The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆
For the proper uniaxial ferroelectrics Sn₂P₂Se₆ with the controlled content of different type of impurities the investigations of dielectric permeability temperature dependence are performed with the aim to determine the influence of the crystal structure defects upon: the efficiency of the thermal...
Збережено в:
| Опубліковано в: : | Condensed Matter Physics |
|---|---|
| Дата: | 1999 |
| Автори: | , , , |
| Формат: | Стаття |
| Мова: | English |
| Опубліковано: |
Інститут фізики конденсованих систем НАН України
1999
|
| Онлайн доступ: | https://nasplib.isofts.kiev.ua/handle/123456789/120525 |
| Теги: |
Додати тег
Немає тегів, Будьте першим, хто поставить тег для цього запису!
|
| Назва журналу: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Цитувати: | The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ / Yu.M. Vysochanskii, A.A. Molnar, M.M. Khoma, S.F. Motrja // Condensed Matter Physics. — 1999. — Т. 2, № 3(19). — С. 421-434. — Бібліогр.: 24 назв. — англ. |
Репозитарії
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
nasplib_isofts_kiev_ua-123456789-120525 |
|---|---|
| record_format |
dspace |
| spelling |
Vysochanskii, Yu.M. Molnar, A.A. Khoma, M.M. Motrja, S.F. 2017-06-12T09:29:03Z 2017-06-12T09:29:03Z 1999 The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ / Yu.M. Vysochanskii, A.A. Molnar, M.M. Khoma, S.F. Motrja // Condensed Matter Physics. — 1999. — Т. 2, № 3(19). — С. 421-434. — Бібліогр.: 24 назв. — англ. 1607-324X DOI:10.5488/CMP.2.3.421 PACS: 64.70.Rh, 67.70.Kb, 64.60.Fr https://nasplib.isofts.kiev.ua/handle/123456789/120525 For the proper uniaxial ferroelectrics Sn₂P₂Se₆ with the controlled content of different type of impurities the investigations of dielectric permeability temperature dependence are performed with the aim to determine the influence of the crystal structure defects upon: the efficiency of the thermal memory effect recording in the incommensurate (IC) phase; the second order phase transition (PT) from the paraelectric phase to the IC phase at temperature Ti and upon the first order PT from IC phase to ferroelectric phase at temperature Tc; the anomalous hysteresis of the dielectric properties temperature dependence in the IC phase; the dielectric contribution of the domain walls in the ferroelectric phase. Static defects smear the anomaly at the PT from paraelectric phase to IC phase, increase the anomalous hysteresis in the IC phase and the hysteresis of the lock-in transition temperature Tc, suppress the dielectric contribution of domain walls in the ferroelectric phase and destroy the memory effect in the IC phase. The increase of the charge carrier concentration also suppresses the dielectric output of the domain walls in the ferroelectric phase but at the same time it supports a more clear memory recording in the IC phase. Such a tendency agrees with the estimations in the mean-field approximation for the characteristics of a domain structure in the ferroelectric phase and memory effect in the IC phase in the ferroelectrics-semiconductors investigated. Для власного одновiсного сегнетоелектрика Sn₂P₂Se₆ з неконтрольованим вмiстом домiшок рiзного типу виконанi дослiдження температурної залежностi дiелектричної проникностi для встановлення впливу дефектностi кристалiчної структури на ефективнiсть запису термiчної “пам’ятi” в неспiвмiрнiй (НС) фазi, на фазовий перехiд (ФП) другого роду з параелектричної фази до НС фази при температурi i та на ФП першого роду з НС фази до сегнетоелектричної фази при температурi c, на аномальний гiстерезис температурної залежностi дiелектричних властивостей в НС фазi, на дiелектричний вклад доменних стiнок у сегнетоелектричнiй фазi. Статичнi дефекти розмивають аномалiї при ФП з параелектричної в НС фазу, збiльшують аномальний гiстерезис в НС фазi та гiстерезис температури c lock-in переходу, подавляють дiелектричний вклад доменних стiнок у сегнетофазi та руйнують ефект “пам’ятi” в НС фазi. Зростання концентрацiї носiїв заряду також подавляє дiелектричний вiдклик доменних стiнок у сегнетоелектричнiй фазi, однак поряд з цим сприяє чiткішому запису ”пам’ятi” в НС фазi. Така тенденцiя погоджується з виконаними оцiнками в наближеннi середнього поля для характеристик доменної структури в сегнетоелектричнiй фазi та для ефекту “пам’ятi” в НС фазi для дослiджуваних сегнетоелектрикiв- напiвпровiдникiв. This work has been partially supported by INTAS within the project INTAS– 93–3230–ext. en Інститут фізики конденсованих систем НАН України Condensed Matter Physics The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ Вплив дефектів і провідності на властивості доменної структури та ефект пам’яті в сегнетоелектриках-напівпровідниках Sn₂P₂Se₆ Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ |
| spellingShingle |
The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ Vysochanskii, Yu.M. Molnar, A.A. Khoma, M.M. Motrja, S.F. |
| title_short |
The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ |
| title_full |
The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ |
| title_fullStr |
The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ |
| title_full_unstemmed |
The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ |
| title_sort |
influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors sn₂p₂se₆ |
| author |
Vysochanskii, Yu.M. Molnar, A.A. Khoma, M.M. Motrja, S.F. |
| author_facet |
Vysochanskii, Yu.M. Molnar, A.A. Khoma, M.M. Motrja, S.F. |
| publishDate |
1999 |
| language |
English |
| container_title |
Condensed Matter Physics |
| publisher |
Інститут фізики конденсованих систем НАН України |
| format |
Article |
| title_alt |
Вплив дефектів і провідності на властивості доменної структури та ефект пам’яті в сегнетоелектриках-напівпровідниках Sn₂P₂Se₆ |
| description |
For the proper uniaxial ferroelectrics Sn₂P₂Se₆ with the controlled content
of different type of impurities the investigations of dielectric permeability
temperature dependence are performed with the aim to determine the influence of the crystal structure defects upon: the efficiency of the thermal
memory effect recording in the incommensurate (IC) phase; the second
order phase transition (PT) from the paraelectric phase to the IC phase
at temperature Ti and upon the first order PT from IC phase to ferroelectric phase at temperature Tc; the anomalous hysteresis of the dielectric
properties temperature dependence in the IC phase; the dielectric contribution of the domain walls in the ferroelectric phase. Static defects smear
the anomaly at the PT from paraelectric phase to IC phase, increase the
anomalous hysteresis in the IC phase and the hysteresis of the lock-in
transition temperature Tc, suppress the dielectric contribution of domain
walls in the ferroelectric phase and destroy the memory effect in the IC
phase. The increase of the charge carrier concentration also suppresses
the dielectric output of the domain walls in the ferroelectric phase but at
the same time it supports a more clear memory recording in the IC phase.
