb3 2 (STEL) centers in PbCl2 Center
T (K)
gx
gy
gz
a
b
c
(PbTl)2 in PbCl2 a STEL in PbCl2 b
20 10
1.083 1.550
0.994 1.374
1.431 1.719
24 0
36 0
)12 10
a b
Estimated experimental errors in the g components and orientation angles are smaller than 0:002 and 2, respectively. From Ref. [6]
Table 2 Principal values (in mT, absolute values) and orientation (in degrees) of the A(Tl) and A(Pb) hf tensors, given as the Euler angles of a Rzy0 z00 ai ; bi ; ci rotation from the cba crystal axes of the (even PbTl)2 and (207 PbTl)2 dimer centers, respectively, in comparison with those of the bent Pb3 2 (STEL) center in PbCl2 Center
T (K)
Ax
Ay
Az
ai
bi
ci
(even PbTl)2 in PbCl2 a (207 PbTl)2 in PbCl2 a STEL in PbCl2 b
20 20 10
65.9 266.6 )82.8
64.5 273.6 )85.3
85.5 223.5 111.7
47.8 41 0
)4.5 32 33
)33.9 )44 0
a b
Estimated experimental errors in Ax;y;z and bi are smaller than 0:4 and 2, respectively. Data for 207 A(Pb) from Ref. [6].
540
S.V. Nistor et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 538±541
ent. However, contrary to the STEL center, which has the gz component along the a crystal direction, parallel to the axis connecting the two Pb nuclei, the Pb±Tl axis of the (PbTl)2 center is tilted away from the a direction by an angle b 36°, suggesting substantial structural modi®cations compared to the STEL center. Two simple structural models of the (PbTl)2 center seem to be the most likely [9]. The ®rst one represents a STEL center with one of the Pb nuclei replaced by a Tl nucleus. In the other one the Pb2 ion is connected to the closest substitutional Tl impurity ion accommodated in the next lattice layer. In both cases the presence of a neighboring charge compensating anion vacancy next to the Tl ion is required to explain the observed tilting of the Pb±Tl bond. Such a tilting, together with the expected asymmetry of the electron cloud at the two nuclei, results in dierent tilting angles of the two A(Tl) and A(Pb) hf tensors with respect to the a axis. Pulse annealing experiments up to 290 K on samples irradiated at T 80 K show (Fig. 1) strong variations in the concentration of the observed paramagnetic centers. Both STEL and (PbTl)2 centers decay in the same temperature range, a process accompanied by the formation of a new paramagnetic center called PbX, with yet unidenti®ed structure. In the same temperature range takes place the trapped-hole Tl2 I ! Tl2 II centers conversion, attributed
[8] to the thermally activated release of the neighboring charge compensating anion vacancy [10]. It means that the ``decay'' of the trapped-electron centers may be in fact a conversion to some other paramagnetic centers, resulting from anion vacancy trapping. Additional information concerning the properties of irradiation centers in PbCl2 has resulted from optical absorption (OA) measurements on PbCl2 :Tl samples irradiated with 7 MeV electrons at T 77 K. The high penetration depth of the electrons results in intense OA spectra (Fig. 2) which could not be obtained by X-ray irradiation. By performing pulse anneal experiments, as in the case of ESR spectra, it has been possible to identify a set of OA bands peaking at 340, 495, 685 and 920 nm with decreasing intensity and a new set of bands at 440, 540, 650 and 995 nm induced by annealing above 140 K. Because PbCl2 crystals irradiated in similar conditions did not exhibit such OA bands, one assumes that the above mentioned bands belong to thallium type centers. However, due to their similar thermal production properties, as revealed by the ESR measurements, it is hard to distinguish the OA bands of the trapped-electron (PbTl)2 centers from those of the trapped-hole Tl2 centers.
Fig. 1. Temperature induced variation in the concentration of the paramagnetic centers produced at T 20 K in a PbCl2 :Tl crystal, X-ray irradiated at 80 K for 10 min and pulse annealed at increasingly higher temperature.
