close

Вход

Забыли?

вход по аккаунту

?

Dispersion and Spontaneous Current Phenomena in NH4Cl Single Crystals.

код для вставкиСкачать
A N N A L E N D E R PHYSIK
7. Folge. Band 46. 1989. Heft 2, S. 81-160
Dispersion and Spontaneous Current Phenomena
in NH,Ct Single Crystals
By F. EL-KABBANY
Physics Department, Cairo University, Egypt
A b s t r a c t . The effect of frequency and d.c. biasing on the dielectnic constant of NH,Cl single
crystal samples was studied. A dispersion phenomenon was observed in the frequency range lo5lo8 Hz. This was attributed t o a piezoelectric effect.
The present study revealed that a spontaneous electric current of the order 10-l2 A was found t o
accompany the order - disorder process. This current showed a clear singnal in the phase transition
region of the order-disorder transition 111-11 of NH,CI. This singnal was well controlled by the application of a polarizing electric field so that its amplitude could be increased, decreased, vanish or
even reversed by the application of an appropriate external electric field.
Dispersion und spontan fliegender Strom in NaHC1-Einkristallen
Es werden die Frequenzabhangigkeit der Dielektrizitiitskonstanteund der EinfluS einer Vorpolarisation a n NH,Cl-Einkristallproben untersucht. Bei den Frequenzen von lo5- los Hz wird ein
Dispersionsphiinomen beobachtet, was auf einen piezoelektrischen Effekt zuriickgefiihrt wird.
Die Untersuchung zeigt, daB der ubergang in den ungeordneten Zustand bei NH,CI von einem
A begleitet wird. Seine GroBe und Richtung ksnn durch ein
Strom in der GroBenordnung von
polarisierendes, elektrisches Feld beeinfluBt werden.
Introduetion
It is well known that ammonium chloride undergoes a number of structure modifications a t atmospheric pressure a t certain well defined transition temperatures [I].
These modifications are characterized by the presence of ordering and disordering of
ions [l, 21. It is now becoming increasingly clear that reliable understanding of the
various electric properties of NH,C1 during its different phase transitions should be
made in terms of order - disorder phenomenon. This report covers some of these properties and is considered as a n extending part of a previously published work [I, 21.
We report here what we believe to be very interesting observations of dielectric measurements and spontaneous thermal current in the immediate vicinity of a critical phase
transition region.
The sitbstance studied here initially chosen because the nature of the transition was
apparently simple and well understood [3, 41. The lattice here has a cubic symmetry
both above and below the transition point. The stable transformations of ammonium
chloride are :
--3ooc_
Phase (111) F-
185OC
Phase (11)-+
Phase (I).
The crystal structure of the three phases is given in Table 1.
Ann. Physik Leipzig 46 (1989) 2
a2
Experimental
The dielectric constant “ E ’ O of NH,C1 Crystal was measured here by a capacitance
measuring bridge [5] type Philips RM6302.
Specially designed system is used for ionic thermocurrent measurements over it
wide range of temperature. An electrometer capable of measuring
A is used. I n
some case, it is desired t o find the effect of polarizing electric field. I n this case, the
sample was polarized before carrying out the measurements. This was done by applying
a polarizing voltage across the sample (electrometer removed). The externally applied
voltage was then removed, the electrometer was reconnected and the measurements
proceded as before.
Table 1
Phase
Space Group
I
I1
111
~ m 3 m
’
State
Pm3m
disordered
disordered
~2 nmm
ordered
Results
i ) F r e q u e n c y d e p e n d e n c e of d i e l e c t r i c c o n s t a n t
The frequency dependence of the dielectric constant “E” was studied in the frequency range 1kHz t o 1000 MHz. The results of these measurements in the three phases
I, I1 and I11 are shown in Fig. 1. One can see that the variation of the dielectric constant
with frequency has two clearly defined regions of dispersion in the studied frequency
90
-
.
o
-
80
Phase I
Phase
I[
7060
E
-
50-
LO 3c-
20-
lo103
104
105
106
lo7
F r e q u e n c y , Hz
108
109
Fig. 1. Frequcncy dependence of the dielectric constant of NH,Cl
F. EL-K~BBANY,
Dispersion and Spontaneous Current in Crystals
83
range. The first is connected with the clamping out of the piezoelectric oscillations of
the crystals as a whole in the range lo5- lo6 Hz. This dispersion of ‘‘G” has a resonance
character occnring a t m 106 Hz. The second dispersion region lies in the region lo74 x 108 Nz.
ii) S p o n t a n e o u s t h e r m a l c u r r e n t
It was found that when NH,Cl samples were thermally cycled through the low temperature phase transition 111-11, a spontaneous or self polarization current is produced. This current is attributed to the rearrangement of charges in the samples which
occur tiiiririg the phase transition and not due to an externally applied field [GI.
1
I
-1
c
L I
-60
I
-50 -4;
I
-30
I
-20
I
-10
I
0
I
10
1.T
Fig, 2. Variation of spontaneous current with temperature during order-disorder transition 111-11
at different polarizing fields
The variation of spontaneous polarization current with temperature (at zero external
polarizing field E p ) is shown in Fig. 2 curve 1. The delay in the onset of the signal peak
after going through the transition point may be due to the existence of slight teniperature gradient in the sample such that the actual temperature is lower than that of the
thermocouple.
The effect of applying a polarizing electric field “EP” is shown in Fig. 2, curves 2 and 3.
After applying “EP”,the polarizing current curve is slightly modified. The maxima
of the observed current signal are shifted to higher temperatures in the presence of
