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NEW CLASSES OF FERROELECTRIC S OF DISPLACEMENT TYPE
G. Smolensky, V. Isupov
To cite this version:
G. Smolensky, V. Isupov. NEW CLASSES OF FERROELECTRIC S OF DISPLACEMENT TYPE.
Journal de Physique Colloques, 1972, 33 (C2), pp.C2-25-C2-31. �10.1051/jphyscol:1972207�. �jpa-
00214943�
JOURNAL DE PHYSIQUE
Colloque C2, suppI&ment au no 4, Tome 33, Avril 1972, page C2-25
NEW CLASSES OF FERROELECTRIC S OF DISPLACEMENT TYPE
G . A. SMOLENSKY and V. A. ISUPOV
Institute of Semiconductors of the Academy of Sciences of the U. S. S. R., Leningrad, U. S. S. R.
RBsumC.
-Nous passons en revue ici les renouveaux ferroelectriques du type de dkplacement.
Nous les decrivons a l'aide de certains phCnom6nes physiques qui les caracterisent. Nous dkcrivons rapidement certains ferroklectriques, ses domaines a 180°, des ferroi?lectriques-ferromagnetiques, ferroelectriques-semiconducteurs, des ferroelectriques impropres et des nouveaux antiferroelec- triques. Nous discutons des proprietes des ferroelectriques a transition de phase diffuske. Nous envisageons aussi des proprietks de composes en couches du type perovskite, K. W. bronzes et aussi des niobiate de lithium, biomithonobiate de cadmium, des antimoniotantalates, fluorides et niobiate de strontium.
Abstract. - A review of new ferroelectrics of displacive type is given. The description is made corresponding to some physical phenomena which characterize the new ferroelectrics. Some ferro- electrics without 1800 domains,
ferroelectric-ferromagnetics, ferroelectric-semiconductors,impoper ferroelectrics, and new antiferroelectrics are shortly described. The properties of ferroelectrics with diffused phase transition are discussed. There is also a short consideration of properties of some perovskite-like layer compounds, perovskites, K. W.-bronzes, and also of lithium niobate cadmium pironiobate, stibiotantalate, fluorides and strontium niobate.
Some more than two and half decades ago the first ferroelectric of the displacement type was discovered.
It was barium titanate, with the perovskite-type structure. A great number of ferroelectrics of the displacement type has been discovered since then. Such ferroelectrics are known now, the crystal structure and properties of which differ very much from those of barium titanate. I t was shown that ferroelectric compounds can be formed by many elements of the Mendeleev periodic system. The family of ferro- and antiferroelectrics of the displacement type includes not only titanates, zirconates, hafnates, niobates and tantalates which are known for a long time, but also borates, manganates, molybdates and tungstates.
Among the ferroelectrics and antiferroelectrics of this type Ba-aluminate, Pb-Ba-stannate, some halogenides (CsGeCl,, BaMeF,) and chalcogenohalogenides (of SbSJ type) are known.
There is a useful idea utilized during the search of new displacement-type ferro- and antiferroelec- trics [I], [2]. This idea states that the existence of oxygen octahedra chains in crystal lattice, where the octahedra are bound with each other by their corners, and the presence of Pb2+, Bi3+ and TIi ions are favourable for ferroelectricity. These ions have great electron polarizability which is evidently caused by an unshared couple of 6 s-electrons which shows high stereochemical activity in some compounds. Accord- ing to [3] some odd hybrid states including 6 s- and 6 p-orbitals appear in T l f , Pb2+ and Bi3+ compounds as the result of the antisymmetric excitations, and these states can produce a considerable dipole moment.
That leads to high ionic polarizability. The before
mentioned idea about the important role of Pb2+
and Bi3+ ions was widely used in the course of our investigations. The consideration of ferroelectrics shows that more than a half of their number are compounds of Pb2+ and Bi3+.
During the selection of materials for our review we classified ferroelectrics as those of
((displacement type
))or
<(order-disorder type
>)on the basis of tradi- tional ideas on already known ferroelectrics, and in case of new ferroelectrics we assumed the absence of permanent dipole moments in the paraelectric phase of selected substances.
