Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals
Temperature dependences of longitudinal and transverse dielectric permeability of KDP and DKDP crystals are studied at different values of hydrostatic pressure in order to determine the pressure behaviour of the isotropic point for these crystals. The isotropic point temperature in KDP crystals...
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| Цитувати: | Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals / V.M. Kedyulich, A.G. Slivka, E.I. Gerzanich, A.M. Guivan, P.M. Lukach // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 271-280. — Бібліогр.: 20 назв. — англ. |
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irk-123456789-1207142017-06-13T03:06:42Z Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals Kedyulich, V.M. Slivka, A.G. Gerzanich, E.I. Guivan, A.M. Lukach, P.M. Temperature dependences of longitudinal and transverse dielectric permeability of KDP and DKDP crystals are studied at different values of hydrostatic pressure in order to determine the pressure behaviour of the isotropic point for these crystals. The isotropic point temperature in KDP crystals at atmospheric pressure is Ti = 186 K at the measuring field frequency 1 kHz and Ti = 176 K at the frequency of 1 MHz. In DKDP crystals the isotropic point is achieved at the temperature Ti = 300 K ( 1 kHz) and Ti = 253 K ( 1 MHz). The hydrostatic pressure increase results in the reduction of the isotropic point temperature with the pressure coefficients ∂Ti/∂p = −4.3 K/kbar for KDP and ∂Ti/∂p = −2.9 K/kbar for DKDP. The analysis of the experimental results in the framework of the Blinc-Zeks pseudospin formalism has shown a good agreement between the calculated and the experimentally obtained temperature of the isotropic point for KDP crystals. Проведено дослідження температурних залежностей повздовжньої та поперечної діелектричної проникності кристалів KDP та DKDP при різних величинах гідростатичного тиску з метою визначення баричної поведінки температури ізотропної точки для цих кристалів. Температура ізотропної точки в кристалах KDP при атмосферному тиску становить Ti = 186 K при частоті вимірювального поля 1 кГц та Ti = 176 K на частоті 1 МГц. В кристалах DKDP ізотропна точка реалізується при температурах Ti = 300 K (1 кГц) та Ti = 253 K ( 1 МГц). Збільшення гідростатичного тиску приводить до пониження температури ізотропної точки з баричними коефіцієнтами ∂Ti/∂p = −4.3 К/кбар для KDP та ∂Ti/∂p = −2.9 К/кбар для DKDP. Аналіз експериментальних результатів в рамках псевдоспінового формалізму Блінца-Жекша показав, що існує добре узгодження між розрахованою та експериментально визначеною температурою ізотропної точки для кристалів KDP. 2003 Article Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals / V.M. Kedyulich, A.G. Slivka, E.I. Gerzanich, A.M. Guivan, P.M. Lukach // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 271-280. — Бібліогр.: 20 назв. — англ. 1607-324X PACS: 77.80.Bh, 77.22.Ch, 77.84.Fa DOI:10.5488/CMP.6.2.271 http://dspace.nbuv.gov.ua/handle/123456789/120714 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
Temperature dependences of longitudinal and transverse dielectric permeability
of KDP and DKDP crystals are studied at different values of hydrostatic
pressure in order to determine the pressure behaviour of the isotropic
point for these crystals. The isotropic point temperature in KDP crystals
at atmospheric pressure is Ti = 186 K at the measuring field frequency
1 kHz and Ti = 176 K at the frequency of 1 MHz. In DKDP crystals
the isotropic point is achieved at the temperature Ti = 300 K ( 1 kHz)
and Ti = 253 K ( 1 MHz). The hydrostatic pressure increase results in the
reduction of the isotropic point temperature with the pressure coefficients
∂Ti/∂p = −4.3 K/kbar for KDP and ∂Ti/∂p = −2.9 K/kbar for DKDP.
