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Electric field effects in positron annihilation in phenanthrene
Volume 187, number 5
CHEMICAL PHYSICS LETTERS
20 December I99 I
Electric field effects in positron annihilation in phenanthrene W. Gbmiak a, T. Goworek a and C. Rybka a.b a Marie Curie-Skkniowska University, 20031 Lublin, Poland b Lublin Polytechnic, 20109 Lublin, Poland
Received 20 July 199 I
The variations ofpositronium formation probability at changes oftemperature ofphenanthrene were investigated. All observed effects can be explained as the results of electric field inhibition of Ps formation in a pyroelectric medium.
1. Introduction
In the lifetime spectra of positron annihilation in an insulator, one can distinguish at least three exponential components. The shortest lived of them is ascribed to the annihilation of the singlet bound state of positron and electron (para-positronium, p-Ps), the intermediate one to the free annihilation in collisions, and the longest-lived component to the annihilation of the triplet state (ortho-positronium, oPS). In our early investigations of positronium annihilation in phenanthrene [ 11, we have found that after crossing the phase-transition point at 73°C in an arbitrary direction, from high to low temperature or vice versa, the intensity Is of the long-lived component diminishes, almost without change of the lifetime. This low value of intensity is not stable and increases with the time constant strongly dependent on the temperature. We proposed that the effect could be explained by formation of unstable phases, or preferably, by the pyroelectric effect, i.e. the polarization of the sample at changes of temperature. Experiments with paraffins [ 21 and polymers [ 31 indicate that the electric field reduces the positronium formation probability. The reduction of positronium yield tends to a saturation value at high field, approximately half of its zero-field value (the phenomena occurring at MV/cm fields [ 41 can be neglected in this paper). Polarization phenomena in phenanthrene were observed very early [5,6] in samples biased by an Elsevier Science Publishers B.V.
external field; the polarization currents appeared when the temperature passed the range from the phase-transition point down to about 40°C. Spontaneous polarization at changes of temperature was reported in 1988 by Kroupa et al. [7,8]. The aim of this paper is to collect more data indicating the relation of observed positron annihilation peculiarities in phenanthrene to the pyroelectricity of this solid.
2. Experimental The lifetime spectra of positron annihilation in phenanthrene were measured using a conventional fast-slow-time spectrometer with a resolution (fwhm ) of about 300 ps. The source of positrons, **Na deposited inside a kapton envelope, was sandwiched between two samples of phenanthrene. The temperature of the sample was controlled with the accuracy of 0.2 K. Before annihilation, the positrons can be assumed as thermalized and diffusing in the sample; thus, the effect is independent of field orientation and polycrystalline samples can be used. The lifetime spectra were decomposed, using the POSITRONFIT program [9], into three exponentials convoluted with the Gaussian resolution curve. The longest-lived component, representing the o-Ps decay had the lifetime of about 1 ns. During data processing, the lifetime of the shortest component was fixed at 123 ps. Even if the p-Ps lifetime is misfitted due to the multielectronic character of Ps inside the medium (“quasi-positronium”), it does not 537
20 December 199 I
CHEMICAL PHYSICS LETTERS
Volume 187. number 5 affect
the discussion below, as it concentrates on the long-lived component only.
3. Results and discussion The temperature dependence of the long-lived component intensity and lifetime is shown in fig. 1.
1.6
35
Very slow recovery of the samples in the temperature range below 50°C makes it impossible to wait for full depolarization; thus, the part of the upper curve below 50°C represents the lower limit of IS rather than its real value. The sensitivity of positronium in phenanthrene to the external electric field was investigated at room temperature, i.e. outside the range where the pyroelectric effects appear. In the samples in their “stable” Ps-rich state, reduction of 1, is observed (fig. 2). A sum of an exponential and a constant was fitted to the experimental data. This constant, representing the high-field saturation value of IX,was found equal to 11.3?0.8%, and thus, not far from the I3 observed after the skip from above the phase-transition point to the temperature below 55°C. This means that the electric field in the sample after such a temperature change is of the order of 10’ V/cm or more. The value of (dl,/dE), is approximately 2.9 cm/MV, and thus very similar to that observed in paraffins [ 2 1. The recovery from the low 1, state at room temperature is so slow that the intensity does not change measurably during several days. Thus, it was also
15
10
o
25
50
75
100
Fig. 1. Longest-lived component intensity Z3and lifetime q in the time spectrum ofpositron annihilation in phenanthrene. Full circles represent the stable values, small open triangles the values observed
immediately
after the change of temperature
starting value indicated by a large triangle.
