Growth an dielectric characterization of relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃ single crystals

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Growth an dielectric characterization of relaxor ferroelectric Pb(Mg_1/3 Nb_2/3)O₃ single crystals

SCHMID, Hans, YE, Z.G., RIVERA, Jean-Pierre

SCHMID, Hans, YE, Z.G., RIVERA, Jean-Pierre. Growth an dielectric characterization of relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃ single crystals. In: 1990 IEEE 7th International Symposium on Applications of Ferroelectrics. Piscataway : Institute of Electrical & Electronics Engineers (IEEE) Inc.,US, 1991. p. 482-485

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Growth and Dielectric Characterization of "Relaxor Ferroelectric" Pb(Mgi/3Nb2/3)03 Single Crystals

Z.G.Ye, J.-P. Rivera and H.Schmid

D6partement de Chimie Min^rale, Analytique et Apphquee, University de Gen&ve, CH-1211, Geneva 4, Switzerland.

Abstract

Single crystals of the perovskite Pb(Mg2/3Nb2/3)^3 P ^ ^ ^ were grown by the high temperature solution technique, according to an originally established pseudo-binary phase diagram between PMN and PbO. Temperature(T) and frequency(f) dependence of the complex dielectric permittivity were measured on thin platelets of different crystallographic sections by means of quasi- continueous frequency scanning from 1 kHz to 10 MHz, at temperatures ranging from 12^K to 418K.

The dielectric properties of the PMN platelets are characterized by an intermediate state between relaxation and resonance.

1. Introduction

The perovskite Pb(Mgi/3Nb2 ⁷³⁾⁰³ (PMN) has extensively been studied because or its high dielectric constant (€'>10'^) near room temperature, associated with a "relaxor" dielectric response and a

"diffuse phase transition". The mvestigation of physical phenomena of PMN is both of scientific and practical interest. A lot of work has been undertaken on PMN in order to understand the mechanism of the so-called diffuse phase transition resulting from the local composition fluctuations due to the disordered B-sites (1-7). Single crystals of PMN were grown previously by using different starting compositions (8-12) but without precise knowledge of a relevant phase diagram. In order to find the optimal conditions for the growth process, we have established a pseudo-binary phase diagram between PMN and PbO, according to which single crystals of PMN have been grown by the high temperature solution technique (13). Thereafter, as a first stage of the physical characterizations, dielectric properties of . as-grown crystals v/iU be described and discussed in

this paper.

2. Experimental and Results

Growth of the PMN single crystals was undertaken in sealed platinum crucibles of 30 ml volume, specially shaped to be sealed and opened several tunes. An excess of 30 to 40 wt% PbO was used as solvent. The morphology of the obtained crystals presented either perfect cubes of small size or degenerated cubes of large size. X-ray diffraction confkmed that the crystals belonged effectively to the perovskite PMN. Observation under the polarizing microscope of crystal - platelets showed rather good optical isotropy, suggesting the absence of internal constraints and hence suitability for subsequent physical characterizations.

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Thin platelets with a thickness of about 50^m were cut parallel to (100), (110) and (111) cubic planes and finely polished with 0.25;im diamond paste. Thereafter they were electroded by means of evaporated semi-transparent gold layers, permitting to avoid mechanically induced strain due to surface contacts and to realize a simultaneous visual control of the domain states during planned subsequent studies in presence of an applied electric field. The electroded samples were mounted in a special optical Oxford He-flow cryostat for low temperature measurements in a helium atmosphere or in a furnace for temperatures above 293IC Dielectric measurements were realized by means of a HP4192A Impedance Analyzer controlled by a computer program permitting automatic quasi-continuous frequency scanning (80 points/decade) from 1 kHz to 10 MHz. The excitation voltage was fixed to 1 V^^^. The "two point" technique was used to minimize the parasitic capacitance effect due to cables, although it turned out to be negHgible in comparison with the strong capacitance of PMN platelets.

The real part of the dielectric constant of a

Pb(Mg]^^3Nb9^3)03 platelet of (100),cut presents an unsharp maximum of €'^QJ^=1S.SXIO^ at 260K (upon cooling) at 1 kHz (Fig.l). A thermal hysteresis of

^'max between cooling and heating is found to be of five degrees for a temperature variation of 3 deg.min"-'-. Upon increasing frequency e'^ax shifts in the direction of higher temperatures. In the frequency dependences'of e' and of the imaginary part e" at different temperatures (Fig.2), it can be seen that e' (or the parallel capacitance Cp) decreases upon increasing frequency and changes sign for a certain frequency (fg), at which the dissipation factor (tg5) diverges (Fig.3). A maximum of e" appears below fg. A change of fn with temperature is noted, the lowest value lying close to the temperature of e'^^ (T^=260K). A three dimensional representation of the temperature and frequency dependence of the dielectric constant is given in figure 4.