Such a tendency agrees with the estimations in the mean-field approximation for the characteristics of a domain structure in the ferroelectric phase
and memory effect in the IC phase in the ferroelectrics-semiconductors investigated.
Для власного одновiсного сегнетоелектрика Sn₂P₂Se₆ з неконтрольованим вмiстом домiшок рiзного типу виконанi дослiдження температурної залежностi дiелектричної проникностi для встановлення
впливу дефектностi кристалiчної структури на ефективнiсть запису
термiчної “пам’ятi” в неспiвмiрнiй (НС) фазi, на фазовий перехiд (ФП)
другого роду з параелектричної фази до НС фази при температурi i
та на ФП першого роду з НС фази до сегнетоелектричної фази при
температурi c, на аномальний гiстерезис температурної залежностi
дiелектричних властивостей в НС фазi, на дiелектричний вклад доменних стiнок у сегнетоелектричнiй фазi. Статичнi дефекти розмивають аномалiї при ФП з параелектричної в НС фазу, збiльшують аномальний гiстерезис в НС фазi та гiстерезис температури c lock-in переходу, подавляють дiелектричний вклад доменних стiнок у сегнетофазi та руйнують ефект “пам’ятi” в НС фазi. Зростання концентрацiї
носiїв заряду також подавляє дiелектричний вiдклик доменних стiнок
у сегнетоелектричнiй фазi, однак поряд з цим сприяє чiткішому запису ”пам’ятi” в НС фазi. Така тенденцiя погоджується з виконаними
оцiнками в наближеннi середнього поля для характеристик доменної структури в сегнетоелектричнiй фазi та для ефекту “пам’ятi” в НС
фазi для дослiджуваних сегнетоелектрикiв- напiвпровiдникiв.
|
| issn |
1607-324X |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/120525 |
| citation_txt |
The influence of defects and conductivity on the domain structure properties and the memory effect in the ferroelectrics-semiconductors Sn₂P₂Se₆ / Yu.M. Vysochanskii, A.A. Molnar, M.M. Khoma, S.F. Motrja // Condensed Matter Physics. — 1999. — Т. 2, № 3(19). — С. 421-434. — Бібліогр.: 24 назв. — англ. |
| work_keys_str_mv |
AT vysochanskiiyum theinfluenceofdefectsandconductivityonthedomainstructurepropertiesandthememoryeffectintheferroelectricssemiconductorssn2p2se6 AT molnaraa theinfluenceofdefectsandconductivityonthedomainstructurepropertiesandthememoryeffectintheferroelectricssemiconductorssn2p2se6 AT khomamm theinfluenceofdefectsandconductivityonthedomainstructurepropertiesandthememoryeffectintheferroelectricssemiconductorssn2p2se6 AT motrjasf theinfluenceofdefectsandconductivityonthedomainstructurepropertiesandthememoryeffectintheferroelectricssemiconductorssn2p2se6 AT vysochanskiiyum vplivdefektívíprovídnostínavlastivostídomennoístrukturitaefektpamâtívsegnetoelektrikahnapívprovídnikahsn2p2se6 AT molnaraa vplivdefektívíprovídnostínavlastivostídomennoístrukturitaefektpamâtívsegnetoelektrikahnapívprovídnikahsn2p2se6 AT khomamm vplivdefektívíprovídnostínavlastivostídomennoístrukturitaefektpamâtívsegnetoelektrikahnapívprovídnikahsn2p2se6 AT motrjasf vplivdefektívíprovídnostínavlastivostídomennoístrukturitaefektpamâtívsegnetoelektrikahnapívprovídnikahsn2p2se6 AT vysochanskiiyum influenceofdefectsandconductivityonthedomainstructurepropertiesandthememoryeffectintheferroelectricssemiconductorssn2p2se6 AT molnaraa influenceofdefectsandconductivityonthedomainstructurepropertiesandthememoryeffectintheferroelectricssemiconductorssn2p2se6 AT khomamm influenceofdefectsandconductivityonthedomainstructurepropertiesandthememoryeffectintheferroelectricssemiconductorssn2p2se6 AT motrjasf influenceofdefectsandconductivityonthedomainstructurepropertiesandthememoryeffectintheferroelectricssemiconductorssn2p2se6 |
| first_indexed |
2025-11-27T03:09:12Z |
| last_indexed |
2025-11-27T03:09:12Z |
| _version_ |
1850796171642011648 |
| fulltext |
Condensed Matter Physics, 1999, Vol. 2, No. 3(19), pp. 421–434
The influence of defects and
conductivity on the domain structure
properties and the memory effect in the
ferroelectrics-semiconductors Sn2P2Se6
Yu.M.Vysochanskii, A.A.Molnar, M.M.Khoma, S.F.Motrja
Institute of Solid State Physics and Chemistry Uzhgorod University,
46 Pidgirna Str., 294000 Uzhgorod, Ukraine
Received July 6, 1998
For the proper uniaxial ferroelectrics Sn2P2Se6 with the controlled content
of different type of impurities the investigations of dielectric permeability
temperature dependence are performed with the aim to determine the in-
fluence of the crystal structure defects upon: the efficiency of the thermal
memory effect recording in the incommensurate (IC) phase; the second
order phase transition (PT) from the paraelectric phase to the IC phase
at temperature Ti and upon the first order PT from IC phase to ferroelec-
tric phase at temperature Tc; the anomalous hysteresis of the dielectric
properties temperature dependence in the IC phase; the dielectric contri-
bution of the domain walls in the ferroelectric phase. Static defects smear
the anomaly at the PT from paraelectric phase to IC phase, increase the
anomalous hysteresis in the IC phase and the hysteresis of the lock-in
transition temperature Tc, suppress the dielectric contribution of domain
walls in the ferroelectric phase and destroy the memory effect in the IC
phase. The increase of the charge carrier concentration also suppresses
the dielectric output of the domain walls in the ferroelectric phase but at
the same time it supports a more clear memory recording in the IC phase.
