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Dielectric measurements of OH- centers in silver chloride
Volume 59A, number 3
PHYSICS LETTERS
DIELECTRIC MEASUREMENTS OF 0H
29 November 1976
CENTERS IN SILVER CHLORIDE
F.J. STERK and J.C. BRETNEY * Department of Physics, University of Nevada, Las Vegas, Las Vegas NV 89154, USA Received 23 August 1976
Dielectric measurements on silver chloride doped with hydroxyl ions show the defect is oriented in the (111) direction with a dipole moment of 1.1 debye. It reorients by a classical rate process with an activation energy of 94 cm’.
Paraelectric defect centers in alkali halide crystals have attracted a considerable amount of theoretical and experimental interest during the last decade. The properties of the molecular and off-center defect have been reviewed extensively by Bridges [1] and Narayanamurti and Pohi [2]. Because of this interest, other crystal systems have been examined for the possibiity of exhibiting paraelectric phenomena. It is the purpose of this letter to report the effect of temperature on the dielectric properties of silver chloride crystals doped with hydroxyl ions. Silver chloride was selected as a promising system, because of its similarity in crystal structure and lattice constant to the alkali halides. However, the higher degree of anharmonicity of the silver chloride crystals was cxpected to significantly effect the properties of the hy. droxyl defect. Initial infrared absorption measurements on these crystals by Sterk and Hanson [3] established the validity of these assumptions. However, the experimental data did not determine whether this crystal system exhibited paraelectric behavior. Although the infrared spectra was interpreted on the basis of the hindered rotor [4,5] model, which allows for quantum mechanical tunneling between equivalent orientations, no direct evidence for tunneling was observed. Furthermore, the orientation of the hydroxyl defect and its dipole moment were not uniquely established. It was the purpose of the present work to answer these questions. These crystals were preparedby adding one or two mole percent of cesium hydroxide to an ingot of silver chloride. Cesium hydroxide was chosen since silver hy*
Present address: Aeronutronics Ford, Newport Branch, CA USA.
216
droxide is an unstable compound. Then the ingot of silver chloride and the dopant were vacuum sealed in a quartz ampule, and grown by the Bridgeman technique. The resulting crystals were oriented along one of the main crystallographic axes and cut in the form of thin plates 0.4 mm—0.8 mm thick and 1 cm2 in area. Chrome-gold electrodes were vapor deposited on the sample faces. It was anticipated that the larger ionic radius of cesium as compared to the silver ion would cause its rejection from the crystal during growth. Subsequent atomic absorption measurements verified that the cesium was rejected during crystal growth. The hydroxyl concentration was determined by dissolving the crystal in potassium iodide and then titrating the resulting solution. Capacitance and loss measurements were made in the frequency range 0.1 kllz to 100 kHz with a General Radio Model 1615-A Capacitance Bridge. A PAR Model 129 two phase lock-in amplifier was used as the detector. The real (e’) and the imaginary part (e”) of the dielectric constant were calculated from the sample capacitance and loss, after correcting for the change in the sample dimensions due to the thermal expansion [6]. The polarizability of the defect was determined from the Clausius-Mossotti expression ~ 1 -~-irNa= e’ + 2 e +2 (1) —
‘
0
where N is the number of dipoles per cm3, a is the polarizabiity of the hydroxyl ion, e’is the dielectric constant of the doped sample, and e 0 is the dielectric constant 2of pure silver chloride [71. or PE1oc ~ /3kT, where p is the dipoleFmoment ofkT thethen deafect, = PE 10~is the local electric field, and k is the Boltzman
Volume 59A, number 3
PHYSICS LETTERS
constant. In the temperature range of this study one expects the polarizabiity to be inversely proportional to the temperature. In fig. 1 the right side of eq. (1) is plotted against the inverse temperature. From the slope of this curve and the concentration of defect centers one obtains the dipole moment for the hydroxyl defect. The maximem value of the dipole moment occurs for the crystal oriented in the (111> direction. This value is 1.1 debye, whereas for the (100) and (110> orientation the values are 0.6 debye and 0.8 debye, respectively. These values are in approximately the correct ratio for the hydroxyl dipole to be oriented in the (111> direction. This result suggests that the infrared absorption work of Sterk and Hanson where a (100) orientation was assumed, must be reexamined. This small magnitude for the dipole moment is unusual in that for most alkali halides doped with the hydroxyl ion the value of the dipole moment is in the neighborhood of 4.8 debye. Since the dipole moment depends on the dipole concentration, an accurate determination of this concentration is important. However, it is known that divalent impurities in the alkali halides can immobilize a fraction of the hydroxyl dipoles, and it is possible that the dipoles can be tied up in clusters and other complex defects. The net affect is to reduce the number of dipoles that are dielectrically active. Hence this value of the dipole moment represents a lower bound. Furthermore, when the dipole __________________________________________ AgCI
6’
OW
2 0’
2
2 -
‘°
-
________________________________ 0.10
OIl
012
0,13
014
Temperature’
015
016
017
(SKI)
The temperature dependence of the relaxation rate. moment calculated by Cade [8] is shifted to a centerof-mass coordinate, it is found that a 15% change in the internuclear distance can account for the observed dipole moment. This distortion of the defect center could be due to the larger anharmonicity of the silver chloride crystal. Fig. 2.