The analysis of the experimental results in the framework of the Blinc-Zeks
pseudospin formalism has shown a good agreement between the calculated
and the experimentally obtained temperature of the isotropic point for
KDP crystals. |
| format |
Article |
| author |
Kedyulich, V.M. Slivka, A.G. Gerzanich, E.I. Guivan, A.M. Lukach, P.M. |
| spellingShingle |
Kedyulich, V.M. Slivka, A.G. Gerzanich, E.I. Guivan, A.M. Lukach, P.M. Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals Condensed Matter Physics |
| author_facet |
Kedyulich, V.M. Slivka, A.G. Gerzanich, E.I. Guivan, A.M. Lukach, P.M. |
| author_sort |
Kedyulich, V.M. |
| title |
Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals |
| title_short |
Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals |
| title_full |
Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals |
| title_fullStr |
Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals |
| title_full_unstemmed |
Temperature and pressure effect on the anisotropy of dielectric permeability in KDP and DKDP crystals |
| title_sort |
temperature and pressure effect on the anisotropy of dielectric permeability in kdp and dkdp crystals |
| publisher |
Інститут фізики конденсованих систем НАН України |
| publishDate |
2003 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120714 |
| citation_txt |
Temperature and pressure effect on the
anisotropy of dielectric permeability in
KDP and DKDP crystals / V.M. Kedyulich, A.G. Slivka, E.I. Gerzanich, A.M. Guivan, P.M. Lukach // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 271-280. — Бібліогр.: 20 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
AT kedyulichvm temperatureandpressureeffectontheanisotropyofdielectricpermeabilityinkdpanddkdpcrystals AT slivkaag temperatureandpressureeffectontheanisotropyofdielectricpermeabilityinkdpanddkdpcrystals AT gerzanichei temperatureandpressureeffectontheanisotropyofdielectricpermeabilityinkdpanddkdpcrystals AT guivanam temperatureandpressureeffectontheanisotropyofdielectricpermeabilityinkdpanddkdpcrystals AT lukachpm temperatureandpressureeffectontheanisotropyofdielectricpermeabilityinkdpanddkdpcrystals |
| first_indexed |
2025-07-08T18:27:16Z |
| last_indexed |
2025-07-08T18:27:16Z |
| _version_ |
1837104352517423104 |
| fulltext |
Condensed Matter Physics, 2003, Vol. 6, No. 2(34), pp. 271–280
Temperature and pressure effect on the
anisotropy of dielectric permeability in
KDP and DKDP crystals
V.M.Kedyulich, A.G.Slivka, E.I.Gerzanich, A.M.Guivan,
P.M.Lukach
Uzhgorod National University,
32 Voloshina Str., 88000 Uzhgorod, Ukraine
Received October 24, 2002, in final form April 4, 2003
Temperature dependences of longitudinal and transverse dielectric perme-
ability of KDP and DKDP crystals are studied at different values of hydro-
static pressure in order to determine the pressure behaviour of the isotrop-
ic point for these crystals. The isotropic point temperature in KDP crystals
at atmospheric pressure is Ti = 186 K at the measuring field frequen-
cy 1 kHz and Ti = 176 K at the frequency of 1 MHz. In DKDP crystals
the isotropic point is achieved at the temperature Ti = 300 K ( 1 kHz)
and Ti = 253 K ( 1 MHz). The hydrostatic pressure increase results in the
reduction of the isotropic point temperature with the pressure coefficients
∂Ti/∂p = −4.3 K/kbar for KDP and ∂Ti/∂p = −2.9 K/kbar for DKDP.
The analysis of the experimental results in the framework of the Blinc-Žekš
pseudospin formalism has shown a good agreement between the calculat-
ed and the experimentally obtained temperature of the isotropic point for
KDP crystals.
Key words: ferroelectrics, dielectric permeability, hydrostatic pressure,
isotropic point
PACS: 77.80.Bh, 77.22.Ch, 77.84.Fa
1. Introduction
KH2PO4 (KDP) and KD2PO4 (DKDP) crystals above the temperature of the
ferroelectric first-order phase transition are tetragonal, while in the ferroelectric
phase they are rhombic [1]. The anomalies of dielectric permeability, related to the
phase transition, are observed both along the spontaneous polarization axis and
normally to it due to a specific mechanism of the phase transition in these crystals.
Proton ordering at hydrogenic bonds in the ab plane causes the displacement of ions
along the c axis and the spontaneous polarization.