Such a tendency agrees with the estimations in the mean-field approxima-
tion for the characteristics of a domain structure in the ferroelectric phase
and memory effect in the IC phase in the ferroelectrics-semiconductors in-
vestigated.
Key words: domain structure, ferroelectrics-semiconductors, influence of
defects
PACS: 64.70.Rh, 67.70.Kb, 64.60.Fr
1. Introduction
Interesting relaxation phenomena are observed [1] for the proper ferroelectrics-
semiconductors Sn2P2S6 and Sn2P2compound, the temperature dependence of di-
c© Yu.M.Vysochanskii, A.A.Molnar, M.M.Khoma, S.F.Motrja 421
Yu.M.Vysochanskii et al.
electric susceptibility at the second order ferroelectric PT after a long term temper-
ature stabilization in the paraelectric phase near T0 splits into two anomalies and
probably there appears an intermediate IC phase. For selenium compound in the
IC phase of the memory effect recording becomes more efficient at the increase of
the concentration of charge carriers. The dielectric output of domain walls in the
ferroelectric phase of Sn2P2Se6 also depends on the specimen conductivity [2]. At
the increase of the concentration of static defects, which occurs in the solid solutions
(PbySn1−y)2P2Se6 and Sn2P2(SexS1−x)6, the smearing of dielectric constant temper-
ature anomalies is observed at the PT (Ti) from paraelectric phase to IC phase and
at a lock-in transition (Tc) from IC phase to ferroelectric phase. At the same time,
both the hysteresis of the value of Tc at cooling and at heating and the anomalous
temperature hysteresis of the thermodynamic properties of the crystal in the IC
phase are increasing [3].
It is interesting to compare the influence of static and dynamic defects on the
nonequilibrium behaviour of proper ferroelectrics-semiconductors in the incommen-
surate and ferroelectric phases. To this end, we investigated the dielectric properties
of Sn2P2Se6 crystals with different impurities and the effectiveness of the recording
of a thermal memory effect in the IC phase of these crystals. The atoms Pb, S, and
Mn were used as the impurities.
In case of Sn by Pb substitution the continuous solid solutions
(PbySn1−y)2P2Se6 are realized. In the paraelectric phase of these crystals the atoms
of metal occupy the central symmetric positions. Therefore, the Pb atoms in the
structure of Sn2P2Se6 play the role of the impurities of “random temperature” type
[4]. The two-mode concentration evolution of the Raman spectra of the low energy
optical vibrations with participation of metal translations testifies to the fact that
Pb atoms are quite “rigid” impurities [5]. This fact determines the decrease of T i
and Tc till 0 K at y increasing to 0.4 and 0.65, correspondingly [3]. At Se by S substi-
tution in the solid solutions Sn2P2(SexS1−x)6 the one-mode concentration behaviour
of the soft optic mode spectral line is observed [5]. Thus, the substitution of atoms
induces deformation of the anions (P2Se6)
4−. The center of mass of these anions in
the paraelectric phase coincides with the symmetry center of the elementary cell.
So, atoms of S in the structure of Sn2P2Se6 serve as defects of a ”random field”
type [4]. Such defects probably are quite weak because static dipoles of acentric an-
ion complexes obviously have little effect on the dynamics of the ferroactive cation
sublattice.
According to the EPR data [6] the ions Mn2+ in crystal Sn2P2Se6 serve as impu-
rities of impression. They occupy vacant places in the elementary cell of the crystal,
which coincide with the center inversion in the paraelectric phase. The ions Mn2+
obviously appear as impurities of a “random temperature” type. It is important that
introducing the Mn in the Sn2P2Se6 structure strongly destroys the electro-neutrality
and changes the electro-conductivity of the crystals.
422
Domain structure properties of Sn2P2Se6
2. Experimental results
The vapour-transport and Bridgeman methods were used for growing the crystals
[3]. The crystals Sn2P2Se6 obtained by Bridgeman method had the specific resistance
was ρ ∼5·1010 Ω·m. For (Pb0.05Sn0.95)2P2Se6 crystals obtained by vapour-transport
method the specific resistance was ρ ∼3·1010 Ω·m. Nominally clean Sn2P2Se6 crys-
tals obtained by vapour-transport had ρ ∼ 4.2·108 Ω·m, and for Sn2P2(S0.005Se0.995)6
crystals with impurity of Mn prepared using the latter method ρ ∼5.5·107 Ω·m. The
specimens had the dimensions about 4×3×1 mm3. On the normal to polar direction
[100] faces, the gold was evaporated for preparing the electric contacts. The tem-
perature dependencies of the dielectric permeability are measured by the computer
controlled equipment [1]. The measurement accuracy was 0.1% for the real part ε ′
and 0.5% for the imaginary part ε′′ of complex permeability. The measurements were
performed at the frequency 104Hz. The value of electric field was 0.05 V/cm. The
samples were measured in conditions of darkness or at a controlled illumination. Be-
fore taking measurements the samples were being annealed in the paraelectric phase
for three hours. At the thermal memory recording the temperature was stabilized
to within 0.002 K. At taking the measurements, the temperature changing rate was
0.1 K/min.
The dielectric properties of two kinds of Sn2P2Se6 crystals – Bridgeman type
(B) and vapour-transport type (V) – differ noticeably. We observe the Curie Weiss
like ε′(T) dependence in the paraelectric phase of crystal B (figure 1). In the case
of crystal V the electric conductivity strongly effects the ε ′(T) behaviour (figure 2).