This review considers ferroelectrics mostly accord- ing t o those new physical properties which differ this or that group of ferroelectrics from classical ferro- electrics.
1. Ferroelectrics without 1800 domains.
-Accord- ing to Shuvalov [4] and Aizu [5] some kinds of ferro- electrics have spontaneous polarization which can be turned only at some definite angle not equal to 180°.
Naturally, in this case the 1800 domains are impossible.
Theoretically, 33 kinds of such ferroelectrics are known, but only one kind is found among the displace- ment-type ferroelectrics experimentally. This type includes Fe-C1-, Fe-Br-, Fe-J-, Co-C1- and Zn-C1- boracites. They have cubic lattice of T: symmetry a t high temperature and trigonal symmetry c:, at low temperature. In the low temperature phase the sponta- neous polarization
- --can exist only in four cubic direc- tions
:[l 1 11, [i 1 I], [1 i 1] and [l 1 I], i. e. in the direc- tions which are oriented from the tetrahedron centre t o its corners. P, can jump over from one of these directions
3
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972207
C2-26 G. A. SMOLENSKY AND V. A. ISUPOV
to another under the influence of electric field or
mechanical stress. In this case the coercitive fields are high (from 50 to 800 kV/cm) and the hysteresis loops as a rule are asymmetric [6].
2. Ferroelectrics-ferromagnetics.
-The substances possessing simultaneously the spontaneous polariza- tion P, and the spontaneous magnetization Ms were predicted in [7] and found in [8]. Unfortunately, there was no measurable magneto-electric interaction, which was prognosticated theoretically [9].
Ni-J-boracite investigated by Ascher and Schmid is the most interesting ferroelectric-ferromagne- tic [lo], [Ill. Its P, and M , coexist below 64 OK.
P, r 0.08 x c/cm2 at 4.2 OK and is oriented on cubic direction [OOI]. Then M , is oriented either on [110]
or on [1T 01. The direction of M, changes by 900 by the change of the P, direction by 180°. The remagnetiza- tion of a crystal (the turn of M, by 900) causes the repolarization (the turn of Ps by 1800). The magneto- electric susceptibility (X me) is equal to -- 3.3 x at 15 OK.
After speaking about the interesting properties of boracites it would be useful to discuss the problem of their Curie temperatures. For this purpose let us consider their crystal structure. From figure 1 where 118
@ a 00
@ M . q.s
FIG. 1. - Crystal structure of boracite type (118 of unit cell).
of a boracite unit cell is shown, it is seen that this structure can be conveniently considered, as the structure built of x-M~:' octahedra which are bound with each other by their corners, as well as octahedra Ti06 in perovskite-type structure. The rest of the atoms situated between the octahedra can be considered as complex ions (B,05)5-. Then the boracite formula can be written as ABC, and have the same appearance as perovskite compounds formulae [9].
The distance Mg-C1 in octahedron is equal to 3.02 A.
The sum of Mg and C1 ionic radii is equal to 2.55 A.
So the ionic polarizability of C1- must be exceptionally great. If C1 is replaced by Br, the radii sum accounts for 2.70 A, if replaced by J, this sum is equal to 2.94 A.
Then the ionic polarizability of halogen decreases sharply. That must lead to the lowering of the phase transition temperature.
The dependence of Curie temperatures of boracites on the lattice period is shown in figure 2. There are three branches there
:for C1-, Br- and J-boracites. It is seen that C1-boracites have the most Curie temperature, and the J-boracites have the least ones. Only the point for Zn-J-boracite falls out of common course of curves.
Our opinion is that it is desirable to check the phase transition temperature in this boracite.
FIG. 2. - The dependence of ferroelectric phase transition in boracites on crystal lattice period.
3. Ferroelectrics-semiconductors.
-Chalcogeno- halogenides of antimony and bismuth properties which were described by Nitsche, Merz et al. represent an important class of ferroelectrics, which shows some new physical phenomena. The crystal lattice of these compocnds is orthorombic both in paraelectric and in ferroelectric state. The direction of Ps is axis c.