The temperature dependence of longitudinal (along the spontaneous polarization
direction) dielectric permeability of both crystals in the paraelectric phase obeys the
c© V.M.Kedyulich, A.G.Slivka, E.I.Gerzanich, A.M.Guivan, P.M.Lukach 271
V.M.Kedyulich et al.
Curie-Weiss law [2]. In the ferroelectric phase a shoulder is observed in this plot, re-
lated to the effect of domain structure freezing [3,4]. The studies of hydrostatic pres-
sure effect on the longitudinal dielectric permeability have shown that the increase
of the all-round pressure causes the decrease of the phase transition temperatures
with the coefficients ∂Tc/∂p = −4.6 K/kbar for KDP and ∂Tc/∂p = −2.4 K/kbar
for DKDP [2] and the downward shift of the εc(T ) curves in the temperature scale.
At the pressure near 18 kbar, the phase transition temperature in the KDP crystal
is zero and the value ∂Tc/∂p tends to infinity, while the (p, T ) phase diagram of
the DKDP crystal is linear. Such a behaviour of the pressure variation of the phase
transition temperature in KDP crystals is related to the proton tunnelling effect,
and the increase of the hydrogen atom mass in DKDP crystals sharply decreases
the tunnelling probability [5,6].
The temperature dependence of transverse (normally to the spontaneous polar-
ization axis) dielectric permeability in the KDP crystals at atmospheric pressure is
given in [7]. At the phase transition temperature Tc = 122 K, the dependence εa(T )
reveals a step. Above the phase transition point in the paraelectric phase there is
a maximum in the εa(T ) plot. The similar behaviour of the dielectric permeability
was observed for antiferroelectric KMnF3, NH4H2AsO4 crystals [8].
The specific feature of the anisotropy of dielectric permeability in the paraelectric
phase of KDP and DKDP crystals is the presence of an isotropic point where the
longitudinal and transverse dielectric permeability values are equal [9]. At this point
the crystals become isotropic with respect to the dielectric permeability εa = εb = εc.
The aim of the present paper is to study the temperature dependences of longitudinal
and transverse dielectric permeability of KDP and DKDP crystals at different values
of external hydrostatic pressure as well as to trace the pressure behaviour of the
isotropic point temperature.
2. Results
Figure 1 shows the temperature dependences of longitudinal and transverse di-
electric permeability of KDP crystals at atmospheric pressure at the measuring field
frequency 1 kHz (curve 1) and 1 MHz (curve 2). No dispersion of the transverse
dielectric permeability at these frequencies has been found (curve 3). At the fre-
quency of 1 kHz the dependence of the longitudinal dielectric permeability has two
anomalies: a maximum of εc(T ) at the phase transition temperature Tc = 122 K
and a shoulder in the ferroelectric phase in the temperature range from 91 to 122 K,
related to the domain structure transformation [4]. At the measuring field frequency
of 1 MHz a number of extrema is observed in the εc(T ) plot in the vicinity of the
phase transition point (curve 2). Such feature is the evidence of the piezoelectric res-
onance in KDP crystals [10] and cannot be related to the order parameter relaxation
time tending to infinity in the order/disorder-type ferroelectrics since in this case
one minimum at the temperature dependence of the dielectric permeability should
be observed at the phase transition temperature.
272
Anisotropy of dielectric permeability in KDP and DKDP
Figure 1. Temperature dependences of the longitudinal dielectric permeability
at the frequency of 1 kHz (1) and 1 MHz (2) and transverse permeability (3) of
the KDP crystal.
At room temperature the transverse dielectric permeability value is higher than
the longitudinal one (see figure 1). The temperature decrease results in the increase
of both values. However, the longitudinal dielectric permeability undergoes essential
anomalous changes, its value sharply increasing while approaching the phase transi-
tion point. Therefore, at the temperature Ti = 186 K (f = 1 kHz) the longitudinal
and transverse dielectric permeability values become equal. The isotropic point tem-
perature at the measuring field frequency of 1 MHz is Ti = 176 K (see the insert to
figure 1). At the temperatures T < Ti the longitudinal dielectric permeability value
exceeds the transverse one.