Some difference also exists for the dielectric constant temperature dependencies in
the IC phase of these two samples. Big difference is observed in the ferroelectric
phase. Here we can see a very strong dielectric output from the domain walls.
For B crystal, the memory effect does not occur at the temperature stabilization
in darkness up till 12 hours. This effect occurs with small amplitude only at the same
time temperature stabilization at white light illumination (figure 3). The memory
effect is very distinctly pronounced for V crystal already after a two hour exposure
Figure 1. The temperature dependen-
cies of the dielectric constant at cool-
ing and heating for Sn2P2Se6 crystal
with low electric conductivity (grown
by Bridgeman method).
Figure 2. The temperature depen-
dencies of the dielectric constant at
cooling for Sn2P2Se6 crystal with
high electric conductivity (grown by
vapour-transport method).
423
Yu.M.Vysochanskii et al.
Figure 3. The thermal memory recording in the IC phase of the Bridgeman type
crystal Sn2P2Se6 at white light illumination during the temperature stabilization
of 12 hours. On insert: the reduced anomalous part of the dielectric constant that
is related to the thermal memory effect.
in darkness (figure 4). Both the amplitude and temperature interval of the memory
effect increase if we illuminate the sample with a white light at the process of tem-
perature stabilization (figure 4a). The spectral sensitivity of memory recording has
been observed as well (figure 4b).
The substitution of Sn by Pb in solid solution (PbySn1−y)2P2Se6 significantly
smears the anomaly of ε’(T) at Ti, increases the hysteresis of the temperature Tc
of PT from IC phase to ferroelectric phase and decreases the dielectric output of
the domain walls in the ferroelectric phase (figure 5). The influence of the cation
substitutions is very strong – for example, the hysteresis of Tc increases from 0.5 K
at y=0 to 12 K at y=0.2 [8].
Figure 4. The reduced anomalous part of the dielectric constant that is related
to the thermal memory effect in the IC phase of the vapour transport type crystal
Sn2P2Se6 at the temperature stabilization time of 2 hours: a) under white light
illumination (1) and in darkness (2); b) under illumination by the light with
different wavelength (1 – 1.000 µm , 2 – 0.560 µm , 3 – 0.666 µm).
424
Domain structure properties of Sn2P2Se6
Figure 5. The temperature de-
pendencies of the dielectric con-
stant at cooling and heating for
(Pb0.05Sn0.95)2P2Se6 crystal.
Figure 6. The result of the thermal
memory recording in the IC phase
of the (Pb0.05Sn0.95)2P2Se6crystal at
white light illumination during the
temperature stabilization for 5 hours.
It was found that for (Pb0.05Sn0.95)2P2Se6 crystals at the temperature stabiliza-
tion in the IC phase for 5 hours upon white light illumination of the sample, the
anomalies on the temperature dependencies of ε′(T), which are characteristic to the
memory effect recording, don’t occur (figure 6). Earlier [1] it was shown that at the
same conditions for the nominally clean Sn2P2Se6 crystals with similar level of a
specific electrical conductivity the memory effect occurs clearly enough.
Partial substitution of Se by S slightly increases the anomalous hysteresis of
ε′ (T) in IC phase and hysteresis Tc (from 0.5 K at x=1 to 1.5 K at x=0.5) [3].
Introducing the impurity atoms into the anion sublattice also decreases the dielectric
output of domain walls in the ferroelectric phase. Moreover, introducing the Mn
impurity into the array of Sn2P2Se6 significantly raises the electro-conductivity of
specimens. Therefore, we even don’t observe the anomaly at Ti on the temperature
dependence of dielectric constant (figure 7). Along with this the dielectric output of
domain walls in the ferroelectric phase is strongly suppressed.
In crystals Sn2P2(S0.005Se0.995)6 with the impurity of Mn, the “memory” develops
in the strongest way. For these crystals, the illumination at the temperature of
stabilization significantly refines the recording of the named effect (figure 8).
Generally, it was determined that the concentration increment (Pb in Sn2P2Se6)
of the static defects makes the recording of the memory effect impossible. At an
increase of the concentration of the free charge carriers (crystals Sn2P2(S0.005Se0.995)6
with impurity of Mn) the recording of “memory” becomes better. At the same time,
the static defects smear the anomaly ε’(T) at Ti. They significantly amplify the
anomalous hysteresis and deform the anomaly of ε ′(T) in the vicinity of Tc. But
even at a large concentration of the static defects the lock-in PT at Tc looks like a
sharp first order transition. Upon heating the clear jump on ε′(T) dependence at Tc
is observed. The increase of the concentration of the free charge carriers decreases the
dielectric output of the domain walls in the ferroelectric phase more effectively than
the increase of a quantity of static defects. At a large concentration of free carriers
the first order PT at Tc clearly occurs on ε’(T) dependencies, and the anomalous
hysteresis slightly increases.
425
Yu.M.Vysochanskii et al.
3. Discussion of the results
Figure 7. The temperature dependen-
cies of the dielectric constant at cool-
ing and heating for Sn2P2(S0.005Se0.995)6
crystal with the impurity of Mn.
Figure 8. The reduced anomalous part
of the dielectric constant that is related
to the thermal memory effect in the
IC phase of Sn2P2(S0.005Se0.995)6 crystal
with the impurity of Mn at the tempera-
ture stabilization time of 2 hours in dark-
ness (1) and under white light illumina-
tion (2).
