Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions
The dielectric properties of (PbySn₁−y)₂P₂(SexS₁−x)₆ mixed crystals in the vicinity of the incommensurate phase transition have been studied. It has been found that due to the defect action, the dielectric anomalies at the phase transitions are smeared and instead of two anomalies bounding the i...
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Maior, M.M. Molnar, Sh.B. Vrabel, V.T. Gurzan, M.I. Motrja, S.F. Vysochanskii, Yu.M. 2017-06-12T17:41:23Z 2017-06-12T17:41:23Z 2003 Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions / M.M. Maior, Sh.B. Molnar, V.T. Vrabel, M.I. Gurzan, S.F. Motrja, Yu.M. Vysochanskii // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 301-306. — Бібліогр.: 8 назв. — англ. 1607-324X PACS: 77.84.Dy, 77.80.Bh DOI:10.5488/CMP.6.2.301 https://nasplib.isofts.kiev.ua/handle/123456789/120719 The dielectric properties of (PbySn₁−y)₂P₂(SexS₁−x)₆ mixed crystals in the vicinity of the incommensurate phase transition have been studied. It has been found that due to the defect action, the dielectric anomalies at the phase transitions are smeared and instead of two anomalies bounding the incommensurate phase there is observed a single broad peak. The character of the dielectric dispersion as well as nonlinear dielectric behavior in the crystals studied suggest that in the chaotic state due to the action of the defect on the IC phase, the crystals possess the properties of ferroelectrics relaxors. В роботі досліджувалися діелектричні властивості змішаних кристалів типу (PbySn₁−y)₂P₂(SexS₁−x)₆ в околі неспівмірних фазових переходів. Було встановлено, що під впливом дефектів, індукованих за- міщеннями, діелектричні аномалії при фазових переходах розмиваються і замість двох аномалій, що обмежують неспівмірну фазу, спостерігається один розмитий пік. Характер діелектричної дисперсії і нелінійна діелектрична поведінка в досліджуваних кристалах свідчать про те, що хаотичний стан, який виникає внаслідок впливу дефектів на неспівмірну фазу, володіє властивостями сегнетоелектриків-релаксорів. en Інститут фізики конденсованих систем НАН України Condensed Matter Physics Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions Розмиті фазові переходи в твердих розчинах (PbySn₁−y)₂P₂(SexS₁−x)₆ Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| title |
Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions |
| spellingShingle |
Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions Maior, M.M. Molnar, Sh.B. Vrabel, V.T. Gurzan, M.I. Motrja, S.F. Vysochanskii, Yu.M. |
| title_short |
Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions |
| title_full |
Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions |
| title_fullStr |
Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions |
| title_full_unstemmed |
Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions |
| title_sort |
diffusion phase transitions in (pbysn₁−y)₂p₂(sexs₁−x)₆ solid solutions |
| author |
Maior, M.M. Molnar, Sh.B. Vrabel, V.T. Gurzan, M.I. Motrja, S.F. Vysochanskii, Yu.M. |
| author_facet |
Maior, M.M. Molnar, Sh.B. Vrabel, V.T. Gurzan, M.I. Motrja, S.F. Vysochanskii, Yu.M. |
| publishDate |
2003 |
| language |
English |
| container_title |
Condensed Matter Physics |
| publisher |
Інститут фізики конденсованих систем НАН України |
| format |
Article |
| title_alt |
Розмиті фазові переходи в твердих розчинах (PbySn₁−y)₂P₂(SexS₁−x)₆ |
| description |
The dielectric properties of (PbySn₁−y)₂P₂(SexS₁−x)₆ mixed crystals in the
vicinity of the incommensurate phase transition have been studied. It has
been found that due to the defect action, the dielectric anomalies at the
phase transitions are smeared and instead of two anomalies bounding the
incommensurate phase there is observed a single broad peak. The character
of the dielectric dispersion as well as nonlinear dielectric behavior in
the crystals studied suggest that in the chaotic state due to the action of the
defect on the IC phase, the crystals possess the properties of ferroelectrics
relaxors.