The orientation dependence of the dielectric permeability, which is a tensor in
an anisotropic crystal, is given by
ε = εijlilj , (1)
where εij are the components of the dielectric permeability tensor, li, lj are cosines
of the angles between the direction in which the dielectric permeability is being
determined, and the Cartesian coordinate axes corresponding to the tensor nota-
tion, respectively. Using equation (1), for tetragonal KDP and DKDP crystals, the
cross-section of the dielectric permeability indicative surface by the ab plane in the
principal axes, is given by
ε(ϕ) = εa sin2(ϕ) + εc cos2(ϕ), (2)
where ϕ is the angle between the direction in which ε is determined and the ferro-
electric axis c. Using equation (2), the cross-sections of the KDP crystal dielectric
permeability indicative surface by the ab plane were built. The temperature trans-
formation of these cross-sections in the vicinity of the isotropic point is shown in
273
V.M.Kedyulich et al.
Figure 2. Temperature transformation of the cross-section of the dielectric per-
meability indicative surface by the ac plane of the KDP crystal in the vicinity of
the isotropic point: 1 – T = 275 K, 2 – T = 186 K, 3 – T = 150 K.
figure 2. As it is seen in the figure, at the temperature T > Ti the major semiax-
is of the dielectric permeability tensor is directed along the a axis, at T = Ti the
longitudinal and transverse dielectric permeability values are equal and the indica-
tive surface cross-section is circular, at T < Ti the major semiaxis of the tensor is
directed along the ferroelectric axis c.
The temperature dependences of the longitudinal and transverse dielectric per-
meability of DKDP crystals at atmospheric pressure at two frequencies of the mea-
suring field are given in figure 3. As in the case of KDP crystals, there is no dispersion
of transverse dielectric permeability in DKDP, and in the temperature dependence of
the longitudinal dielectric permeability the piezoresonance effects are seen to be re-
vealed at the frequency of 1 MHz in the vicinity of the phase transition (Tc = 221 K)
(see curve 2). The specific feature of DKDP crystals is a considerable dispersion of
the longitudinal dielectric permeability. Therefore, unlike KDP crystals, the isotropic
point temperature strongly depends on the measuring field frequency. At the fre-
quency of 1 kHz the isotropic point is achieved at the temperature Ti = 300 K, while
at the frequency of 1 MHz the Ti value is 253 K.
The behaviour of the temperature dependences of the longitudinal and transverse
dielectric permeability under external hydrostatic pressure is illustrated in figure 4
(for KDP) and in figure 5 (for DKDP). The all-round pressure is seen to result in
the decrease of the phase transition temperature in both crystals and in the shift
of the εa(T ) and εc(T ) dependences towards lower temperatures. This leads to the
isotropic point temperature decrease in these crystals. For KDP crystal, the pressure
274
Anisotropy of dielectric permeability in KDP and DKDP
Figure 3. Temperature dependences of longitudinal dielectric permeability at the
frequencies of 1 kHz (1) and 1 MHz (2) and transverse dielectric permeability (3)
of the DKDP crystal.
Figure 4. Temperature dependences of longitudinal (solid circles) and transverse
(open circles) dielectric permeability of the KDP crystal at different values of the
external hydrostatic pressure p, kbar: 1 – 0; 2 – 1.7; 3 – 2.5; 4 – 4.25.
The insert shows the pressure dependence of the isotropic point temperature at
the frequency of 1 kHz (open circles) and 1 MHz (solid circles).
275
V.M.Kedyulich et al.
Figure 5. Temperature dependences of longitudinal (solid circles) and transverse
(open circles) dielectric permeability of the DKDP crystal at different values of
the external hydrostatic pressure p, kbar: 1 – 0; 2 – 4.1.
coefficient of the isotropic point temperature is ∂Ti/∂p = −4.3 K/kbar and does not
depend on the measuring field frequency (see the insert to figure 4). For DKDP
crystal ∂Ti/∂p = −2.9 K/kbar.