For ferroelectrics-semiconductors ba-
sed on Sn2P2Se6 compound, the mem-
ory effect in the IC phase occurs due
to the emerging of the density wave of
charge carriers. This wave is caused by
the drift of charge carriers in the non-
homogenous electric field of the sponta-
neous polarization wave at a long time
temperature stabilization and following
the localization of these carriers on the
trapping levels with the energy about
0.7 eV [1]. The theory of the memory
effect caused by trapped charge carri-
ers was developed in [9] and has been
used for the investigated crystals ear-
lier [1,10]. The data of this work regard-
ing the increment of the memory effect
at the increase of the concentration of
charge carriers agree with the previous
analysis.
We can try to qualitatively explain
the disappearing of the memory effect in
the (PbySn1−y)2P2Se6 series the follow-
ing way. The rise of the concentration of
static defects promotes the destruction
of the coherent wave of the spontaneous
polarization. At a large concentration of
such defects one can consider even the
evolution of the IC phase to “chaotic”
phase or to a state of dipole glass. The
peculiarities of such a state are clearly seen in the solid solutions (Pb ySn1−y)2P2Se6
with 0.4 < y < 0.65 for which the IC phase occurs in the region of 100–0 K [3,11,12].
Simultaneous substitution of Sn by Pb and Se by S destroys the long-range order
in all the temperature range of IC phase and even at a high temperature [8]. Thus,
conditions for the relaxation emerging of a density wave of charge carriers disappear
and we have no possibility to record the “memory”.
Here at analysing the experimental data we will pay most attention to explaining
the observed correlation between the efficiency of the thermal memory recording in
the IC phase and to the domain structure dielectric properties in the ferroelectric
phase.
Both factors, the increase of the concentration of static defects and the increase of
the concentration of free charge carriers, change the concentration and the mobility
426
Domain structure properties of Sn2P2Se6
of the domain walls in the ferroelectric phase and, as a result, their dielectric response
[13] is changed.
The increase of the concentration of the static defects increases the quantity of
domain walls which appear in the sample at PT from IC phase to ferroelectric phase.
However, in this case, their dielectric contribution into ε ′ slightly decreases due to
decrease in their mobility.
At the same time, the increase of the concentration of the free charge carriers
promotes the compensation of a depolarizing field. By this, at PT from IC phase
to ferroelectric phase, a small quantity of the domain walls appears and even the
monodomain state can occur. If domain walls are charged, they can be effectively
compensated by free carriers. This induces a significant decrease of the mobility of
the domain walls. The piezoelectric interaction between spontaneous polarization
and deformation also changes the characteristics of the domain structure.
The crystals Sn2P2S(Se)6 are proper uniaxial ferroelectrics with the symmetry
changing P21/c – Pc at phase transition from paraelectric to ferroelectric phase. The
180◦ domains with anti-collinear orientation of the spontaneous polarization vector−→
Ps occur in the ferroelectric phase. Earlier, the domains in Sn2P2S6 were observed by
the optic method using the liquid crystals [14] and by the non-linear optic method
based on the two-beam interaction [15]. According to the data [16] regarding the
directed light scattering by the domain walls in the ferroelectric phase of Sn 2P2S6,
these walls are oriented at some angle relative to the spontaneous polarization vector
and, consequently, they are charged.
The analysis of the properties of domain structures in uniaxial ferroelectrics in
the mean-field approximation has been performed by many authors [17–20]. Follow-
ing these works we will use the thermodynamic potential density
F = F0 +
α
2
· P 2 +
β
4
· P 4 +
γ
6
· P 6 +
δ
2
(
∂P
∂z
)2
+ ..., (1)
where α = αT (T − T0) and all other coefficients are positive. From (1) after mini-
mization we find
P 2 = P 2
0 · sinh2K · z
cosh2K · z + γ·P 2
0
2·γ·P 2
0
+
3
2
β
, (2)
where
K = P0 ·
√√√√
(
γ · P 2
0 +
β
2
) /
δ (3)
and P0 follows from the equation α+ β · P 2
0 + γ · P 4
0 = 0. The domain wall width is
rc = K−1. At γ = 0 we find
P = P0 · tanh
(
z
rc
)
, rc =
√
2 · δ
α
, P0 =
√
−α
β
. (4)
In this case the expression for the specific surface energy of the domain wall has the
following form [18]
σ0 =
4 · δ · P 2
0
3 · rc
. (5)
427
Yu.M.Vysochanskii et al.
After adding to (1) the density of kinetic energy
T =
ρ
2
(
∂u
∂t
)2
=
m
2
·
(
∂P
∂t
)2
, m =
ρ · a6
e∗2
, (6)
where: ρ – the density of crystal, u – the shift of the ferroactive ions, a – the
dimension of an elementary cell, e∗ – the effective charge which is connected with
the equilibrium polarization P0 and with the equilibrium shift u0 by the relation
P0 =
e∗ · u0
a3
. (7)
In this case the surface energy of the domain wall moving with velocity ν is
σ (ν) =
σ0√
1− ν2
c2
= m (ν) · c20. (8)
Here
c0 =
√
δ
m
, m (ν) =
σ0
c20
(√
1− ν2
c20
)−1
= m0 ·
(
1− ν2
c20
)− 1
2
(9)
is an effective mass of a unit of the square of the domain wall. At ν → 0 we can find
[18]
m0 =
σ0
c20
≡ 4 · P 2
0 ·m
3 · rc
. (10)
The equation of the domain wall motion in a small electric field E can be written
as follows:
Γ · dP
dt
= E, (11)
where Γ is the kinetic coefficient. The mobility of the domain wall is
µ =
ν
E
=
rc
P0 · Γ
. (12)
Taking into account the depolarization energy and the full energy of the domain
walls in the specimen with the thickness ξ in the polar direction X we can find the
equilibrium width d of the domains in the ferroelectric phase of the crystal
d =
√
ε0 · ξ · σ0
k · P 2
0
. (13)
Here k = 3.4
1+
√
εx·εz
, εx and εz – the dielectric permeability of the crystal in longitu-
dinal (polar) and transverse directions, respectively.