В роботі досліджувалися діелектричні властивості змішаних кристалів типу (PbySn₁−y)₂P₂(SexS₁−x)₆ в околі неспівмірних фазових переходів. Було встановлено, що під впливом дефектів, індукованих за-
міщеннями, діелектричні аномалії при фазових переходах розмиваються і замість двох аномалій, що обмежують неспівмірну фазу, спостерігається один розмитий пік. Характер діелектричної дисперсії і
нелінійна діелектрична поведінка в досліджуваних кристалах свідчать про те, що хаотичний стан, який виникає внаслідок впливу дефектів на неспівмірну фазу, володіє властивостями сегнетоелектриків-релаксорів.
|
| issn |
1607-324X |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/120719 |
| citation_txt |
Diffusion phase transitions in (PbySn₁−y)₂P₂(SexS₁−x)₆ solid solutions / M.M. Maior, Sh.B. Molnar, V.T. Vrabel, M.I. Gurzan, S.F. Motrja, Yu.M. Vysochanskii // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 301-306. — Бібліогр.: 8 назв. — англ. |
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Condensed Matter Physics, 2003, Vol. 6, No. 2(34), pp. 301–306
Diffusion phase transitions in
(PbySn1−y)2P2(SexS1−x)6 solid solutions
M.M.Maior, Sh.B.Molnar, V.T.Vrabel, M.I.Gurzan, S.F.Motrja,
Yu.M.Vysochanskii
Institute for Solid State Physics and Chemistry,
Uzhgorod National University, Uzhgorod, Ukraine
Received December 5, 2002, in final form April 2, 2003
The dielectric properties of (PbySn1−y)2P2(SexS1−x)6 mixed crystals in the
vicinity of the incommensurate phase transition have been studied. It has
been found that due to the defect action, the dielectric anomalies at the
phase transitions are smeared and instead of two anomalies bounding the
incommensurate phase there is observed a single broad peak. The char-
acter of the dielectric dispersion as well as nonlinear dielectric behavior in
the crystals studied suggest that in the chaotic state due to the action of the
defect on the IC phase, the crystals possess the properties of ferroelectrics
relaxors.
Key words: diffusion phase transition, relaxors, dielectric properties
PACS: 77.84.Dy, 77.80.Bh
1. Introduction
Sn2P2Se6 crystal exhibits a sequence of phase transitions: at Ti ≈ 220 K, from
the paraelectric to the incommensurate phase, and at Tc ≈ 192 K, from the incom-
mensurate phase to the ferroelectric phase. The substitution of tin by lead atoms in
the cation sublattice of Sn2P2Se6 leads to a decrease of the incommensurate phase
transition temperatures. At the same time, the region of the occurrence of the in-
commensurate phase increases [1].
With an increase of lead content in the solid solutions, the ε′ peak at paraelectric-
incommensurate phase gets gradually smeared and, at y > 0.4ε′(T ) dependence,
demonstrates only one very broad maximum. The (PbySn1−y)2P2Se6 solid solutions
with y > 0.4 exhibit only a paraelectric-incommensurate phase transition with the
incommensurate phase extending down to 0 K.
When substitution is made simultaneously in both cation and anion sublattices,
the effect of phase transition smearing is substantially enhanced.
At a relatively high concentration of the substituted component in the mixed
crystals of (PbySn1−y)2P2(SexS1−x)6, the temperature dependence of the dielectric
c© M.M.Maior, Sh.B.Molnar, V.T.Vrabel, M.I.Gurzan, S.F.Motrja, Yu.M.Vysochanskii 301
M.M.Maior et al.
constant shows a single broad peak instead of two anomalies bounding the incom-
mensurate phase [1]. In the vicinity of the maxima, the pronounced dielectric dis-
persion with a broad spectrum of relaxation times occurs. This makes the phase
transition very similar to the phase transition observed in the so-called ferroelectrics-
relaxors.