Fridkin 1121 predicted that the Curie temperatures of these ferroelectrics must depend on the light exposure of crystals. This dependence is caused by the contribu- tion which the electronic subsystem gives in free energy of crystal if the nonequilibric carriers concentra- tion n is high. This contribution is proportional to nEg(P), where Eg is the width of the forbidden band.
The screening of the electric fields by free carriers influences the Curie temperature location (temperature hysteresis) too [13], [14].
The existence of the light exposure dependence of the Curie temperature is confirmed by experiment [15].
Due to the light exposure by which n increases by two
orders of magnitude, the Curie temperature becomes
lower than 1 OC.
NEW CLASSES O F FERROELECTRICS O F DISPLACEMENT TYPE C2-27
If we speak about SbSJ, we must note the work of Balkansky et al. [16], where the lattice dynamics of this substance was investigated. The modes which were infrared and Raman-active were determined there. It was shown that soft optical modes were connected with Sb and S displacement along axis
z.The decrease of the soft optical mode frequency in paraelectric phase by nearing the Curie temperature was found.
4. Ferroelectrics with diffused phase transition. - The attention of a number of investigators is presently attracted by the so-called ferroelectrics with diffused phase transition [9], [17]. These ferroelectrics are characterized by the dependence on the temperature of
E'and E" maxima in the phase transition region on the frequency of the measuring electric field. Their inverse dielectric permittivity changes above the Curie temperature corresponding to square rule
:All these ferroelectrics are characterized by the presence of two or more kinds of ions in octahedrical sets of the lattice. The diffusion of phase transition is cleared by the frozen composition (concentration) fluctuations and by the concentration dependence of the transition temperature of the individual districts in a crystal. The crystal is composed of polar and unpolar districts in the phase transition region. The polar districts can be depolarized by the warm fluctua- tions and be polarized again. The polarization- depolarization phenomena in these districts lead to the relaxation character of dielectric polarization of these ferroelectrics.
The activation energy determined according to the slope of the curve In o
=f (IlT,) (Fig. 3), where T,,, is the temperature of maximum on the curve err(T) is
FIG. 3. - The dependence of the temperature (frequency) of E"
maximum of PbMg1.3Nb2.303 single crystal on frequency (temperature).
several electron-volts by its order of magnitude, and the resonance frequency of relaxation elements v o is some lo3' cps. It is clear that these values have no physical sense. That is caused by the fact that the number of relaxation elements in these ferroelectrics sharply depends on temperature and has a maximum in the phase transition region. The other results were obtained [18] when the dependence In co, = f (IlT) was built, where om was frequency by which
E"had a maximum by constant temperature. In this case
Eis measured for the constant number of relaxation ele- ments and the temperature dependence of these ele- ments does not influence the location of the
'Emaxi- mum. Now the activation energy estimated by this dependence slope is some 10-I eV by its order of magnitude and the resonance frequency of relaxation elements is 1 0 ~ ~ - 1 0 ~ ' CPS, that is it has a reasonable value. We can propose that this resonance frequency v o is close to Cochran frequency of a soft optical mode by q - 0.
The obtained result is a convincing confirmation of the proposed mechanism of the phase transition diffu- sion and of the relaxation character of the dielectric polarization. By the way, the mentioned picture of the phenomena clears the square temperature dependence of inverse dielectric permittivity at the temperature above the average temperature of the phase transition.
5.
<(Improper
))ferroelectrics.
-Ferroelectrics of
M~;+(MOO,), type, where Me3+
=Sm, Eu, Gd, Tb, have a pseudotetragonal lattice at room temperature with a r b r 7.3 A,
cr 10.6 A. Their Curie tempera- tures qre in the range 157-190 OC. The spontaneous polarization is equal to (0.14-0.24) x c/cm2 and is directed along axis
c[19], [20]. These ferroelectrics have a low dielectric constant
( E ,r 10).
