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Self-trapped holes in ammonium silver bromide crystal
Nuclear Instruments and Methods in Physics Research B 91 (1994) 227-229 North-Holland
Self-trapped
RIUMI B
Beam Interactions with Materials 8 Atoms
holes in ammonium silver bromide crystal
T. Awano a**and T. Matsuyama b aDepartment of Applied Physics, Tohoku Gakuin
University, Tagajo 985, Japan b Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-04, Japan
irradiated by y-rays at 77 K. These sorts of self Self-trapped holes of the forms of Bra and Br,- are stable in (NH,),AgBr, trapped holes are the analogues of those in (NH,).&&. It is common in alkali or ammonium silver halides that (halogen)’ is stable at 77 K. The Bra decayed at 160 K in an annealing process. The Br,- was converted into another form of Br,- at 250 K, which decayed at 310 K
1. Introduction
We previously reported that a self-trapped hole of the form of I0 was stable in Rb,AgI, and K,AgI, among alkali silver halides [l]. The I0 was weakly coupled with the nearest alkali ion and it was denoted by RbI+ or KIf. On the other hand, both of I,- and I0 were stable in (NH,),AgI, among ammonium silver halides [2]. In the present work we have investigated the structure of defects in (NH,),AgBr, by ESR measurements. Although no study has been executed on the crystal structure of (NH,),AgBr,, the structure seems to be the same as that of Rb,AgI,. One of the reasons is that the crystal structure of (NH&Ag13 is the same as that of Rb,AgI, and K,AgI, [3]. Another reason is that the powder X-ray diffraction pattern of (NH,), AgBr, can be analyzed on the assumption that (NH,), AgBr, has the same structure as Rb,AgI,. Single crystal diffraction analysis has been applied and the details of this work will be described elsewhere. Fig. 1 shows the’crystal structure assuming that it is the same as that of Rb,AgI,. The crystal structure is orthorhombit and belongs to the space group of Dx. A silver ion is, surrounded by four bromide ions which form approximately a tetrahedron. The lines dl and d2 in Fig. 1 show directions between the Br-(I) ion or the Br-(II) ion and the nearest NH4+ ion, respectively.
2. Experimental The single crystal of (NH&AgBr, was prepared from a saturated hydrobromic acid solution of stoichio-
* Corresponding author, tel. +81 22 368 1115, fax +81 22 368 7070, email [email protected]. 0168-583X/94/$07.00
metric compounds. The dimension of the obtained crystal after crystal growth for one month was 4 X 4 X 15 mm3. y-ray irradiation was done by the 6oCo facility at the Research Reactor Institute of Kyoto University. The dose was about 50 Mrad. The irradiation temperature was 77 K. 3. Results and discussion Fig. 2 shows examples of ESR spectra applying a magnetic field in the b-c plane of y-irradiated (NH,),AgBr, crystals at 77 K. The magnetic field was applied along the direction parallel to the b-axis of the crystal at the top and parallel to the c-axis at the bottom of Fig. 2. When the magnetic field was applied along the direction parallel to the b-axis of the crystal, 4 sets of several lines with an equal hyperfine splitting and an almost equal strength were observed besides the large str,lcture centered at g = 2. The former signal is due to i\r”. The super-hyperfine structure shows that each isotope of bromine is weakly coupled with a nitrogen nuclenz; 2nd with hydrogen nuclei. The hyperfine tensor of Br- has axial symmetry. Fig. 3 shows the angular dependence of the position of the center of each set of signals of Br’. Dotted lines are fitting curves calculated assuming axial symmetry of the spin Hamiltonian. The parameters of the spin Hamiltonians are; I A ,,/gp I = 42 mT, I A I /gp I = 21 mT, g,, = 2.00, g I = 2.11 for Bra which is tilted by 35” from the b-axis in the b-c plane; I A,,/gP I = 39 mT, I A I/gp I = 18 mT, g,, = 1.98, g, = 2.08 for Bra which is tilted by 25 from the b-axis in the b-a plane. The axial directions of the Br’s coincide with the direction between each Br- ion located on site (I) or (II) in Fig. 1 and its nearest NH, + ion. The angle between the b-axis of the crystal and dl in Fig. 1 is 47”. The angle between the b-axis of the crystal and d2 is 41”.