In the presence of the intermediate IC phase in proper uniaxial ferroelectric in
the expression (1) for the thermodynamic potential density the coefficient δ < 0 and
we should also account for the invariants
g
2
(
∂2P
∂z2
)2
+
λ
2
· P 2
(
∂P
∂z
)2
. (14)
428
Domain structure properties of Sn2P2Se6
For Sn2P2Se6, two coefficients are negative (δ and β) and all the other coeffi-
cients in (1) are positive [21]. In such a case we can find the solution for the space
distribution of the spontaneous polarization in the form [18]
P = P0 · tanhK · z. (15)
Then, from (1) and (14) we obtain
P 2
0 = − β
2 · γ ·
1 +
√√√√1−
√
4 · γ · α
β2
(16)
and
K2 =
k2
i
8
(
1− λ
2 · δ · P 2
0
)
+
(
1− λ · P 2
0
2 · δ
)2
− 4 · α
αi
0.5
, (17)
where
k2
i = − δ
2 · g , αi = α +
δ2
4 · g . (18)
Now the surface energy of the domain wall in the ferroelectric phase is σ = h · Ff
K
.
Here h – some coefficient, and the energy of the ferroelectric phase
Ff =
−β3 + 6 · α · β · γ − (β2 − 4 · α · γ)
3
2
24 · γ2
. (19)
In the approximation γ = 0 from the equations (4) and (5) we get the following
temperature dependencies for the domain wall width, energy and mobility
rc ∼ |T − T0|−
1
2 , σ ∼ |T − T0|
3
2 , µ ∼ |T − T0|−1
. (20)
Near the Lifshitz point, where δ = 0 , these dependencies have the following form
rLPc =
(
g
α
) 1
4 ∼ |TLP − T |−
1
4 ,
σLP ∼ g
1
4 ·α
7
4
µ
∼ |TLP − T |
7
4 ,
µ = rc
P0·Γ
∼ |TLP − T |−
3
4 .
(21)
The contribution of the domain walls into a dielectric permeability is determined by
formula [17]
ε =
2 · P0 · ξ
ε0 ·E · d, (22)
where ε0 is a dielectric constant, ξ is a parameter of translation displacements of the
domain walls. The frequency dependence of the dielectric permeability follows from
the equation of motion
M · d
2ξ
dt2
+N · dξ
dt
+ K̃ · ξ = 2 · P0 · E, (23)
429
Yu.M.Vysochanskii et al.
where: m0 ≈ m·P 2
0
rc
≡ M is the effective mass of the unit of square of the domain
wall, N ≈ L·P 2
0
rc
is the dissipation coefficient. Here L – the soft optic mode damping,
K̃ – the rigidity of the pinning of the domain walls. So, the spectrum of the domain
wall real part dielectric output is
ε′ =
4 · P 2
0
M · ε0 · d
· (ω2
0 − ω2)
(ω2
0 − ω2) + ω2 · τ 2 , (24)
where
τ ∼= N
M
and ω0 =
(
K̃
M
)0.5
(25)
are, consequently, the damping coefficient and the proper frequency of the domain
wall vibrations. At the approximation τ → 0 and ω → 0 from (24) one can find
K̃ =
4 · P 2
0
ε0 · d · ε′
. (26)
In the ferroelectric phase , far from the temperature of phase transition, we can
estimate the characteristics of the domain walls in Sn2P2S6. We will use the data for
300 K. Here, volume of the elementary cell V= 4.56·10−28 m3 , ρ= 3.56·103 kg·m−3,
P0= 0.15 C·m−2, u0= 0.26·10−10 m, εx ≈200 and εz ≈50. The coefficients of the
thermodynamic potential (1) αT ∼1.6·106 J·m·C−1·K−1, β ∼7.4·108 J·m5·C4,
γ ∼3.5·1010 J·m9·C5, δ ∼1.5·10−10 J·m3·C2 [21]. According to the ultrasound data
[22] Γ ∼12.8·10−6 J·m·s·C−2.
For the listed parameters using the above shown formulas we can estimate:
e∗ ≈2.6·10−18 C, m ≈1.1·10−16 Kg·m3·C−2, c0 ≈1.2·103 m·s−1, σ ≈1.1·10−3 J·m−2,
rc ≈4.2·10−9 m, d ≈1.6·10−7 m, µ ≈2.2·10−3 C·m2·J−1·s−1, M ≈7.6·10−10 kg·m−2,
K̃ ≈3.2·1014 N·m−2. In this case we can estimate the frequency of the vibrations of
the domain walls ̟0 ≈1010 Hz.
According to the experimental data of dielectric [23] and ultrasound [22] spec-
troscopy in the ferroelectric phase of the crystals Sn2P2S6 and Sn2P2Se6 the vibra-
tions of the domain walls have got frequencies in the interval of 107–108 Hz.
Such small values of the discussed frequencies observed here can be caused by the
piezoelectric interaction between polarization and deformation in the polar phase.
Such interaction increases the mass of the domain walls and, consequently, decreases
the frequency of their vibrations [19]. To account for this effect we should add to
(1) the energy
Fel = cijkl · uij · ukl + qijkl · uij · P 2
k , (27)
where: uij – elastic deformations; cijkl – elastic modulus; qijkl ≡ qel- coefficients
of electrostriction. Thus we find the next expression for the effective mass of the
domain wall [19]
Mef =
2 · P 4
0 · l2 · q2el
ε2x · ρ · c4 · d
. (28)
430
Domain structure properties of Sn2P2Se6
For the parameters listed above and for the values of sound velocity
c ≈2.5·103 m·s−1, electrostriction coefficients qel ≈3.5·109 J·m·C−2 [21], the sam-
ple dimension l ≈2·10−3 m, using the relations (25), (26) and (28) we found the
frequency of the domain wall vibrations ̟0 ≈3.8·108 Hz. This value coincides with
the experimental data [22],[23].