E,of a free crystal has a very little maximum in the transition point.
E,of a clamped crystal has no maximum [21].
The elastic constant c,, has a sharp minimum in the transformation point, where C,, decreases to a half of its original value. Such a behaviour was explained by the suggestion [21], that these molybdates are
((ferro- electrics-ferroelastics
))corresponding to the termino- logy introduced by Aizu [22].
A phenomenological theory of Levanyuk and Sannikov [23] allows to explain the phenomena observed in these molybdates. It is suggested that in these substances neither the spontaneous polarization nor the spontaneous deformation are the parameters of the transition. Using a two-component parameter of the transition the authors obtain a good agreement with the experiment. Dvorak and Petzelt [24] offered the name
<(improper ferroelectrics
))for the ferro- electrics with the transition parameter not being the spontaneous polarization.
Recently Pytte [25] suggested that the appearance of
instability leading to the phase transformation in
molybdates is caused by the presence of soft phonon
modes with q # 0.
C2-28
G . A. SMOLENSKY AND V. A. ISUPOVThe investigation of neutron scattering on terbium molybdate [26] shows that the phonon instability at points ($, 3, 0) of the Brilluen zone is the cause of the phase transition. An anharmonic connection with the atom displacements corresponding to this mode, which have antiferroelectric character, produces the sponta- neous deformation. The latter gives rise in its turn to the spontaneous polarization in substances which have piezoelectric properties.
I t is remarkable that the problem of oscillation modes and the instability of lattice is followed by the problem of the atom interaction forces in the lattice. It is these forces which are the original cause of the phase transition appearance, and improper ferroelectric transitions, too.
6. Antiferroelectrics.
-During the last few years some new antiferroelectrics of the displacement type were discovered. Particularly, the antiferroelectric phase transformation was found in lead orthovanadate (Pb,V20,) [27]. According to 1281, this substance has a rhombohedric lattice with a
=7.65 A, a
=460 02' built of tetrahedra VO,.
The dielectric constant of Pb,V20, has a maximum near 100 OC (Fig. 4). A twinning structure observed below 100 OC in a polarized light beam along a three- fold axis. These twins disappear above 100OC. In transformation point the samples show sharp volume
FIG. 4. - The temperature dependence of dielectric permittivity of Pb3Vz08 at 0.5 Mcps in the field perpendicular to cleavage
planes.
changes. These phenomena plus the absence of the dielectric hysteresis loops allowed the authors to propose the antiferroelectric properties of lead ortho- vanadate.
The antiferroelectric properties were found in lead silicate Pb,SiO, too [29]. The dielectric permittivity of the polycrystalline samples goes through a maximum at - 160 OC (Fig. 5). When the sample is heated its volume decreases at the phase transition point. The Curie-Weiss law is observed above the phase transition
point, its constants being
8,= 33.3, 0 = 277 OK, C
=lo3 OK. I t is known that the ferro- and antiferro- electrics of the displacement type have usually
FIG. 5. - The temperature dependence of dielectric permittivity e at 0.5 Mcps (I), I/(€ - €0) (2) and warm dilation of polycrystal-
line lead silicate Pb4SiO6.
7. Miscellaneous.
-Coming to other ferroelectrics we shall first discuss the layer perovskite-like com- pounds 191. These compounds are characterized by the perovskite-like layers and the bismuth-oxygen layers.
Different compounds of this type can have different number of oxygen octahedra in the unit cell. These substances are tetragonal in paraelectric phase.
Bismuth titanate Bi,Ti,O,, is the most investigated layer ferroelectric. Probably it will be used for the quantum optic and memory cells. Its spontaneous polarization makes the angle of - 50 with the layers [30]. The component of P, along the
caxis is nearly 4 x lo-' c/cm2, the component along a axis is nearly 30 x lo-' c/cm2.