538
10
15
M
25
30
EL.FIELD kV/cm
TEMPERATURE ,“C
from the
Fig. 2. The ortho-positronium intensity 1, at room temperature as a function of applied external electric field: (a) the sample in its “stable” state (after long storage); (b) the sample immediately after the change of temperature from 90°C to room temperature.
20 December 199 I
CHEMICAL PHYSICS LETTERS
Volume 187, number 5
possible to make the measurements with the external electric field applied to the samples exhibiting the low I, value. As can be seen in fig. 2 (curve b), the l3 value is practically independent of the applied field. If the reduction of I3 is due to strong internal fields up to the saturation of the effect, the addition of a much smaller external field cannot influence the result. The change of temperature from T, to T, produces the polarization P=J;::p( T) dT, where p(T) is the pyroelectric coefficient. In phenanthrene, this coefficient is practically zero in the high-temperature phase; it is peaked slightly below the phase-transition point and diminishes gradually in the range down to 30°C [ 7,8], Thus, the reduction of II is not directly related to the phase transition and it should be observed without crossing the phase-transition point. To demonstrate this, the starting points for the change of temperature were chosen to be 65, 60, 55 and 5O”C, and for every starting point, a series of measurements were performed with various end points. The results are shown in fig. 3. The decrease of the long-lived component is quite visible and diminishes with the reduction of the initial temperature; the effect almost disappears when the starting point is 50°C.
Note that the temperature dependence of hl,/l, (curve a in fig. 3) is very similar to that of the polarization P and birefringence An reported in refs. [ 7,8 1, but is more flattened at low temperatures. That is easily explainable by the saturation of Ps-formation effects at high fields. The temperature dependence of I, for the starting point 90” C was measured several times and for various samples, and was reasonably reproducible, while in the paper by Kroupa et al. [ 81, poor reproducibility of the ordinate scale was reported. Similarly as above, the saturation effect renders the annihilation parameters almost insensitive to the variation of high fields; thus, the good reproducibility in our case can be somewhat artificial. The change of temperature in one direction generates the electric field; thus, the skip in the opposite direction between the same temperatures should restore the unpolarized state, if there is no hysteresis. Fig. 4a shows the I, values measured during one cycle of temperatures. The data for 68°C and for the final temperature were accumulated during a short 0.5 h run to avoid the substantial compensation of pyroelectric charges. After the full cycle, the f3 value was almost identical with the initial one. When, after lowering the temperature, the sample is stored and the cycle of temperature is then completed, the electric charge is overcompensated and
0.5
0.4
\
,” 0.3 0 ‘;j ’ 0.2
0.1
0 I
I
30
40
I
50
60
TEMPERATURE
I
70
I
BO
0
I
90
TEMPERATURE ,“C
I
I
L
6
CYCLE NUMBER
, “C Fig. 4. Temperature
Fig. 3. Relative decrease of the long-lived component I, after the change of temperature.
I
2
The starting value of temperature
is
lived component 78°C;
cycling; (a) the intensities I, of the
measured in a cycle of temperatures
long-
7%68-
(b) the I, intensity measured after every cycle in a series
marked by doubling the symbol; the abscissa scale corresponds
of procedures analogous to that in part “a” of the figure. Open
to the final temperature. The lines represent visual guides only.
circles denote I, after the sixth cycle and 24 h storage at 78°C.