According to the relations (4) the width of domain wall under the cooling de-
creases from ∼4.3·10−9 m at T0–T=20 K to ∼1.3·10−9 m at T0–T=100 K. So, at
temperature decreasing the width of the domain walls decreases up to the dimen-
sions of the elementary cell of the crystal lattice. For this matter, we observe the
well known effect of “freezing” of the domain walls, which appears in Sn2P2S6 as
the maximum of the dielectric losses at T0–T∼100 K and a decrease of the domain
contribution to dielectric permeability at T0–T>100K [24].
Earlier we have analyzed the domain structure in the ferroelectric phase of di-
electric crystals. In fact Sn2P2S(Se)6 crystals are ferroelectrics-semiconductors. The
change of the concentration of the free charge carriers and the variation of the den-
sity of donor or acceptor impurity states can change the configuration of the domain
structure. So, the dimension of domains d depends on the concentration of the free
carriers of the charge n and also depends on the density of impurity state on the
surface of the sample Ni [20]:
d =
(
2 · π · √π
l ·
√
ε0 · εz · δ
− 1
ε0 · εz · λ2
)− 1
2
, λ̃2 =
kB · T
4 · π · e2 · n, (29)
and
d = π ·
(
2 · π · √π√
ε0 · εz · δ · l
− 32 · π ·Ni · e2
∆E · ε0 · εz · l
)− 1
2
. (30)
From the relations (29) and (30) it follows that at the critical concentrations
ncr =
kB · T
4 · e2 · l ·
√
π · ε0 · εz·
δ
(31)
and
N cr
i =
√
π · ε0 · εz
16 · e2 · ∆E√
δ
(32)
the sample becomes monodomain (d → ∞). For Sn2P2S6 platelet sample with the
thickness l =2·10−3 m and using the above listed parameters and taking the energy
of impurities level ∆Ei ≈0.7eV [1] for temperature 330K we estimate ncr ≈6.8·1019
m−3 and N cr
i ≈0.9·1018 m−2.
For the crystals Sn2P2Se6 and Sn2P2S6, the two relaxation effects (i.e., the ther-
mal memory in the incommensurate phase of Sn2P2Se6 and the second order fer-
roelectric PT splitting after a long term temperature stabilization near T 0 in the
paraelectric phase of Sn2P2S6) are coincidentally explained using the following set
of the semiconductor parameters of these compounds: the conduction electron con-
centration n ≈1014 – 1016 m−3; the attachment level concentration Na ≈1024 m−3
431
Yu.M.Vysochanskii et al.
[1]. From the named values of Na it follows the estimation of the concentration of
surface impurities levels – Ni ≈1016 m−2.
As we can see, the values of convenient concentrations n and Ni estimated based
on the experimental data, are much smaller than their critical values n cr and N cr
i .
Herein, the mean domain width is probably close to the earlier estimated value d.
In the investigated samples the specific resistance ρ changes from 5·1010 Ω·m for
the crystals Sn2P2Se6 obtained by the Bridgeman method up to 5.5·107 Ω·m for
the crystals Sn2P2(S0.005Se0.995)6 with the impurity of Mn prepared by a vapour-
transport method. For different samples investigated the conductivity differs by
three orders. At the same time we have estimated the concentrations n of the charge
carriers in the conduction zone for the most conductive specimens of Sn2P2Se6.
From this we can conclude that in all the investigated specimens the concentrations
of charge carriers are quite smaller relative to the concentration ncr at which the
sample becomes monodomain.
So, we can believe that the decrease of the dielectric contribution domain walls
to the ferroelectric phase of Sn2P2Se6 crystals at an increase of their electric con-
ductivity is essentially caused by the decrease of the mobility of the domain walls.
The mobility decreases as the result of the charged domain wall compensation by
the free charge carriers.
4. Conclusion
For ferroelectrics-semiconductors, taking as an example the Sn2P2Se6 crystals,
we have determined the interrelation between changing of the domain structure di-
electric properties in the ferroelectric phase and the efficiency of the thermal memory
recording in the incommensurate phase at varying concentrations of mobile defects
(charge carriers) and static defects.
The memory effect strongly depends on the concentration of the charge carriers
(higher efficiency of the memory recording in the Sn2P2Se6 samples with larger
electric conductivity; the improvement of this effect under illumination of the sample
in the process of temperature stabilization as well as its sensitivity to the light
wavelength). The dielectric response of the domain walls mobility effectively changes
at the variations of the electric conductivity of the samples. These peculiarities can
be explained by considering the dominant role of redistribution of the charge carriers
in the field of the spontaneous polarization wave with their subsequent localization
on the levels of the impurities.
The destruction of the memory effect occurs at the increase of the concentra-
tion of the static defects. In this case the anomalous temperature hysteresis of the
dielectric constant in the IC phase improves, the dielectric anomalies at the phase
transitions become more smear and the dielectric response of the domain walls in
the ferroelectric phases becomes suppressed.
Generally, there was found the correlation between the shape of the temperature
anomaly of the dielectric permeability at a lock-in transition as well as the dielectric
properties of the domain structure of ferroelectric phase of Sn2P2Se6 ferroelectric-
432
Domain structure properties of Sn2P2Se6
semiconductor on the one hand, and thermal memory effect in the incommensurate
phase, on the other hand. The background of the mechanisms of these relaxation
phenomena is the redistribution of the charge carriers in the inhomogeneous field
of the spontaneous polarization (in the incommensurate or ferroelectric phase) and
the fixation of these carriers on the centers of the impurities.
5. Acknowledgements
This work has been partially supported by INTAS within the project INTAS–
93–3230–ext.
References
1. Vysochanskii Yu. , Molnar A. // J. of Physical Studies, 1997, vol. 4, p. 535–543.
2. Maior M.M., Molnar S.B., Slivka V.Yu. // Ferroelectrics, vol. 192, p. 161–166.
3. Vysochanskii Yu.M., Gurzan M.I., Maior M.M., Motrja S.F., Perechinskii S.I., Po-
torij M.V., Salo L.A., Khoma M.M., Slivka V.Yu., Voroshilov Yu.V. // Sov. Phys.