All ferroelectrics with the layer perovskite structure are characterized by the high Curie temperatures (150-950 OC). Such a rule was observed : the less is the radius of ions in positions with the twelwe-fold coordi- nation in perovskite-like layers, the higher is the Curie temperature. The dependence of the Curie temperature of a number of layer compounds on the average lattice period perpendicular to axis
c(that is (a + b)/2) is
given in figure 6. It is seen that the Curie temperature is the higher, the less is (a + b)/2. The experimental points both for niobates and tantalates fall in the same curve. There are different curves for the families of compounds with different thickness of perovskite- like layers. We connect the obtained dependence with the increasing of the internal fields in bismuth-oxygen layers when the lattice period decreases.
The considerable success was achieved in the research
NEW CLASSES OF FERROELECTRICS OF DISPLACEMENT
TYPE
C2-29FIG. 6. - The dependence of Curie temperature of layer perov- skite-like compounds on average lattice period perpendicular to
z-axis (i. e. on (a - b)/2).
of the perovskite ferroelectrics (table I). It is seen from this incomplete table that the perovskite-type ferro- electrics of complex composition are very diverse.
Speaking about them it should be mentioned that a very large number of these perovskites was studied by Fesenko and Venevtsev. Some of them are ferro- and antiferroelectrics. Kupriyanov and Fesenko
Compound
-
BaTiO, PbTiO, PbZrO, KNbO, NaNbO, Nao .,Bio .,TiO, PbMgl/3Nb2/303 PbNil/3Nb2/303 PbCol/,Nb2/3O, PbZnl/3Nb2/303 PbSc0.5Nb0.503 PbFe0.5Nb0.503 PbFe2/3W1/303
PBC00.5W0.503 PbNi0.5W0.503
discovered also some ferroelectrics with formula A(B~.< B:.<)o~.~~, which have perovskite structure, too 1311.
The number of ferroelectrics with the structure of tetragonal K-W-bronze increases considerably after discovering the ferroelectric solid solutions (Ba, Sr)Nb,O, in our laboratory 1321, when it became clear that it was important to use two atom A kinds with different sizes. Some of the new ferroelectrics of this type are given in Table 11. It is shown that many of these substances have good electro-optic properties.
Ferroelectric crystals LiNbO, and LiTaO, are of great importance now. They are widely studied. Some properties of LiNbO, are given in Table 111. LiNbO, is used in quantum optics and hypersonics measurements.
Some interesting properties were shown by cadmium pyroniobate Cd2Nb,0, [33] which has a cubic pyro-
FIG. 7. -The temperature dependence of 8, I/& and tg 6 of Cd2Nb2O7 single crystal in the field
of
80 V/cm with frequency of1
kcps along[III].
TABLE I
Perovskite-type ferroelectrics and antiferroelectrics State
-
FE FE AFE FE AFE FE FE FE FE FE FE FE FE AFE AFE
Compound
-
SrTiO, CdTiO, PbHfO, KTaO, BiFeO, Ko.5Bio.5TiO3 PbMg1/3Ta2/303 PbNil/3Taz/3O3
PbC01/3Ta2/303
PbCdl/,Nb2/3O,
PbSc0.5Ta0.503 PbFe0.5Ta0.503
PbMgo.
5Wo
- 5 0 3 PbCd0.5WlJ.503CsGeC1,
State
-
FE
AFE
AFE
FE
FE
FE
FE
FE
FE
FE
AFE
AFE
FE
G. A. SMOLENSKY A N D V. A. ISUPOV
Some properties of LiNbO,
sf1 = 0.581 x 10-12 cmZ/dyne sf3 = 0.495 x 10-12 cmz/dyne sf4 = 1.481
x
10-12cm2/dyne d31 = - 0.097 X 10-6 CGSE d33 = 0.213 X 10-6 CGSE dzz = 0.488 x 10-6 CGSE ne = 2.303 8, no = 2.414 4 ne = 2 . 0 5 6 4 , n 0 = 2 . 1 1 9 3 r33 = 32.2 X 10-10, f'l3 = 10 rsl = 32.6 x 10-10 cm/Vchlorine structure at room temperature. The tempera- ture dependence of
&,118 and tg 6 of Cd2Nb207 single crystal along [ l l l ] is shown in figure 7. The dielectric constant passes through a maximum at
-88 OC and it is possible to suggest that this temperature is the Curie point. But the dependence l/&(T) is linear only till
-
73 OC. The slope of this dependence grows below - 73 OC. The sharp increase of tg 6 begins below
-
73 OC, too. And the polysynthetic twinning is also observed below - 73 OC. We consider it as domain structure. So, the temperature of the ferroelectric phase transition does not coincide here with the dielec- tric constant maximum.