Fig. 2. Optical absorption spectra of a PbCl2 :Tl sample irradiated with 7 Mev electrons at T 77 K and pulse annealed at increasingly higher temperatures. For clarity reasons, only the most signi®cant spectra are shown.
S.V. Nistor et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 538±541
3. Trapped-hole centers in PbCl2 :Tl Based on ESR and optical experiments on pure and thallium doped PbCl2 crystals we suggest that the A-centers represent in fact Vk -like hole selftrapped centers. Indeed, from pulse anneal experiments on pure PbCl2 crystals we found the decay of the STEL centers to be accompanied by a decrease in the concentration of the A-centers, which may be due to an electron±hole recombination process. Moreover, we found the concentration of the A-centers produced by low temperature irradiation and pulse annealing at 200 K strongly decreasing with increase in the concentration of Tl2 centers. This result suggests that A-center is a competing but less ecient hole trapping center. Finally, the main seven hf components structure of the A-center spectrum can be related to the similar structure of the Vk centers in alkali halides [7]. The absence of a clear isotope hf structure, which seems to be hidden in the large linewidth of the A-center hf components and the intensity ratio very dierent from the 1:2:3:4:3:2:1 ratio of the Vk centers could be attributed to the presence of several such centers with slightly dierent spin Hamiltonian parameters, resulting from the hole being self-trapped at dierent pairs of neighboring Clÿ ions in the PbCl2 lattice. 4. Conclusions The ESR and optical absorption measurements have revealed both electron and hole trapping processes in the PbCl2 crystals, which seem to play an essential role in the photolysis under UV-irradiation. In the presence of impurity ions such as Tl new trapped-electron and trapped-hole centers
541
are produced, which may compete with the selftrapping processes. The formation of heteronuclear trapped-electron dimer centers suggests the possibility of employing speci®c cation impurities as activating centers in the photolytic formation of colloidal lead in PbCl2 . Moreover, such impurities may play an important role as localization sites in the formation of trapped excitons, with in¯uence on their properties. Acknowledgements This work was performed under the auspices of a bilateral Flemish-Romanian cooperative scientific research project to which the authors are greatly indebted. One of the authors (E.G.) thanks the Fund for Scienti®c Research Flanders (F.W.O.) for ®nancial support. References [1] J.F. Verwey, J. Phys. Chem. Sol. 31 (1970) 163 and references cited therein. [2] W.C. De Gruijter, J. Kerssen, J. Sol. State. Chem. 5, 467 (1972); ibid. Solid State Comm. 10 (1972) 837. [3] J. Kerssen, W.C. De Gruijter, J. Volger, Physica 70 (1973) 375 and references cited therein. [4] E. Goovaerts, S.V. Nistor, D. Schoemaker, Phys. Rev. B 28 (1983) 3712. [5] S.V. Nistor, D. Schoemaker, Phys. Stat. Sol. B 190 (1995) 339. [6] S.V. Nistor, E. Goovaerts, D. Schoemaker, Phys. Rev. B 48 (1993) 9575; Rad. E. Def. Solids 136 (1995) 157 . [7] D. Schoemaker, Phys. Rev. B 7 (1973) 786. [8] S.V. Nistor, M. Stefan, I. Ursu, Sol. State Comm. 85 (1993) 983. [9] S.V. Nistor, E. Goovaerts, D. Schoemaker, Phys. Rev. B 57 (1998) 1. [10] K.J. De Vries, J.H. Santen, Physica 29 (1963) 482.