2. Experimental
(PbySn1−y)2P2(SexS1−x)6 mixed crystals were grown using vapor transport tech-
nique [2]. The samples in the form of platelets perpendicular to [100] polar direction
with a typical size of 3 × 3 × 1 mm were used. Dielectric measurements were per-
formed in a liquid helium cryostat (UTRECS) operated in a quasistatic regime (with
a temperature variation rate ≈ 0.5 K/min) using a capacitance bridge.
3. Results and discussion
Figure 1. Temperature dependence of
real and imaginary part of the dielectric
constant for (PbySn1−y)2P2(SexS1−x)6
solid solutions: 1 – y = 0.1, x = 0.1; 2 –
y = 0.25, x = 0.25; 3 – y = 0.3, x = 0.3;
4 – y = 0.5, x = 0.2.
Temperature dependence of the di-
electric constant for mixed crystals
(PbySn1−y)2P2(SexS1−x)6 with various
compositions is represented in figure 1.
In the crystals with x, y > 0.2 at
the paraelectric-incommensurate phase
transition there occurs only a weak kink
on the temperature dependence of ε′. At
the dielectric losses, this phase transi-
tion manifests itself as a beginning of
a sharp increase below Ti. As shown
in figures 2 and 3, the dielectric con-
stant demonstrates a pronounced dielec-
tric dispersion in the vicinity of the
smeared peak. From the high temper-
ature side the dielectric response be-
comes frequency dependent below the
temperature corresponding to the kink
on ε′(T ) and ε′′(T ). The character of
the dielectric dispersion observed in
(PbySn1−y)2P2(SexS1−x)6 crystals is as
follows: smooth decrease of ε′ and in-
crease of ε′′ with increase in frequency,
which is very similar to the dispersion
found in (PbySn1−y)2P2Se6 at low temperatures [3]. In the latter, the dispersion is
related to the freezing of the activation dynamics of the incommensurate modulation
pinned by defects.
302
Diffusion phase transitions in (PbySn1−y)2P2(SexS1−x)6
Figure 2. Temperature depen-
dence of the dielectric constant for
(Pb0.25Sn0.75)2P2(Se0.75S0.25)6 solid
solution at various frequencies.
Figure 3. Frequency dependence
of the dielectric constant for
(Pb0.25Sn0.75)2P2(Se0.75S0.25)6 solid
solution at various temperatures: 1 –
68.7 K; 2 – 76.5 K; 3 – 89.8 K; 4 –
95.1 K; 5 – 100.2 K; 6 – 104.5 K; 7 –
109.4 K.
It seems likely that the dispersion in
(PbySn1−y)2P2(SexS1−x)6 crystals is al-
so caused by the relaxation dynamics of
the incommensurate structure and fer-
roelectric domain walls (since the dis-
persion occurs in the ferroelectric phase
as well).
It is evident from the structural data
performed for Sn2P2Se6 crystals [4] that
there are no higher harmonics in the
polarization distribution of the incom-
mensurate phase. Therefore, there is no
soliton structure in these crystals. Thus,
in respect to coupling with the defects,
the incommensurate modulation could
be imagined as a sequence of broad do-
main walls separating the area with an
opposite direction of polarization.
In the presence of high density of
the defects, the broad domain walls are
pinned by fluctuation in the defect den-
sity. This kind of pinning is consid-
ered to be weak [5]. In the weak pin-
ning regime, there is a minimal length
scale at which the domain walls or soli-
tons feel the potential barrier between
metastable states. In such systems the
relaxation time is a function of the
length scale. The minimal length scale
in which domain walls are pinned de-
pend on time and temperature [5]:
Lc ∝
[
1 +
T
T + Tε
ln
(
t
t0
)]1/θ
,
where Tε corresponds to the height of
the barriers between metastable states
EB, θ is an exponent depending on the
system dimension and the character of
coupling of the wall with the defects of
random field or random-bond type and
t0 is a time constant for the relaxation of
domain walls in length scale L(τ(L) =
t0 exp(−EB(L)/kBT ).