539
Volume 187, number 5
CHEMICAL PHYSICS LETTERS
the non-zero polarization remains in the sample. Even if the storage time is short (e.g., the time of measurement ), after every further cycle of this kind, the surplus of charge increases; as a result, the intensity of the long-lived component is systematically reduced. This is shown in fig. 4b. Six cycles were performed; after the last cycle, the sample was stored at 78°C and then I3 recovered to its normal value. While the reproducibility of 13versus T, including the results of all jumps of temperature, is high, the reproducibility of sample recovery is not. In the hightemperature region, the recovery curve could be well approximated by an exponential, the time constant of charge compensation process was relatively stable, but at low temperatures it varied substantially from run to run and from sample to sample (at the same storage temperature) and the recovery curves deviated from a simple exponential. The reciprocal of the time constant can be assumed proportional to the charge-detrapping rate. In spite of time-constant instabilities, a very rough estimate of trap depth could be made (if one assumes one trap level). This estimate is 0.5 eV. In phenanthrene, one observes rather continuous distribution of charge traps with a peak at 0.43 eV [ lo], and thus not far from our estimate. 4. Conclusions All observed effects in positron annihilation in
540
20 December I99 1
phenanthrene can be explained by the action of the electric field generated in the pyroelectric medium by changes of temperature. The reduction of positronium yield is not connected strictly with the phase transition, but it appears in a relatively broad temperature range in the low-temperature phase. No hysteresis of polarization effects was observed.
References [ 1] T. Goworek, W. Gbmiak, R. Wasiewicz, J. Wawryszczuk and C. Rybka, Chem. Phys. Letters 120 ( 1985) 223. [2] W. Brandtand J. Wilkenfeld, Phys. Rev. B 12 (1975) 2579. [ 31 A. Bisi, F. Bisi, A. Fasana and L. Zappa, Phys. Rev. I22 (1961) 1709. 141A. Bisi, G. Gambarini and L. Zappa, Lettere Nuovo Cimento 31 (1981) 58. [ 51R.A. Amdt and A.C. Damask, J. Chem. Phys. 45 (1966) 4627. [6] A.N. Gubkin and A.M. Kozhevnikov, Zh. Fiz. Khim. 42 (1968) 2106. [7] J. Kroupa, I. Fousek, N.R. Ivanov, B. Biezina and V. Lhotsk& Ferroclectrics 79 ( 1988) 189. [ 81 J. Kroupa, J. Fousek, N.R. Ivanov, B. Biezina, M. Pave& A. Fouskova, V. LhotskB, V. PetfiEek and 1. Cisaiova, Solid StateCommun. 66 ( 1988) 1003. [ 91 P. Kirkegaard and M. Eldrup, Computer Phys. Commun. 3 (1972) 240. [ IO] Z. Dreger, J. Kalinowski, 1. Davoli, S. Stizza and S. Feliziani, Phys. StatSol. 149(b) (1988) 363.
CHEMICAL PHYSICS LETTERS
20 December I99 I
Electric field effects in positron annihilation in phenanthrene W. Gbmiak a, T. Goworek a and C. Rybka a.b a Marie Curie-Skkniowska University, 20031 Lublin, Poland b Lublin Polytechnic, 20109 Lublin, Poland
Received 20 July 199 I
The variations ofpositronium formation probability at changes oftemperature ofphenanthrene were investigated. All observed effects can be explained as the results of electric field inhibition of Ps formation in a pyroelectric medium.
1. Introduction
In the lifetime spectra of positron annihilation in an insulator, one can distinguish at least three exponential components. The shortest lived of them is ascribed to the annihilation of the singlet bound state of positron and electron (para-positronium, p-Ps), the intermediate one to the free annihilation in collisions, and the longest-lived component to the annihilation of the triplet state (ortho-positronium, oPS). In our early investigations of positronium annihilation in phenanthrene [ 11, we have found that after crossing the phase-transition point at 73°C in an arbitrary direction, from high to low temperature or vice versa, the intensity Is of the long-lived component diminishes, almost without change of the lifetime. This low value of intensity is not stable and increases with the time constant strongly dependent on the temperature. We proposed that the effect could be explained by formation of unstable phases, or preferably, by the pyroelectric effect, i.e. the polarization of the sample at changes of temperature. Experiments with paraffins [ 21 and polymers [ 31 indicate that the electric field reduces the positronium formation probability. The reduction of positronium yield tends to a saturation value at high field, approximately half of its zero-field value (the phenomena occurring at MV/cm fields [ 41 can be neglected in this paper). Polarization phenomena in phenanthrene were observed very early [5,6] in samples biased by an Elsevier Science Publishers B.V.
external field; the polarization currents appeared when the temperature passed the range from the phase-transition point down to about 40°C. Spontaneous polarization at changes of temperature was reported in 1988 by Kroupa et al. [7,8]. The aim of this paper is to collect more data indicating the relation of observed positron annihilation peculiarities in phenanthrene to the pyroelectricity of this solid.