Electron and hole trapping in PbCl2 and PbCl2 :Tl crystals S.V. Nistor
a,b,*
, E. Goovaerts a, M. Stefan b, D. Schoemaker
a
a
b
Physics Department, University of Antwerp (U.I.A.), B-2610 Antwerpen-Wilrijk, Belgium Institute of Atomic Physics (N.I.M.P.), P.O. Box MG 7 Magurele, Bucuresti 76900, Romania
Abstract Formation of primary paramagnetic point defects under low temperature X-ray irradiation have been studied by ESR and optical absorption in pure and thallium doped PbCl2 single crystals. Besides Pb3 self-trapped electron 2 (STEL) centers the PbCl2 :Tl crystals exhibit trapped-electron (PbTl) -type centers. Based on production properties of paramagnetic centers it is suggested that besides forming Tl2 centers the holes are self trapped at pairs of neighbouring Clÿ anions resulting in Vk type centers with various orientation and length of the Cl±Cl axis. Ó 1998 Published by Elsevier Science B.V. All rights reserved. PACS: 61.72.Ji; 76.30.Mi; 65.70.+y Keywords: ESR; Paramagnetic; Centers; Irradiation; Electron; Hole; Self-trapping
1. Introduction Single crystals of PbCl2 exhibit under UV excitation at room temperatures photochemical changes resulting in formation of colloidal lead and desorption of halogen [1]. The primary defect involved in the photolytic formation of lead colloids has not yet been clearly determined, although it has been suggested that the ®rst step in this process involves the trapping of electrons at lead cations [1,2]. Early Electron Spin Resonance (ESR) studies on low temperature UV, X-ray or c-ray irradiated PbCl2 single crystals resulted in the observation of the so-called A, B and C paramagnetic centers with g 2. The identi®cation of the *
Corresponding author.
A spectrum, as being due to trapped electron Pb and Pb3 2 centers [3], disagrees with the results of more recent, extensive studies [4,5] on such centers in alkali halides. The identi®cation of the socalled C spectrum as some sort of anion trappedhole center does seem very unlikely as well. Recently we have shown [6] that conduction electrons produced by illumination above the band gap in pure PbCl2 crystals are trapped at pairs of nearest-neighbor Pb2 cations along the a axis, resulting in paramagnetic self-trapped-electron (STEL) centers. The STEL center represents a paramagnetic Pb3 2 molecular ion with a strongly bent molecular bond and electron con®guration complementary to that of the Xÿ 2 (X halogen) self-trapped hole (STH) center in alkali halides [7]. Its observation suggests that electron self-trapping may represent the initial step in the formation
0168-583X/98/$19.00 Ó 1998 Published by Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 0 0 5 1 - 2
S.V. Nistor et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 538±541
539
of lead clusters by photochemical decomposition of lead halides at temperatures where diusion of anion vacancies occurs. Here we present the results of an ESR and optical study concerning the electron and hole trapping properties of low temperature irradiated thallium doped PbCl2 crystals. It reports the simultaneous formation, besides the already known [8] trapped-hole Tl2 centers of a new heteronuclear dimer type (PbTl)2 trapped electron center with bent structure. Moreover, it suggests that the Atype ESR spectrum is in fact due to the presence of several Vk type hole self-trapped centers with slightly dierent spin-Hamiltonian parameters.