. The relationship between the real and imaginary part of the dielectric constant, e"=f(e'), at different frequencies and various temperatures, presents a snail shell variation (Fig.5)

In the admittance plane, the variation of the susceptance (B) versus the conductance (G) is characterized by a series of admittance circles, as in the case of a piezoelectric resonance (Fig.6).

482

o 10

^ 5 -

IkHz ^ lOkHz /A 100kHz //

250kHz / 350 kHz yV / /

SOOkHz / I / / ^ —

-

5 0 100 150 2 0 0

T E M P E R A T U R E (K)

2 5 0 3 0 0

Fig.l.Temperature dependence of the dielectric permittivity (£') of a PMN (100) platelet (thickness=44Mm,

area=631nm2^) at different frequencies.

Measurements undertaken on other crystallographic sections, (110) and (111), showed a similar dielectric dispersion behaviour.

3. Dicussion and Conclusions

The dielectric dispersion at high measuring frequencies appears rather unexpected in comparison with reported previous results (14-17).

Above 1 MHz the measured PMN platelets behave like a self-induction coil rather than a capacitor at low frequencies (see Fig.2,3,4). Different examinations showed that the unusual behaviour of the grown PMN platelets was in fact of intrinsic nature.

The maximum of and e" (Fig.1,2) as a function of temperature of PMN is found to shift to higher temperatures with an increase of measuring frequency, as is the case for typical relaxor- dielectrics, but a second ma.ximum appears above 350 kHz for e'. According to a Debye-like relaxation mechanism(14), which is characterized by lnu=

_ -(lnro)-E/(kTj^), where u=27tf, Tjn[K] the tempera-ture of e" maximum, T q the natural relaxation time, E the activation energy and k Boltzmann's constant, an enormously high natural oscillation frequency of about 10"^^'Hz is obtained. This suggests that the dielectric relaxation in PMN is more complicated than in ordinary relaxor dielectrics and that it cannot be characterized by a Debye model.

On the other hand, the circle form found in the admittance plane (B(G)) (Fig.6) suggests a piezoelectric-like resonance, which should be described by an equivalent circuit of a piezoelectric oscillator composed of a series circuit Rg, and Cg in parallel with Cp (e.g. 18). However, the large frequency interval of that "resonance" and the fact that the center of the impedance circles shifts inside the negative part of the susceptance, are not consistent with a pure piezoelectric resonance.

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Fig.2.Frequency dependence of the real part (a,c) and the imaginary part (b,d) of the dielectric permittivity of the PNGs^ (100) platelet at different temperatures. a),b):

125K ^ T <264K; c),d): 275K < T <418K.

0

Fig.5 .Frequency dependence of the dissipation factor (tgS) of the PMN (100) platelet at various temperatures.

In fact, it is known that the complex susceptibility x for a damped harmonic oscillator follows the equation x=V(l-^"'+i^)j where x is the (reduced) frequency and k the damping coefficient.

Figure 4.4 (chapter 4) of reference 19, shows the variation

x"{x')

for different values of k (see also Fig.3.13 of the same ref.). In that figure 4.4, the plot gives a circle centered on the x" axis, near x' > 0, for k=0.1, which is typical of a resonance phenomenon and a semi-circle centered on the x' axis, near x">0, for k=10, which represents, in the limiting case, a Debye relaxation. Between these two limiting cases a whole set of intermediate situations takes place, with a continuous deformation of the "resonance" circle, ending with the "Debye" semi-circle. Such curves look like "snail shells"!

12

z o _I

2

⁰

u

^f — 275K ^{b )} IMl-_^^^^ 293K~^

326K. \0

\ w

^10kHz

418K 378K IkHz

0 4 8 J2 16 E P S I L O N ' / 1 0 0 0 2 0

Fig^.Dependence £"=f(e') at different temperamres for the PMN (100) platelet a):12.5K < T <264K; b)-275K < T

<418K.