0 1994 - Elsevier Science B.V. All rights reserved
SSDZ 0168-583X(93)E1024-G
III. HALIDES
228
T. Awano, T. Matsuyama /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 227-229
40 60
b 3
II
20
4
40
Fig. 1. Crystal structure of (NH,),AgBr,[3]. A unit cell is projected on the a-c plane from the direction parallel with the b-axis. Figures show b-coordinates of each ion.
60
250
300 MAGNETIC
350 FIELD (mT)
400
Fig. 3. Angular dependence of the ESR signal of Br’. Circles show the center of each set of signals. Dotted lines show curves fitted with the spin Hamiltonian parameters given in the text. The fitting curve is shifted by 22” toward the a-axis in the bottom part and is considered with the experimental error.
When the crystal was annealed up to 160 K, the signal of Bra disappeared and the main structure of the ESR signal for the magnetic field parallel to the b-axis became a set of 7 lines of the same splitting and binomial strengths. This structure was already observed in the spectra at 77 K. But it was superimposed with the signal of Bra therefore not so clear. Fig. 4
I
250
I
300 MAGNETIC
t
I
I
II
I
350 FIELD (mT)
!
(1
400
Fig. 2. ESR spectra of y-irradiated (NH&AgBrs. The magnetic field was applied in the direction of the b-c plane of the crystal. Angles from the b-axis are shown in the figure. The temperature of irradiation and measurement was 77K. The microwave frequency was 9.290 GHz.
shows ESR spectra of (NHJ,AgBr, y-radiated at 77 K and then annealed at 168 K. The magnetic field was applied in the b-c plane of the crystal. This is the hyperfine structure of Br,-. The Br,- has its axial direction along the b-axis of the crystal. The parameters of the spin Hamiltonian are 1A,,/gP 1 = 15.5 mT, IA,/gp ( = 6.5 mT, g,, = 2.05, g, = 2.16. The axial direction of Br- coincides with the direction of each set of bromines in the neighboring unit cell along the b-axis of the crystal. The magnitudes of these hyperfine tensors are smaller than the corresponding values for the V, centers in alkali halides ( I A,,/gp I of the V, center in KBr is 45 mT [4]>. This small hyperfine interaction indicates that the trapped hole is delocal-
T. Awano,
T. Matsuyama /Nucl.
Instr. and Meth. in Phys. Res. B 91 (1994) 227-229
229
width as this Br-. But the direction of I,- was parallel with the u-axis of the crystal, which is contrary to the case of (NH,),AgBr,.
4. Conclusion
Bra and Br,- are stable in (NH,),AgBr, as in (NHJ2Ag13. The character of thermal decay and conversion of Bra and Br, - in (NH,),AgBr, is almost the same as I0 and I, - in (NH,),AgI,. The self-trapped hole of the form of (halogen)’ is stable in alkali and ammonium silver halides. A part of this may be attributed to their electronic band structure. The Ag(4d) band in these crystals is close to the halogen p band which forms the top of the valence band.
Acknowledgements I
250
I
300 MAGNETIC
I
I
I
I
I
I
350 FIELD (mT)
I
I
II
400
Fig. 4. ESR spectra of (NH,),AgBr, y-irradiated at 77 K and annealed at 168 K. The magnetic field was applied in the directions of the b-c plane of the crystal. Angles from the b-axis are shown in the figure. The temperature of measurement was 77 K The microwave frequency was 9.290 GHz.
We would like to thank Prof. M. Ikezawa of Tohoku University for the guidance and encouragement. This work was executed under the visiting researcher program of the Research Reactor Institute, Kyoto University.
References ized from the Br,- molecular ion, which is parallel to the b-axis. The full width at half maximum of this signal is 14 mT. The line width is also too large comparing with those of the Vx centers in alkali halides. In the case of (NH,),AgI, the ESR spectrum of crystals annealed at 190 K consists of 11 lines of the same splitting and binomial intensity [2]. The I,- has the same characteristic hyperfine parameters and line
[l] T. Awano, T. Nanba, M. Ikezawa, T. Matsuyama and H. Yamaoka, .I. Phys. Sot. Jpn. 58 (1989) 2570. [2] T. Awano, M. Ikezawa, T. Matsuyama and H. Yamaoka, in: Defects in Insulating Materials, eds. 0. Kanert and J.-M. Spaeth (World Scientific, Singapore, 1993) p. 491. [3] C. Blink and H.A.S. Kroase, Acta Crystallogr. 5 (1951) 433. [4] D. Schoemaker, Phys. Rev. 149 (1966) 693.