ment and procedures employed in the previous studies [5,6]. As in the case of pure PbCl2 crystals strong ESR spectra are produced after relatively short (typically 20 min) X-ray irradiation at T 80 K. Besides the lines attributed to STEL centers [6] and to Tl2 centers [8], several additional strongly anisotropic lines with identical production/bleaching properties were observed. A careful analysis [9] of the ESR transitions recorded with the magnetic ®eld rotated in the three main crystal planes shows that the EPR spectrum is described by the following spin-Hamiltonian:
2. Trapped-electron centers in PbCl2 :Tl
with S I1 I2 1=2. The spin-Hamiltonian Eq. (1) fully describes the ESR spectrum of a dimer type paramagnetic center (PbTl)2 containing pairs of Tl and Pb2 ions, called (PbTl)2 which have trapped an electron in a bonding molecular orbital. The examination of (PbTl)2 center's ESR parameters (Tables 1 and 2) strongly suggests that a strong bending of its molecular bond is also pres-
1 1 H H g S S A Tl I1 SA Pb I2 g0 lB g0
The samples employed in the present study were cut from oxygen-free PbCl2 single crystals doped in the melt with either 0.1 or 0.2 mol% TlCl, prepared by earlier described procedures [8]. The irradiation as well as the optical and ESR measurements were performed with the same equip-
1
Table 1 Principal values and orientation (in degrees) of the g tensors, as the Euler angles for a Rzy0 z00 a; b; c rotation from the cba crystal axes, for the dimer PbTl2 and Pb3 2 (STEL) centers in PbCl2 Center
T (K)
gx
gy
gz
a
b
c
(PbTl)2 in PbCl2 a STEL in PbCl2 b
20 10
1.083 1.550
0.994 1.374
1.431 1.719
24 0
36 0
)12 10
a b
Estimated experimental errors in the g components and orientation angles are smaller than 0:002 and 2, respectively. From Ref. [6]
Table 2 Principal values (in mT, absolute values) and orientation (in degrees) of the A(Tl) and A(Pb) hf tensors, given as the Euler angles of a Rzy0 z00 ai ; bi ; ci rotation from the cba crystal axes of the (even PbTl)2 and (207 PbTl)2 dimer centers, respectively, in comparison with those of the bent Pb3 2 (STEL) center in PbCl2 Center
T (K)
Ax
Ay
Az
ai
bi
ci
(even PbTl)2 in PbCl2 a (207 PbTl)2 in PbCl2 a STEL in PbCl2 b
20 20 10
65.9 266.6 )82.8
64.5 273.6 )85.3
85.5 223.5 111.7
47.8 41 0
)4.5 32 33
)33.9 )44 0
a b
Estimated experimental errors in Ax;y;z and bi are smaller than 0:4 and 2, respectively. Data for 207 A(Pb) from Ref. [6].
540
S.V. Nistor et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 538±541
ent. However, contrary to the STEL center, which has the gz component along the a crystal direction, parallel to the axis connecting the two Pb nuclei, the Pb±Tl axis of the (PbTl)2 center is tilted away from the a direction by an angle b 36°, suggesting substantial structural modi®cations compared to the STEL center. Two simple structural models of the (PbTl)2 center seem to be the most likely [9]. The ®rst one represents a STEL center with one of the Pb nuclei replaced by a Tl nucleus. In the other one the Pb2 ion is connected to the closest substitutional Tl impurity ion accommodated in the next lattice layer. In both cases the presence of a neighboring charge compensating anion vacancy next to the Tl ion is required to explain the observed tilting of the Pb±Tl bond. Such a tilting, together with the expected asymmetry of the electron cloud at the two nuclei, results in dierent tilting angles of the two A(Tl) and A(Pb) hf tensors with respect to the a axis. Pulse annealing experiments up to 290 K on samples irradiated at T 80 K show (Fig. 1) strong variations in the concentration of the observed paramagnetic centers. Both STEL and (PbTl)2 centers decay in the same temperature range, a process accompanied by the formation of a new paramagnetic center called PbX, with yet unidenti®ed structure. In the same temperature range takes place the trapped-hole Tl2 I ! Tl2 II centers conversion, attributed
[8] to the thermally activated release of the neighboring charge compensating anion vacancy [10]. It means that the ``decay'' of the trapped-electron centers may be in fact a conversion to some other paramagnetic centers, resulting from anion vacancy trapping. Additional information concerning the properties of irradiation centers in PbCl2 has resulted from optical absorption (OA) measurements on PbCl2 :Tl samples irradiated with 7 MeV electrons at T 77 K. The high penetration depth of the electrons results in intense OA spectra (Fig. 2) which could not be obtained by X-ray irradiation. By performing pulse anneal experiments, as in the case of ESR spectra, it has been possible to identify a set of OA bands peaking at 340, 495, 685 and 920 nm with decreasing intensity and a new set of bands at 440, 540, 650 and 995 nm induced by annealing above 140 K. Because PbCl2 crystals irradiated in similar conditions did not exhibit such OA bands, one assumes that the above mentioned bands belong to thallium type centers. However, due to their similar thermal production properties, as revealed by the ESR measurements, it is hard to distinguish the OA bands of the trapped-electron (PbTl)2 centers from those of the trapped-hole Tl2 centers.