In fig. 1 it is also observed that the reorientation of the dipole is a thermally activated process. The relaxalion of therelaxation defect decreases increasing ture. The time r iswith expressed by tempera-
-
23Oppm OH ulIlu
0r,sntotion
0
=
0.2 kHZ
~~.::~:
4
T 2
.
I
0
o.os
29 November 1976
o.a
0.10 Temperature-’
0.20
~<‘~
Fig. 1. The temperature and frequency dependence of the defect contribution to the polarizabiity of AgU: 0H.
exp (—U/kfl,
(2)
where Uis the activation energy between two equivalent positions of the defect. From the slope of fig. 2 the activation energy is determined to be 94 cm~,and the attempt frequency 2.2 X 1012 sect. The value for the barrier to rotation is in substantial disagreement with the infrared absorption measurements of Sterk and Hanson in which the barrier to reorientation was determined to be 492 cm1. Part of this large discrepancy is due to assuming the wrong orientation of the defect. At present it is not clear if it will account for all of the discrepancy. In conclusion the hydroxyl ion in silver chloride does 217
Volume 59A, number 3
PHYSICS LETTERS
not exhibit the quantum mechanical tunneling behavior similar to that seen in the alkali halides. Rather the defect responds to a classical rate process with an activation energy of 94 cm~.The hydroxyl moment is 1.1 debye and it is oriented in the (111) direction. Currently work is in progress to correlate the optical and dielectric results. The authors would like to thank the Research Council of the University of Nevada, Las Vegas for their fmancial support.
218
29 November 1976
References [1] F. Bridges, C.R.C.
Critical Reviews in Solid State Sciences
1. [21 ~V.~Naxayanamurti and R.O. Pohi, Rev. Mod. Phys. 42 (1970) 201. [3] F..’. Sterk and R.C. Hanson, Solid State Commun. 9 (1971) 1473. [4] A.F. Devonshire, Proc. Roy. Soc. (London) A153 (1936) 601. [5] P. Sauer, Z. Physik 194 (1966) 360. [6] R.M. Nicklow and R.A. Young, Phys. Rev. 129 (1963) 1936. [7] R.P. Lowndes, Phys. Rev. B 6 (1972) 4667. [8] P.E. Cade, J. chem. Phys. 47 (1967) 2390.
PHYSICS LETTERS
DIELECTRIC MEASUREMENTS OF 0H
29 November 1976
CENTERS IN SILVER CHLORIDE
F.J. STERK and J.C. BRETNEY * Department of Physics, University of Nevada, Las Vegas, Las Vegas NV 89154, USA Received 23 August 1976
Dielectric measurements on silver chloride doped with hydroxyl ions show the defect is oriented in the (111) direction with a dipole moment of 1.1 debye. It reorients by a classical rate process with an activation energy of 94 cm’.
Paraelectric defect centers in alkali halide crystals have attracted a considerable amount of theoretical and experimental interest during the last decade. The properties of the molecular and off-center defect have been reviewed extensively by Bridges [1] and Narayanamurti and Pohi [2]. Because of this interest, other crystal systems have been examined for the possibiity of exhibiting paraelectric phenomena. It is the purpose of this letter to report the effect of temperature on the dielectric properties of silver chloride crystals doped with hydroxyl ions. Silver chloride was selected as a promising system, because of its similarity in crystal structure and lattice constant to the alkali halides. However, the higher degree of anharmonicity of the silver chloride crystals was cxpected to significantly effect the properties of the hy. droxyl defect. Initial infrared absorption measurements on these crystals by Sterk and Hanson [3] established the validity of these assumptions. However, the experimental data did not determine whether this crystal system exhibited paraelectric behavior. Although the infrared spectra was interpreted on the basis of the hindered rotor [4,5] model, which allows for quantum mechanical tunneling between equivalent orientations, no direct evidence for tunneling was observed. Furthermore, the orientation of the hydroxyl defect and its dipole moment were not uniquely established. It was the purpose of the present work to answer these questions. These crystals were preparedby adding one or two mole percent of cesium hydroxide to an ingot of silver chloride. Cesium hydroxide was chosen since silver hy*
Present address: Aeronutronics Ford, Newport Branch, CA USA.