2. Experimental The lifetime spectra of positron annihilation in phenanthrene were measured using a conventional fast-slow-time spectrometer with a resolution (fwhm ) of about 300 ps. The source of positrons, **Na deposited inside a kapton envelope, was sandwiched between two samples of phenanthrene. The temperature of the sample was controlled with the accuracy of 0.2 K. Before annihilation, the positrons can be assumed as thermalized and diffusing in the sample; thus, the effect is independent of field orientation and polycrystalline samples can be used. The lifetime spectra were decomposed, using the POSITRONFIT program [9], into three exponentials convoluted with the Gaussian resolution curve. The longest-lived component, representing the o-Ps decay had the lifetime of about 1 ns. During data processing, the lifetime of the shortest component was fixed at 123 ps. Even if the p-Ps lifetime is misfitted due to the multielectronic character of Ps inside the medium (“quasi-positronium”), it does not 537
20 December 199 I
CHEMICAL PHYSICS LETTERS
Volume 187. number 5 affect
the discussion below, as it concentrates on the long-lived component only.
3. Results and discussion The temperature dependence of the long-lived component intensity and lifetime is shown in fig. 1.
1.6
35
Very slow recovery of the samples in the temperature range below 50°C makes it impossible to wait for full depolarization; thus, the part of the upper curve below 50°C represents the lower limit of IS rather than its real value. The sensitivity of positronium in phenanthrene to the external electric field was investigated at room temperature, i.e. outside the range where the pyroelectric effects appear. In the samples in their “stable” Ps-rich state, reduction of 1, is observed (fig. 2). A sum of an exponential and a constant was fitted to the experimental data. This constant, representing the high-field saturation value of IX,was found equal to 11.3?0.8%, and thus, not far from the I3 observed after the skip from above the phase-transition point to the temperature below 55°C. This means that the electric field in the sample after such a temperature change is of the order of 10’ V/cm or more. The value of (dl,/dE), is approximately 2.9 cm/MV, and thus very similar to that observed in paraffins [ 2 1. The recovery from the low 1, state at room temperature is so slow that the intensity does not change measurably during several days. Thus, it was also
15
10
o
25
50
75
100
Fig. 1. Longest-lived component intensity Z3and lifetime q in the time spectrum ofpositron annihilation in phenanthrene. Full circles represent the stable values, small open triangles the values observed
immediately
after the change of temperature
starting value indicated by a large triangle.
538
10
15
M
25
30
EL.FIELD kV/cm
TEMPERATURE ,“C
from the
Fig. 2. The ortho-positronium intensity 1, at room temperature as a function of applied external electric field: (a) the sample in its “stable” state (after long storage); (b) the sample immediately after the change of temperature from 90°C to room temperature.
20 December 199 I
CHEMICAL PHYSICS LETTERS
Volume 187, number 5
possible to make the measurements with the external electric field applied to the samples exhibiting the low I, value. As can be seen in fig. 2 (curve b), the l3 value is practically independent of the applied field. If the reduction of I3 is due to strong internal fields up to the saturation of the effect, the addition of a much smaller external field cannot influence the result. The change of temperature from T, to T, produces the polarization P=J;::p( T) dT, where p(T) is the pyroelectric coefficient. In phenanthrene, this coefficient is practically zero in the high-temperature phase; it is peaked slightly below the phase-transition point and diminishes gradually in the range down to 30°C [ 7,8], Thus, the reduction of II is not directly related to the phase transition and it should be observed without crossing the phase-transition point. To demonstrate this, the starting points for the change of temperature were chosen to be 65, 60, 55 and 5O”C, and for every starting point, a series of measurements were performed with various end points. The results are shown in fig. 3. The decrease of the long-lived component is quite visible and diminishes with the reduction of the initial temperature; the effect almost disappears when the starting point is 50°C.