Dielectric response for such disor-
dered systems is defined by the scale in
303
M.M.Maior et al.
Figure 4. Temperature dependen-
ce of the dielectric constant for
(Pb0.25Sn0.75)2P2(Se0.75S0.25)6 solid so-
lution under bc fields: 1 – 0 V/cm; 2 –
900 V/cm; 3 – 1800 V/cm.
Figure 5. Field dependence of ∆ε for
(Pb0.25Sn0.75)2P2(Se0.75S0.25)6 solid so-
lution at various temperatures: 1 –
109.4 K, 2 – 104.0 K, 3 – 114.7 K, 4
– 124.5 K, 5 – 98.3 K.
which domain walls can relax and ε is expected to have logarithmic frequency de-
pendence. The theory prediction correlate well with the experimentally observed
frequency behavior of the dielectric constant (figure 3).
Temperature dependence of ε for (PbySn1−y)2P2(SexS1−x)6 crystals is shown in
figure 4 at various bias voltages. With lowering temperature in the intermediate
phase, the effect of the bias field increases (figure 5). Nonlinear dielectric contribution
∆ε = ε(E)−ε(0) has a quadratic field dependence. In the low temperature region of
the intermediate phase starting from a certain value of the bias, the field dependence
changes from growing to diminishing. This is an indication of the presence of the
polar state induced by an external electric field.
Dielectric measurements demonstrate the occurrence of positive nonlinear di-
electric contribution in the IC phase under the effect of the external bias field [6].
It should be mentioned that the positive nonlinear dielectric contribution occurs
in Sn2P2Se6 throughout the temperature range of the existence of incommensurate
phase.
At the same time, in (PbySn1−y)2P2Se6, solid solutions ∆ε have a positive sign
in the IC phase only close Ti of the temperatures of phase transition from the
paraelectric to the IC phase. At low temperature boundary of IC phase, ∆ε changes
a sign to negative, which is manifested in suppression of ε′(T ) maxima at lock-in
phase transition under the effect of the bias field. The latter suggests that in the
low temperature region of the IC phase in such solid solutions under the action of
the defects induced by substitutions in a cation sublattice, the polar areas appear
which are increased with the increase of the external bias.
On the contrary, the character of the bias field effect in (PbySn1−y)2P2(SexS1−x)6
solid solutions (nonlinear dielectric effect is positive throughout the intermediate
phase) testifies to the absence of macroscopic polar region.
304
Diffusion phase transitions in (PbySn1−y)2P2(SexS1−x)6
Considering the substituted atoms in solid solutions as the defects, the inter-
action of IC polarization wave with these defects could be interpreted as a weak
pinning, characteristic of a large defect concentration [5].
In (PbySn1−y)2P2Se6 solid solutions, both IC phase transitions remain reason-
ably sharp [1]. This means that the defects induced by substitutions in a cation
sublattice, can be considered as the defects saving symmetry. At the same time,
even at a relatively small concentration of sulfur atoms in an anion sublattice, the
phase transition from the paraelectric to the IC phase in (PbySn1−y)2P2(SexS1−x)6
becomes hardly distinguishable considering a temperature dependence of the dielec-
tric properties. It should be mentioned that in Sn2P2(SexS1−x)6 solid solutions, this
phase transition is not sharp as well. These facts make it possible to assume that
the defects induced by substitutions in the anion sublattice can be considered as the
defects destroying the symmetry.
The fact that the wave vector of the IC modulation in Sn2P2(S0.2Se0.8)6 solid
solution shows a temperature independent behavior due to the pinning effect [4]
in several temperature ranges, is a considerable evidence in favor of this sugges-
tion. Thus, one can assume that mainly the defects induced by substitutions in
an anion sublattice destroy the long-range order in the IC phase in solid solutions
(PbySn1−y)2P2(SexS1−x)6 resulting in the formation of the so-called chaotic state.