III. HALIDES
Self-trapped
RIUMI B
Beam Interactions with Materials 8 Atoms
holes in ammonium silver bromide crystal
T. Awano a**and T. Matsuyama b aDepartment of Applied Physics, Tohoku Gakuin
University, Tagajo 985, Japan b Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-04, Japan
irradiated by y-rays at 77 K. These sorts of self Self-trapped holes of the forms of Bra and Br,- are stable in (NH,),AgBr, trapped holes are the analogues of those in (NH,).&&. It is common in alkali or ammonium silver halides that (halogen)’ is stable at 77 K. The Bra decayed at 160 K in an annealing process. The Br,- was converted into another form of Br,- at 250 K, which decayed at 310 K
1. Introduction
We previously reported that a self-trapped hole of the form of I0 was stable in Rb,AgI, and K,AgI, among alkali silver halides [l]. The I0 was weakly coupled with the nearest alkali ion and it was denoted by RbI+ or KIf. On the other hand, both of I,- and I0 were stable in (NH,),AgI, among ammonium silver halides [2]. In the present work we have investigated the structure of defects in (NH,),AgBr, by ESR measurements. Although no study has been executed on the crystal structure of (NH,),AgBr,, the structure seems to be the same as that of Rb,AgI,. One of the reasons is that the crystal structure of (NH&Ag13 is the same as that of Rb,AgI, and K,AgI, [3]. Another reason is that the powder X-ray diffraction pattern of (NH,), AgBr, can be analyzed on the assumption that (NH,), AgBr, has the same structure as Rb,AgI,. Single crystal diffraction analysis has been applied and the details of this work will be described elsewhere. Fig. 1 shows the’crystal structure assuming that it is the same as that of Rb,AgI,. The crystal structure is orthorhombit and belongs to the space group of Dx. A silver ion is, surrounded by four bromide ions which form approximately a tetrahedron. The lines dl and d2 in Fig. 1 show directions between the Br-(I) ion or the Br-(II) ion and the nearest NH4+ ion, respectively.
2. Experimental The single crystal of (NH&AgBr, was prepared from a saturated hydrobromic acid solution of stoichio-
* Corresponding author, tel. +81 22 368 1115, fax +81 22 368 7070, email [email protected]. 0168-583X/94/$07.00
metric compounds. The dimension of the obtained crystal after crystal growth for one month was 4 X 4 X 15 mm3. y-ray irradiation was done by the 6oCo facility at the Research Reactor Institute of Kyoto University. The dose was about 50 Mrad. The irradiation temperature was 77 K. 3. Results and discussion Fig. 2 shows examples of ESR spectra applying a magnetic field in the b-c plane of y-irradiated (NH,),AgBr, crystals at 77 K. The magnetic field was applied along the direction parallel to the b-axis of the crystal at the top and parallel to the c-axis at the bottom of Fig. 2. When the magnetic field was applied along the direction parallel to the b-axis of the crystal, 4 sets of several lines with an equal hyperfine splitting and an almost equal strength were observed besides the large str,lcture centered at g = 2. The former signal is due to i\r”. The super-hyperfine structure shows that each isotope of bromine is weakly coupled with a nitrogen nuclenz; 2nd with hydrogen nuclei. The hyperfine tensor of Br- has axial symmetry. Fig. 3 shows the angular dependence of the position of the center of each set of signals of Br’. Dotted lines are fitting curves calculated assuming axial symmetry of the spin Hamiltonian. The parameters of the spin Hamiltonians are; I A ,,/gp I = 42 mT, I A I /gp I = 21 mT, g,, = 2.00, g I = 2.11 for Bra which is tilted by 35” from the b-axis in the b-c plane; I A,,/gP I = 39 mT, I A I/gp I = 18 mT, g,, = 1.98, g, = 2.08 for Bra which is tilted by 25 from the b-axis in the b-a plane. The axial directions of the Br’s coincide with the direction between each Br- ion located on site (I) or (II) in Fig. 1 and its nearest NH, + ion. The angle between the b-axis of the crystal and dl in Fig. 1 is 47”. The angle between the b-axis of the crystal and d2 is 41”.
0 1994 - Elsevier Science B.V. All rights reserved
SSDZ 0168-583X(93)E1024-G
III. HALIDES
228
T. Awano, T. Matsuyama /Nucl. Instr. and Meth. in Phys. Rex B 91 (1994) 227-229
40 60
b 3
II
20
4
40
Fig. 1. Crystal structure of (NH,),AgBr,[3]. A unit cell is projected on the a-c plane from the direction parallel with the b-axis. Figures show b-coordinates of each ion.