The hysteresis loops of Cd2Nb,07 at different temperatures are given in figure 8. This view of the loops was explained by the phase transformation from one ferroelectric phase to another under the influence of the electric field.
Ferroelectric properties of stibiotantalite were recently discovered in Koptsik's laboratory (Moscow State University) [34]. Mineral stibiotantalite has the composition of Sb(Ta, Nb)O, and an orthorombic
FIG.
8. -The dielectric hysteresis loops of CdzNbz07 single crystal in the field of 40 kV/cmwith
frequency of 50 cpsalong [Ill].
lattice whit a
=4.916, b
=5.542, c
=11.78 A, built of octahedra Ta06, Nb06 and Sb06. The dielectric constant
E,of natural crystals is equal to -- 900 at 200C and passes through a sharp maximum near 400 OC, where it reaches 20 000. Above 400 OC it obeys the Curie-Weiss rule with C
=(2.5 + 0.7) x lo5 OC.
P,
=17 x c/cm2 at 20 OC. The piezo-
TABLE 111
Linear electrooptic coeficient of some ,ferroelectrics and antiferroelectrics at room temperature Crystal
-
rij x 10' CGSE
-
mij x 10' CGSE
-
NEW CLASSES OF FERROELECTRICS OF DISPLACEMENT TYPE
C2-3 1moduli d, and d,, are equal to 3.54 x and 7.2 x CGSE, correspondingly. The electro- optical coefficients are rather large : at 1
=6 300 A,
r,,
=78 x lo-',
r1
=a, r,, + a, r2-3
=40.5 x 10-'CGSE.
As shown by Syrkin and Popolitov, the synthetic crystals SbNbO, also have ferroelectric properties. A phase transition of I order near to transitions of I1 order is observed at -- 400 OC.
at 20 0C. The piezomodule d,
=1 x lo-, CGSE.
The recently discovered ferroelectrics of BaMeF, type where Me2+
=Mn, Fe, Co, Ni, Mg and Zn [35]
are of great interest. Some of them show the magnetic ordering. E. g., BaFeF, is an antiferromagnetic below 60-70 OK.
The orthorhombic crystal structure of these com- pounds is built of octahedra MeF, which are connected in noncoplanar layers MeF, (Fig. 9) [36]. Ions of Ba are situated in spaces between these layers. P, is directed along a axis and is in the range of
(6.7-9.7) x c/cm2 .
The dielectric permittivity is not large : different crystals have
E, =8-1 1, cl,
=1 4 - 2 2 , ~ ~
=7-10 at 20 OC.
It is important to note that the ferroelectricity existence in fluorides is unusually, because the electron polariza- bility of F- is much less than that of 0'-.
This year the ferroelectric properties were discovered in Sr2Nb20, [37]. Its lattice seems to be orthorhombic.
The dielectric hysteresis loops are observed along c
FIG.
9.- Crystal structure
ofBaMeF4 compounds
(aprojection on 100)).
axis. P,
=9 x c/cm2, the coercive field is
- 6 kV/cm at 20 OC. e,
=75,
E~ =4 6 , ~ ~
=43 at room temperature. The pyroelectric, piezoelectric and linear electrooptic effects are noted. The Curie temperature is not shown.
There are many ferroelectrics and antiferroelectrics of the displacement type which were studied in detail recently. The limits of the paper size do not allow us to describe their properties in details.
Presently the ferroelectrics are a wide class of substances which have interesting properties and are widely used in technics. Obviously the expansion of the number of ferroelectrics will continue in the nearest future with the increasing rate.
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