216
droxide is an unstable compound. Then the ingot of silver chloride and the dopant were vacuum sealed in a quartz ampule, and grown by the Bridgeman technique. The resulting crystals were oriented along one of the main crystallographic axes and cut in the form of thin plates 0.4 mm—0.8 mm thick and 1 cm2 in area. Chrome-gold electrodes were vapor deposited on the sample faces. It was anticipated that the larger ionic radius of cesium as compared to the silver ion would cause its rejection from the crystal during growth. Subsequent atomic absorption measurements verified that the cesium was rejected during crystal growth. The hydroxyl concentration was determined by dissolving the crystal in potassium iodide and then titrating the resulting solution. Capacitance and loss measurements were made in the frequency range 0.1 kllz to 100 kHz with a General Radio Model 1615-A Capacitance Bridge. A PAR Model 129 two phase lock-in amplifier was used as the detector. The real (e’) and the imaginary part (e”) of the dielectric constant were calculated from the sample capacitance and loss, after correcting for the change in the sample dimensions due to the thermal expansion [6]. The polarizability of the defect was determined from the Clausius-Mossotti expression ~ 1 -~-irNa= e’ + 2 e +2 (1) —
‘
0
where N is the number of dipoles per cm3, a is the polarizabiity of the hydroxyl ion, e’is the dielectric constant of the doped sample, and e 0 is the dielectric constant 2of pure silver chloride [71. or PE1oc ~ /3kT, where p is the dipoleFmoment ofkT thethen deafect, = PE 10~is the local electric field, and k is the Boltzman
Volume 59A, number 3
PHYSICS LETTERS
constant. In the temperature range of this study one expects the polarizabiity to be inversely proportional to the temperature. In fig. 1 the right side of eq. (1) is plotted against the inverse temperature. From the slope of this curve and the concentration of defect centers one obtains the dipole moment for the hydroxyl defect. The maximem value of the dipole moment occurs for the crystal oriented in the (111> direction. This value is 1.1 debye, whereas for the (100) and (110> orientation the values are 0.6 debye and 0.8 debye, respectively. These values are in approximately the correct ratio for the hydroxyl dipole to be oriented in the (111> direction. This result suggests that the infrared absorption work of Sterk and Hanson where a (100) orientation was assumed, must be reexamined. This small magnitude for the dipole moment is unusual in that for most alkali halides doped with the hydroxyl ion the value of the dipole moment is in the neighborhood of 4.8 debye. Since the dipole moment depends on the dipole concentration, an accurate determination of this concentration is important. However, it is known that divalent impurities in the alkali halides can immobilize a fraction of the hydroxyl dipoles, and it is possible that the dipoles can be tied up in clusters and other complex defects. The net affect is to reduce the number of dipoles that are dielectrically active. Hence this value of the dipole moment represents a lower bound. Furthermore, when the dipole __________________________________________ AgCI
6’
OW
2 0’
2
2 -
‘°
-
________________________________ 0.10
OIl
012
0,13
014
Temperature’
015
016
017
(SKI)
The temperature dependence of the relaxation rate. moment calculated by Cade [8] is shifted to a centerof-mass coordinate, it is found that a 15% change in the internuclear distance can account for the observed dipole moment. This distortion of the defect center could be due to the larger anharmonicity of the silver chloride crystal. Fig. 2.
In fig. 1 it is also observed that the reorientation of the dipole is a thermally activated process. The relaxalion of therelaxation defect decreases increasing ture. The time r iswith expressed by tempera-
-
23Oppm OH ulIlu
0r,sntotion
0
=
0.2 kHZ
~~.::~:
4
T 2
.
I
0
o.os
29 November 1976
o.a
0.10 Temperature-’
0.20
~<‘~
Fig. 1. The temperature and frequency dependence of the defect contribution to the polarizabiity of AgU: 0H.
exp (—U/kfl,
(2)
where Uis the activation energy between two equivalent positions of the defect. From the slope of fig. 2 the activation energy is determined to be 94 cm~,and the attempt frequency 2.2 X 1012 sect. The value for the barrier to rotation is in substantial disagreement with the infrared absorption measurements of Sterk and Hanson in which the barrier to reorientation was determined to be 492 cm1. Part of this large discrepancy is due to assuming the wrong orientation of the defect. At present it is not clear if it will account for all of the discrepancy. In conclusion the hydroxyl ion in silver chloride does 217
Volume 59A, number 3
PHYSICS LETTERS
not exhibit the quantum mechanical tunneling behavior similar to that seen in the alkali halides. Rather the defect responds to a classical rate process with an activation energy of 94 cm~.The hydroxyl moment is 1.1 debye and it is oriented in the (111) direction. Currently work is in progress to correlate the optical and dielectric results. The authors would like to thank the Research Council of the University of Nevada, Las Vegas for their fmancial support.
218
29 November 1976
References [1] F. Bridges, C.R.C.
Critical Reviews in Solid State Sciences
1. [21 ~V.~Naxayanamurti and R.O. Pohi, Rev. Mod. Phys. 42 (1970) 201. [3] F..’. Sterk and R.C. Hanson, Solid State Commun. 9 (1971) 1473. [4] A.F. Devonshire, Proc. Roy. Soc. (London) A153 (1936) 601. [5] P. Sauer, Z. Physik 194 (1966) 360. [6] R.M. Nicklow and R.A. Young, Phys. Rev. 129 (1963) 1936. [7] R.P. Lowndes, Phys. Rev. B 6 (1972) 4667. [8] P.E. Cade, J. chem. Phys. 47 (1967) 2390.