3. Analysis
Using the pseudospin formalism [11,12], the authors of [7] have obtained the
relations, describing the temperature dependences of longitudinal and transverse
dielectric permeability in the paraelectric phase of KDP crystals:
χc =
2Nµ1 · tanh
(
Γ
T
)
Γ − J (0) tanh
(
Γ
T
) , (3)
χa =
Nµ2
2
Γ
tanh
(
Γ
T
)
1 + J(0)
3Γ
tanh
(
Γ
T
) , (4)
where Γ is the tunnelling energy, J(0) is the dipolar interaction energy in the mean-
field approximation, µ1 is the dipolar moment along the c axis, µ2 is the dipolar
moment along the a axis, N = 1022 cm−3 [7] is the hydrogenic bond concentration.
Using equations (3), (4), one can obtain the value of temperature Ti, corresponding
to the isotropic point (χa = χc):
Ti = Γ
/
arcth
Γ(µ2
2 − 2µ2
1)
J(0)
(
2
3
µ2
1 + µ2
2
) . (5)
276
Anisotropy of dielectric permeability in KDP and DKDP
Thus, the isotropic point temperature position in this model is given by four val-
ues J(0), Γ, µ1 and µ2 which can be determined from the temperature dependences
of the longitudinal and transverse dielectric permeability of KDP crystal.
The analysis of equation (4) shows the existence of a kink in the εa(T ) de-
pendence, which is clearly revealed experimentally. The temperature of the kink
point for the transverse dielectric permeability Tf can be found from the condition
∂2χa/∂T 2 = 0:
tanh
(
Γ
Tf
)
=
(3Tf − J(0))Γ
3Γ2 − J(0)Tf
. (6)
Using equation (4) and taking into account the relationship between J(0), Γ and
the phase transition temperature Tc, given by [7]
tanh
(
Γ
Tc
)
=
Γ
J(0)
, (7)
the unknown values of the tunnelling energy Γ and dipolar interaction energy J(0)
can be determined knowing the experimentally obtained phase transition tempera-
tures Tc and kink points in the εa(T ) dependence.
The unknown value of the dipolar moment along the a axis µ2 is determined
from the experimental value of transverse susceptibility in the kink point of εa(Tf).
By substituting equation (6) into equation (4) one obtains
εa(Tf) = Nµ2
2
3Tf − J(0)
3Γ2 − J(0)2/3
. (8)
The dipolar moment along the ferroelectric axis µ2 is determined based on the
best fitting of the experimental dependence of the longitudinal dielectric permeabil-
ity εc(T ) by equation (3). The experimental data for Tc, Tf , T exp
i , εa(Tf) and the
calculated values of J(0), Γ, µ1, µ2 for KDP crystal at different hydrostatic pressure
values are given in table 1. In calculations, the regular part of the susceptibility
χ0 = 10/4π [7] was taken into account. As one can see, for these crystals there is
a good agreement between the experimentally obtained isotropic point temperature
and the value calculated according to equation (5).
The unexpected changes in the theoretical parameters Γ and µ1 with pressure
were obtained (table 1). While the experimental behaviour of the dipolar moment
could be related to the existence of the tricritical point in the phase (p, T ) diagram
of the KDP crystal at p ≈ 2 kbar [13], the decrease of Γ with pressure disagrees
with the common views [2].
The absence of the kink point at the temperature dependences of transverse
dielectric permeability in the temperature range under investigation does not make it
possible to use the above calculation techniques in analyzing the pressure behaviour
of the isotropic point in DKDP crystals.
4. Conclusion
Temperature dependences of longitudinal and transverse dielectric permeability
of KDP and DKDP crystals were studied at different values of hydrostatic pressure
277
V.M.Kedyulich et al.