external applied electric field. However, the amplitude of the signal could be increased,
decreased, vanish or even reversed by application of an appropriate external electric
field. Also, it was found experimentally that the current signal is more pronounced
for lower heating rates. However, the observation of this polarization current was
84
Ann. Physik Leipzig 46 (1989) 2
repeated and was found t o be reproducible if the temperature of the sample did not
vary appreciably from the transition temperature. According to the experimental
observations of the spontaneous current created during the order-disorder phase transition, there are two important factors controlling the generation of such current. These
are :
1. The heating rate is a very essential parameter. For fast heating rates the variation
in such current seems to be less sensitive to the order-disorder transition.
2 . The biasing voltage across the sample controls this current completely. However, it
was found that low biasing voltage is preferable.
Discussion
The variation of the dielectric constant with frequency showed the presence of
dispersion behaviour in the three phases I, 11, and IT[. This dispersion is more pronounced in phase 11. Also, as expected, the dielectric constant has greatly increased a t
low frequencies. However, the most important feature of the dispersion, a t the frequencies lo5- lo6 Hz, is the large changing of the dielectric constant. In this frequency range,
the dielectric constant is that of crystal in the state with the complete clamping of
mechanical oscillations of the individual domains. The dispersion of “E” centred a t the
frequencies 107- 4 x 108Hz must be related to the piezoelectric resonance of individual
domains. The dispersion was observed before in many salts but a t different frequencies.
The frequency difference is due to the large tlifferencieP in sizes of domains in the various
crystals. The dielectric constant of the tested NH,Cl crystal reached stationary values
when the frequency exceed los Hz and with subsequent increasing of the frequency,
it was not significantly changed.
The present work includes observations 011 the spontaneous or polarization current
in NH4C1 samples when they are thermally cycled through its order-disorder phase
transition 111-11. This current is due to the rearrangement of charges in the crystal
that occurs during the transition and not rlne to a n externally applied electric field [i’].
It is well known that in the ordered state, NH4C1is piezoelectric [8]. The results showed
a characteristic current peak a t the transition point. This might be related to the orientation or disorientation of the NH4+ions as the NH,Cl crystals was taken through the
transition. The origin of the phenomenon of spontaneous polarization current in NH,Cl
is not widely known. This polarization current is usually associated with the phase
transition temperature. However, two reasons might be suggested which are probably
responsible for this current. These are :
1. P i e z o e l e c t r i c e f f e c t
It is well known that NH4Cl is piezoelectric in its ordered phase [9]. This means
that if NH4Cl sample becomes strained as it goes through the transition, one would
expect to see a spontaneous current due t o this effect. The NH4C1 sample could
become strained as a result of existing temperature gradient. According t o Mohler
[9], there exist piezoelectric domains in ammonium chloride and the domain structure
can be modified by the application of a suitable external field.
2. P e r m a n e n t dipoles
The ammonium ions do not have a dipole moment but dipoles may present in the
crystal as a result of any of the following reasons:
a) molecular defects such as NH, or HC1 due t o the possibility of a proton [lo]
transfer in ammonium salts.
F.EL-KABBANY,
Dispersion and Spontaneous Current in Crystals
85
b) dipole moment of anions due to octopolar interaction of NH, tetrahedra [ll].
Impurity - Vacancy dipoles.
Just the dipoles do exist in the crystal, it is easy to understand how the spontaneous polarization comes about.
C)
However, the present work suggests that the dipoles become aligned as the crystal
changes from the disordered state to the ordered state, and in turn produces a spontaneous current in the external circuit. A reversing of this current is produced as the crystal
is brought from the ordered state to the disordered state. However, it seems possible
that the spontaneous current is due t o dipoles snapping into or out of alignment as the
temperature in the different parts of the crystal pass through the transition temperature.
References
[I] BADR,Y. A.; EL-KABBANY,
I?.; SAID,G.; SALEH,0. W.: Phys. Stat. Sol. (a) 60 (1980) 297.
F.: Sensation of some electric properties for ordering and disordering in NH,CI,
121 EL-KABBANY,
under publication.
[3] WADDINQTON,
T. C.: 5. Chem. SOC.A 4 3 (1958) 40.
J. A.; STEVENSON,
D. P.: J. Chem. Phys. 28 (1958) 929.
[4] IBERS,
[5] EL-KABBANY,
F.: Material Science and Engineering 56 (1982) 149.
161 KESSLER,
A.: J. Electrochem. SOC.123 (1976).
[7] CHARLES,
P.; HORST,S.; PAUL,
S.; GEORQE,
F.: Phys. Rev. 4 (1971) 907.
181 BAHRS,S.: Z. Phys. 106 (1937) 407.
E.; PITKA,
R.: Solid State Common. 14 (1964) 791.
[9] MOHLER,
[lo] HERINQTON,
M.; STAVELY,
L. A. K.: J. Phys. Chem. Solid 26 (1964) 921.
[ll] HULLER,A.: Z. Phys. 264 (1972) 456.
Bei der Redaktion eingegangen am 7. Marz 1986.
Anschr. d. Verf.: Dr. F. EL-KABBANY
Physics Department
Faculty of Science
Cairo University,
Cairo, Egypt
Документ
Категория
Без категории
Просмотров
0
Размер файла
277 Кб
Теги
crystals, current, phenomena, spontaneous, nh4cl, single, dispersion
1/--страниц
Пожаловаться на содержимое документа