Solid State, 1985, vol. 27, p. 529.
4. Levanyuk A.P., Sigov A.S. Defects and Structural Phase Transitions. New York, Gor-
don and Breach, 1987.
5. Vysochanskii Yu.M., Slivka V.Yu. // Sov. Phys. Usp., 1992, vol. 32, p. 123.
6. Geifman I.N., Kozlova I.V., Vysochanskii Yu.M., Kofman V.Ya., Mikailo O.A. // Appl.
Magn. Resonance, 1991, vol. 2, p. 435–441.
7. Maior M.M., Vysochanskii Yu.M., Slivka V.Yu., Gurzan M.I. // Fiz. Tverd. Tel., 1985,
vol. 27, p. 1560–1562.
8. Maior M.M., Vysochanskii Yu.M., Salo L.A., Rizak V.M., Potorij M.V., Slivka V.Yu.
// Fiz. Tverd. Tel., 1989, vol. 31, p. 203–208
9. Mamin R.F. // Sov. Phys. Solid State, 1991, vol. 33, p. 1473.
10. Maior M.M., Rasing Th., Eijt S.W.H., van Loosdrecht P.H.M., van Kempen H., Molnar
S.B., Vysochanskii Yu.M., Motrja S.F., Slivka V.Yu. // J. Phys. C, 1994, vol. 6,
p. 11211–11220.
11. Maior M.M., Molnar S.B., Vysochanskii Yu.M., Slivka V.Yu., Rasing Th., van Loos-
drecht P.H.M., van Kempen H. // Phys. Rev. B, 1995, vol. 51, p. 9325–9328.
12. Maior M.M., Wiegers S.A.J., Rasing Th., Eijt S.W.H., Penning F.C., Vysochan-
skii Yu.M., Motrja S.F., van Kempen H. // Ferroelectrics, 1997, vol. 202, p. 139–147.
13. Lines M., Glass A. Principles and Applications of Ferroelectrics and Related Materials.
Oxford, Clarendon Press, 1987.
14. Vysochanskii Yu.M., Maior M.M., Perechinskii S.I., Tikhomirova N.A. // Sov. J. Crys-
tallogr., 1992, vol. 37, p. 90.
15. Grabar A.A., Bercha A.I., Simchera V.Yu., Stoika I.M. // Ferroelectrics, 1997, vol. 202,
p. 211–217.
16. Grabar A.A. // Tech. Phys. Lett., 1997, vol. 23, p. 635–637.
17. Strukov B.A., Levanyuk A.P. The Physical Principles of Ferroelectric Phenomenon in
Crystals. Moskow, Nauka, 1983 (in Russian).
18. Sidorkin A.S. // Ferroelectrics, 1997, vol. 191, p. 109–128.
19. Sidorkin A.S., Nesterenko L.P. // Fiz. Tverd. Tela (St. Petersburg), 1995, vol. 37,
p. 3747–3752.
433
Yu.M.Vysochanskii et al.
20. Sidorkin A.S., Darinskii B.M., Sigov A.S. // Phys. Solid. State, 1997, vol. 39, p. 823–
825.
21. Vysochanskii Yu.M., Maior M.M., Rizak V.M., Slivka V.Yu., Khoma M.M. // Sov.
Phys. JETF, 1989, vol. 68, p. 782–787.
22. Samulionis V., Valevichius V., Grigas J. // Ferroelectrics, 1990, vol. 105, p. 397–402.
23. Grigas J., Kalesinskas V., Lapinskas S. // Ferroelectrics, 1988, vol. 80, p. 225–228.
24. Maior M.M., Vysochanskii Yu.M., Bovtun V.P., Poplavko Yu.M., Koperles B.M. //
Fiz. Tverd. Tel., 1985, vol. 27, p. 1263–1266.
Вплив дефектів і провідності на властивості
доменної структури та ефект пам’яті в
сегнетоелектриках-напівпровідниках Sn2P2Se6
Ю.М.Височанський, А.А.Молнар, М.М.Хома, С.Ф.Мотря
Інститут фізики та хімії твердого тіла, Ужгородський університет,
294000 Ужгород, вул. Підгірна, 46
Отримано 6 липня 1998 р.
Для власного одновiсного сегнетоелектрика Sn2P2Se6 з неконтро-
льованим вмiстом домiшок рiзного типу виконанi дослiдження тем-
пературної залежностi дiелектричної проникностi для встановлення
впливу дефектностi кристалiчної структури на ефективнiсть запису
термiчної “пам’ятi” в неспiвмiрнiй (НС) фазi, на фазовий перехiд (ФП)
другого роду з параелектричної фази до НС фази при температурi i
та на ФП першого роду з НС фази до сегнетоелектричної фази при
температурi c, на аномальний гiстерезис температурної залежностi
дiелектричних властивостей в НС фазi, на дiелектричний вклад до-
менних стiнок у сегнетоелектричнiй фазi. Статичнi дефекти розмива-
ють аномалiї при ФП з параелектричної в НС фазу, збiльшують ано-
мальний гiстерезис в НС фазi та гiстерезис температури c lock-in пе-
реходу, подавляють дiелектричний вклад доменних стiнок у сегнето-
фазi та руйнують ефект “пам’ятi” в НС фазi. Зростання концентрацiї
носiїв заряду також подавляє дiелектричний вiдклик доменних стiнок
у сегнетоелектричнiй фазi, однак поряд з цим сприяє чiткішому за-
пису ”пам’ятi” в НС фазi. Така тенденцiя погоджується з виконаними
оцiнками в наближеннi середнього поля для характеристик домен-
ної структури в сегнетоелектричнiй фазi та для ефекту “пам’ятi” в НС
фазi для дослiджуваних сегнетоелектрикiв- напiвпровiдникiв.
Ключові слова: доменна структура,
сегнетоелектрики-напівпровідники, вплив дефектів
PACS: 64.70.Rh, 67.70.Kb, 64.60.Fr
434
|