Table 1. The experimental data for Tc, Tf , T exp
i , εa(Tf ) and the calculated values
of J(0), Γ, µ1, µ2, for KDP crystal at different hydrostatic pressure values. The
values of µ1 and µ2 are given in SGS units.
p, kbar Tc, K Tf , K εa(Tf) J(0), K Γ, K µ1, 10−18 µ2, 10−18 T theor
i , K T exp
i , K
0 122.0 171.5 57.3 208 190 0.951 3.94 183 186
1.7 114.3 162.5 57.0 196 180 0.969 3.82 178 181
2.5 110.2 157.0 57.2 189 174 0.976 3.76 174 174
4.25 106.5 147.5 56.8 177 163 0.950 3.62 164 168
and at two measuring field frequencies – 1 kHz and 1 MHz. The isotropic point
temperature in KDP crystal at atmospheric pressure is Ti = 186 K at the measuring
field frequency of 1 kHz, and Ti = 176 K at the frequency of 1 MHz. In DKDP
crystals, the isotropic point is achieved at the temperatures Ti = 300 K (1 kHz) and
Ti = 253 K (1 MHz). The hydrostatic pressure increase causes the decrease of the
isotropic point temperature with the pressure coefficient ∂Ti/∂p = −4.3 K/kbar for
KDP and ∂Ti/∂p = −2.9 K/kbar for DKDP. The pressure variation of the isotropic
point temperature for both crystals is independent of the measuring field frequency.
The analysis of the experimental results has shown that in the framework of the
tunnelling model in the molecular-field approximation, a satisfactory quantitative
description of the temperature behaviour of the transverse dielectric permeability of
the KDP crystal in the paraelectric phase can be obtained, as well as the temperature
coordinate of the isotropic point and its dependence on the external pressure can
be determined. However, unexpected pressure dependences of the parameters of
the tunnelling model such as the tunnelling energy Γ and dipolar moment µ1 were
obtained. This is another evidence for the weakness of the tunnelling model for
all-round description of pressure effects in KDP-type crystals. It has been shown
in a number of papers [14–18] that an essential improvement of the theoretical
description of the wide variety of the experimental data concerning the pressure effect
on the physical properties of KDP-type crystals can be achieved with the account of
short-range configurational interactions. Therefore, it seems important to perform
further theoretical studies of the specific features of the temperature behaviour of
longitudinal εc and transverse εa dielectric permeability in KDP-type crystals under
external pressure, namely to analyze the obtained results in the framework of the
proton ordering model with the account of short-range and long-range interactions
and proton tunnelling at hydrogenic bonds in the four-particle cluster approximation
[19,20]. Such studies could provide information on the physical grounds for the
experimentally observed features at the εc(T ) and εa(T ) dependences discussed in
the present work.
278
Anisotropy of dielectric permeability in KDP and DKDP
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V.M.Kedyulich et al.
Вплив температури і тиску на анізотропію
діелектричної проникності кристалів KDP і DKDP
В.М.Кедюлич, О.Г.Сливка, О.І.Герзанич, Г.М.Гуйван,
П.М.Лукач
Ужгородський національний університет,
88000 Ужгород, вул. Волошина, 32
Отримано 24 жовтня 2002 р., в остаточному вигляді – 4 квітня
2003 р.
Проведено дослідження температурних залежностей повздовжньої
та поперечної діелектричної проникності кристалів KDP та DKDP
при різних величинах гідростатичного тиску з метою визначення
баричної поведінки температури ізотропної точки для цих криста-
лів. Температура ізотропної точки в кристалах KDP при атмосфер-
ному тиску становить Ti = 186 K при частоті вимірювального по-
ля 1 кГц та Ti = 176 K на частоті 1 МГц. В кристалах DKDP ізо-
тропна точка реалізується при температурах Ti = 300 K ( 1 кГц) та
Ti = 253 K ( 1 МГц). Збільшення гідростатичного тиску приводить
до пониження температури ізотропної точки з баричними коефіці-
єнтами ∂Ti/∂p = −4.3 К/кбар для KDP та ∂Ti/∂p = −2.9 К/кбар для
DKDP. Аналіз експериментальних результатів в рамках псевдоспіно-
вого формалізму Блінца-Жекша показав, що існує добре узгоджен-
ня між розрахованою та експериментально визначеною температу-
рою ізотропної точки для кристалів KDP.
Ключові слова: сегнетоелектрики, діелектрична проникність,
гідростатичний тиск, ізотропна точка
PACS: 77.80.Bh, 77.22.Ch, 77.84.Fa
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