Fig. 1. Temperature induced variation in the concentration of the paramagnetic centers produced at T 20 K in a PbCl2 :Tl crystal, X-ray irradiated at 80 K for 10 min and pulse annealed at increasingly higher temperature.
Fig. 2. Optical absorption spectra of a PbCl2 :Tl sample irradiated with 7 Mev electrons at T 77 K and pulse annealed at increasingly higher temperatures. For clarity reasons, only the most signi®cant spectra are shown.
S.V. Nistor et al. / Nucl. Instr. and Meth. in Phys. Res. B 141 (1998) 538±541
3. Trapped-hole centers in PbCl2 :Tl Based on ESR and optical experiments on pure and thallium doped PbCl2 crystals we suggest that the A-centers represent in fact Vk -like hole selftrapped centers. Indeed, from pulse anneal experiments on pure PbCl2 crystals we found the decay of the STEL centers to be accompanied by a decrease in the concentration of the A-centers, which may be due to an electron±hole recombination process. Moreover, we found the concentration of the A-centers produced by low temperature irradiation and pulse annealing at 200 K strongly decreasing with increase in the concentration of Tl2 centers. This result suggests that A-center is a competing but less ecient hole trapping center. Finally, the main seven hf components structure of the A-center spectrum can be related to the similar structure of the Vk centers in alkali halides [7]. The absence of a clear isotope hf structure, which seems to be hidden in the large linewidth of the A-center hf components and the intensity ratio very dierent from the 1:2:3:4:3:2:1 ratio of the Vk centers could be attributed to the presence of several such centers with slightly dierent spin Hamiltonian parameters, resulting from the hole being self-trapped at dierent pairs of neighboring Clÿ ions in the PbCl2 lattice. 4. Conclusions The ESR and optical absorption measurements have revealed both electron and hole trapping processes in the PbCl2 crystals, which seem to play an essential role in the photolysis under UV-irradiation. In the presence of impurity ions such as Tl new trapped-electron and trapped-hole centers
541
are produced, which may compete with the selftrapping processes. The formation of heteronuclear trapped-electron dimer centers suggests the possibility of employing speci®c cation impurities as activating centers in the photolytic formation of colloidal lead in PbCl2 . Moreover, such impurities may play an important role as localization sites in the formation of trapped excitons, with in¯uence on their properties. Acknowledgements This work was performed under the auspices of a bilateral Flemish-Romanian cooperative scientific research project to which the authors are greatly indebted. One of the authors (E.G.) thanks the Fund for Scienti®c Research Flanders (F.W.O.) for ®nancial support. References [1] J.F. Verwey, J. Phys. Chem. Sol. 31 (1970) 163 and references cited therein. [2] W.C. De Gruijter, J. Kerssen, J. Sol. State. Chem. 5, 467 (1972); ibid. Solid State Comm. 10 (1972) 837. [3] J. Kerssen, W.C. De Gruijter, J. Volger, Physica 70 (1973) 375 and references cited therein. [4] E. Goovaerts, S.V. Nistor, D. Schoemaker, Phys. Rev. B 28 (1983) 3712. [5] S.V. Nistor, D. Schoemaker, Phys. Stat. Sol. B 190 (1995) 339. [6] S.V. Nistor, E. Goovaerts, D. Schoemaker, Phys. Rev. B 48 (1993) 9575; Rad. E. Def. Solids 136 (1995) 157 . [7] D. Schoemaker, Phys. Rev. B 7 (1973) 786. [8] S.V. Nistor, M. Stefan, I. Ursu, Sol. State Comm. 85 (1993) 983. [9] S.V. Nistor, E. Goovaerts, D. Schoemaker, Phys. Rev. B 57 (1998) 1. [10] K.J. De Vries, J.H. Santen, Physica 29 (1963) 482.