The positive nonlinear dielectric contribution under the effect of the bias field
was observed in K1−x(NH4)xH2PO4 crystals, in which, in the intermediate phase
between the paraelectric and the polar phase, the existence of a “structural glass”
state is suggested [7]. The similar behavior under the effect of the bias field was also
found in PMN crystals for a crystallographic direction [111] [8]. At the same time, for
the direction [100], the nonlinear dielectric contribution is negative. Such orientation
dependent nonlinear dielectric behavior in relaxors is related to the reorientation of
polar regions [7].
The peculiarity of the solid solutions (PbySn1−y)2P2Se6 (at y > 0.4) with IC
phase extended to very low temperatures is that the amplitude of the nonlinear
dielectric effect steeply decreases below 50 K. The effect is essentially negligible below
the temperature corresponding to the peak of the dielectric losses which is related
to the freezing of the relaxation dynamics of IC modulation. This phenomenon is
an argument in favor of the suggestion that nonlinear dielectric effect in the IC
phase in the crystals studied is associated with the variation of configuration state
of IC modulation pinned by the defects. In conclusion, one can mention that the
character of the dielectric dispersion as well as nonlinear dielectric behavior in the
crystals studied suggest that in the chaotic state due to the action of the defect on
the IC phase, the crystals possess the properties of ferroelectrics relaxors.
References
1. Maior M.M., Vysochanskii Yu.M., Salo L.A., Rizak V.M., Potorij M.V., Slivka V.Yu.
// Fiz. Tver. Tila, 1989, vol. 31, p. 203–211.
2. Gursan M.I. Ph.D.Thesis, Uzhgorod State University, 1983.
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3. Maior M.M., Vysochanskii Yu.M., Slivka V.Yu., Rasing Th., Van Loosdrecht P.H.M.,
Van Kempen H. // Phys. Rev. B., 1995, vol. 50, p. 9235–9239.
4. Parsamian T.K., Khasanov S.S., Shechtman V.Sh. // Fiz. Tver. Tila, 1987, vol. 29,
p. 1665–1670.
5. Natterman T., Shapir Y., Vilfan I. // Phys. Rev. B, 1990, vol. 42, p. 8577–8581.
6. Maior M.M., Vysochanskii Yu.M., Molnar Sh.B., Khoma M.M. // Fiz. Tver. Tila, 1992,
vol. 34, p. 1070–1075.
7. Korotkov L.N. // Phys. Stat. Sol. (b), 2000, vol. R3, p. 222–228.
8. Glazounov A.E., Tagantsev A.K. // Ferroelectrics, 1997, vol. 201, p. 315–321.
Розмиті фазові переходи в твердих розчинах
(PbySn1−y)2P2(SexS1−x)6
M.M.Майор, Ш.Б.Молнар, В.T.Врабель, М.І.Гурзан,
С.Ф.Мотря, Юл.M.Височанський
Ужгородський Національний Університет, Ужгород
Отримано 5 грудня 2002 р., в остаточному вигляді – 2 квітня
2003 р.
В роботі досліджувалися діелектричні властивості змішаних криста-
лів типу (PbySn1−y)2P2(SexS1−x)6 в околі неспівмірних фазових пере-
ходів. Було встановлено, що під впливом дефектів, індукованих за-
міщеннями, діелектричні аномалії при фазових переходах розмива-
ються і замість двох аномалій, що обмежують неспівмірну фазу, спо-
стерігається один розмитий пік. Характер діелектричної дисперсії і
нелінійна діелектрична поведінка в досліджуваних кристалах свід-
чать про те, що хаотичний стан, який виникає внаслідок впливу де-
фектів на неспівмірну фазу, володіє властивостями сегнетоелектри-
ків-релаксорів.
Ключові слова: розмитий фазовий перехід, релаксори,
діелектричні властивості
PACS: 77.84.Dy, 77.80.Bh
306
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