60
250
300 MAGNETIC
350 FIELD (mT)
400
Fig. 3. Angular dependence of the ESR signal of Br’. Circles show the center of each set of signals. Dotted lines show curves fitted with the spin Hamiltonian parameters given in the text. The fitting curve is shifted by 22” toward the a-axis in the bottom part and is considered with the experimental error.
When the crystal was annealed up to 160 K, the signal of Bra disappeared and the main structure of the ESR signal for the magnetic field parallel to the b-axis became a set of 7 lines of the same splitting and binomial strengths. This structure was already observed in the spectra at 77 K. But it was superimposed with the signal of Bra therefore not so clear. Fig. 4
I
250
I
300 MAGNETIC
t
I
I
II
I
350 FIELD (mT)
!
(1
400
Fig. 2. ESR spectra of y-irradiated (NH&AgBrs. The magnetic field was applied in the direction of the b-c plane of the crystal. Angles from the b-axis are shown in the figure. The temperature of irradiation and measurement was 77K. The microwave frequency was 9.290 GHz.
shows ESR spectra of (NHJ,AgBr, y-radiated at 77 K and then annealed at 168 K. The magnetic field was applied in the b-c plane of the crystal. This is the hyperfine structure of Br,-. The Br,- has its axial direction along the b-axis of the crystal. The parameters of the spin Hamiltonian are 1A,,/gP 1 = 15.5 mT, IA,/gp ( = 6.5 mT, g,, = 2.05, g, = 2.16. The axial direction of Br- coincides with the direction of each set of bromines in the neighboring unit cell along the b-axis of the crystal. The magnitudes of these hyperfine tensors are smaller than the corresponding values for the V, centers in alkali halides ( I A,,/gp I of the V, center in KBr is 45 mT [4]>. This small hyperfine interaction indicates that the trapped hole is delocal-
T. Awano,
T. Matsuyama /Nucl.
Instr. and Meth. in Phys. Res. B 91 (1994) 227-229
229
width as this Br-. But the direction of I,- was parallel with the u-axis of the crystal, which is contrary to the case of (NH,),AgBr,.
4. Conclusion
Bra and Br,- are stable in (NH,),AgBr, as in (NHJ2Ag13. The character of thermal decay and conversion of Bra and Br, - in (NH,),AgBr, is almost the same as I0 and I, - in (NH,),AgI,. The self-trapped hole of the form of (halogen)’ is stable in alkali and ammonium silver halides. A part of this may be attributed to their electronic band structure. The Ag(4d) band in these crystals is close to the halogen p band which forms the top of the valence band.
Acknowledgements I
250
I
300 MAGNETIC
I
I
I
I
I
I
350 FIELD (mT)
I
I
II
400
Fig. 4. ESR spectra of (NH,),AgBr, y-irradiated at 77 K and annealed at 168 K. The magnetic field was applied in the directions of the b-c plane of the crystal. Angles from the b-axis are shown in the figure. The temperature of measurement was 77 K The microwave frequency was 9.290 GHz.
We would like to thank Prof. M. Ikezawa of Tohoku University for the guidance and encouragement. This work was executed under the visiting researcher program of the Research Reactor Institute, Kyoto University.
References ized from the Br,- molecular ion, which is parallel to the b-axis. The full width at half maximum of this signal is 14 mT. The line width is also too large comparing with those of the Vx centers in alkali halides. In the case of (NH,),AgI, the ESR spectrum of crystals annealed at 190 K consists of 11 lines of the same splitting and binomial intensity [2]. The I,- has the same characteristic hyperfine parameters and line
[l] T. Awano, T. Nanba, M. Ikezawa, T. Matsuyama and H. Yamaoka, .I. Phys. Sot. Jpn. 58 (1989) 2570. [2] T. Awano, M. Ikezawa, T. Matsuyama and H. Yamaoka, in: Defects in Insulating Materials, eds. 0. Kanert and J.-M. Spaeth (World Scientific, Singapore, 1993) p. 491. [3] C. Blink and H.A.S. Kroase, Acta Crystallogr. 5 (1951) 433. [4] D. Schoemaker, Phys. Rev. 149 (1966) 693.
III. HALIDES