Note that the temperature dependence of hl,/l, (curve a in fig. 3) is very similar to that of the polarization P and birefringence An reported in refs. [ 7,8 1, but is more flattened at low temperatures. That is easily explainable by the saturation of Ps-formation effects at high fields. The temperature dependence of I, for the starting point 90” C was measured several times and for various samples, and was reasonably reproducible, while in the paper by Kroupa et al. [ 81, poor reproducibility of the ordinate scale was reported. Similarly as above, the saturation effect renders the annihilation parameters almost insensitive to the variation of high fields; thus, the good reproducibility in our case can be somewhat artificial. The change of temperature in one direction generates the electric field; thus, the skip in the opposite direction between the same temperatures should restore the unpolarized state, if there is no hysteresis. Fig. 4a shows the I, values measured during one cycle of temperatures. The data for 68°C and for the final temperature were accumulated during a short 0.5 h run to avoid the substantial compensation of pyroelectric charges. After the full cycle, the f3 value was almost identical with the initial one. When, after lowering the temperature, the sample is stored and the cycle of temperature is then completed, the electric charge is overcompensated and
0.5
0.4
\
,” 0.3 0 ‘;j ’ 0.2
0.1
0 I
I
30
40
I
50
60
TEMPERATURE
I
70
I
BO
0
I
90
TEMPERATURE ,“C
I
I
L
6
CYCLE NUMBER
, “C Fig. 4. Temperature
Fig. 3. Relative decrease of the long-lived component I, after the change of temperature.
I
2
The starting value of temperature
is
lived component 78°C;
cycling; (a) the intensities I, of the
measured in a cycle of temperatures
long-
7%68-
(b) the I, intensity measured after every cycle in a series
marked by doubling the symbol; the abscissa scale corresponds
of procedures analogous to that in part “a” of the figure. Open
to the final temperature. The lines represent visual guides only.
circles denote I, after the sixth cycle and 24 h storage at 78°C.
539
Volume 187, number 5
CHEMICAL PHYSICS LETTERS
the non-zero polarization remains in the sample. Even if the storage time is short (e.g., the time of measurement ), after every further cycle of this kind, the surplus of charge increases; as a result, the intensity of the long-lived component is systematically reduced. This is shown in fig. 4b. Six cycles were performed; after the last cycle, the sample was stored at 78°C and then I3 recovered to its normal value. While the reproducibility of 13versus T, including the results of all jumps of temperature, is high, the reproducibility of sample recovery is not. In the hightemperature region, the recovery curve could be well approximated by an exponential, the time constant of charge compensation process was relatively stable, but at low temperatures it varied substantially from run to run and from sample to sample (at the same storage temperature) and the recovery curves deviated from a simple exponential. The reciprocal of the time constant can be assumed proportional to the charge-detrapping rate. In spite of time-constant instabilities, a very rough estimate of trap depth could be made (if one assumes one trap level). This estimate is 0.5 eV. In phenanthrene, one observes rather continuous distribution of charge traps with a peak at 0.43 eV [ lo], and thus not far from our estimate. 4. Conclusions All observed effects in positron annihilation in
540
20 December I99 1
phenanthrene can be explained by the action of the electric field generated in the pyroelectric medium by changes of temperature. The reduction of positronium yield is not connected strictly with the phase transition, but it appears in a relatively broad temperature range in the low-temperature phase. No hysteresis of polarization effects was observed.
References [ 1] T. Goworek, W. Gbmiak, R. Wasiewicz, J. Wawryszczuk and C. Rybka, Chem. Phys. Letters 120 ( 1985) 223. [2] W. Brandtand J. Wilkenfeld, Phys. Rev. B 12 (1975) 2579. [ 31 A. Bisi, F. Bisi, A. Fasana and L. Zappa, Phys. Rev. I22 (1961) 1709. 141A. Bisi, G. Gambarini and L. Zappa, Lettere Nuovo Cimento 31 (1981) 58. [ 51R.A. Amdt and A.C. Damask, J. Chem. Phys. 45 (1966) 4627. [6] A.N. Gubkin and A.M. Kozhevnikov, Zh. Fiz. Khim. 42 (1968) 2106. [7] J. Kroupa, I. Fousek, N.R. Ivanov, B. Biezina and V. Lhotsk& Ferroclectrics 79 ( 1988) 189. [ 81 J. Kroupa, J. Fousek, N.R. Ivanov, B. Biezina, M. Pave& A. Fouskova, V. LhotskB, V. PetfiEek and 1. Cisaiova, Solid StateCommun. 66 ( 1988) 1003. [ 91 P. Kirkegaard and M. Eldrup, Computer Phys. Commun. 3 (1972) 240. [ IO] Z. Dreger, J. Kalinowski, 1. Davoli, S. Stizza and S. Feliziani, Phys. StatSol. 149(b) (1988) 363.