Fig.4.Temperature and frequency dependence of tne dielectric permittivity for a PMN (100) platelet (thickness =44/im, area=6.31mm-).

484

These macroscopic models explain well, at least qualitatively, the dielectric character of the measured PMN crystal platelets (see Fig.5: €"(€') in this paper).

At low temperatures (12K to =100K) the resonance regime dominates, as well as at high temperatures (>

400K). For the other temperatures, the PMN crystals behave in an intermediate way between a "pure"

Debye relaxation and a "pure" resonance phenomenon, giving the snail shell curves in the dependence e"=f{€'). That intermediate character is particularly pronounced between 250 and 300K, a temperature range in which e' passes through a maximum value and where the so-called diffuse phase transion takes place.

A more detailed theoretical and numerical analysis of that intermediate "relaxor"-"resonator"

characteristic, as well as the correlation with the peculiar physical properties of PMN will be presented in a following paper (20).

40 20"

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Fig.6 Admittance circles for different temperatures of the PMN (100) platelet. a):12.5K < T <200K; b).230K < T

<293K; c).326K < T <418K.

Acknowledgement

The authors wish to thank E.Burkhardt, R.Bouteiller and R.Cros for their technical help and the Fonds National Suisse pour la Recherche Scientifique for financial support.

References

1) CASmolenskii, "Physical phenoinena in ferroelectiics with diffuse phase transition", J.Phys.SocJapan. vol.28, Suppl.,p.26 (1970)

2) L.E.Cross, "Relaxor ferroelectiics", Ferroelectrics, vol.76, p.241 (1987)

3) G.Schmidt, "Diffusive phase transitions", Ferro electrics.

vol.78, p.l99(1988)

4) VA.Isupov, "Some problems of diffuse ferroelectric phase transitions", Ferro electrics, vol.90, p.113 (1989)

5) G.Bums and F.H.Dacol, "Glassy polarization behaviour in ferroelectric compounds Pb(Mg-j ^Nb, rt)03 and Pb(ZnT ,oNbort)OV'. SoHd Stateromigun.. vol.48, p.853 (1983)^^"* "^^ ^

6) E.Husson, M.Chubb and AMoreU, "Superstructure in Pb(Mgj^/3Nb2/3)03 ceramics revealed by high resolution elearon microscopy", Mat.Res.Bul].. vol. 23, pJ357 (1988).

/) KUchino, S.Nomura, EE.Cross, R.E.Newnhani, "Soft modes in relaxor ferro electrics", Phase Transitions, vol.2, p.l (1981).

S) I.E.Myi'nikova and VA.Bokov, "Growth and electrical properties of monocivstals of Pb,NiNh^On and :^b,MgNb^OQ", GroWth of Crvst^s. VoD, p309.

Consultants Bureau, New-York,(l959).

9) W.ABonner and L.G.Van Uitert, "Growth of single crvstals of PboMgNh^Oo by the Kyropoulos technique", Mat.Res.BuUr. voL2,~p.T31 (1967).

10) N.Setter and EE.Cross, "Flux growth of lead scandium tantalate (PbSc,, ^Taf, ^00 and lead magnesitmi niobate (PbMg, ,^m-,,-JJ.) smgle crystals",XCrvstal Growth vol.50, f/i55IT580).

11) G.T.Petrovsidi, LABondar, E.MAndreev, EN.Koroleva, "Formation of single crvstals of the perovskite-like ferroelectric PbMg-,y.i^^/X) Tzv.Akad.Nauk SSSR.

Neorg.Mater.. voiiO, p.l067 (1984.)

12) LI.Afanas'ev, AA-Berezhnoi, T.S.Bushneva and S.VJrokof ev, "Growing crystals of lead magnoniobate and maenotantalate", Opt.- Mekh. Prom - st. voL44, p.40 (19^77).

13) Z.G.Ye, P.Tissot and H.Schmid, "Pseudo-bmary PbMg, ,,NbTrtO,-PbO phase diagram and crystal growtS^of PbMg, rtNb^/oO.iPMN]", Mat.Res.Bull.

vol.25, N^.6, (1990) (in'^ress).

14) V.V.Kizillov and V.AIsupov, 'Relaxation polarization of PbMg^ ,3Nb, ,303(PMN) - A ferroelectric with a diffused ohase TJ-ansiuon. Ferro electrics. \ol'5. d.3 (1973).

15) V.ABokov and LE.Myrmkova, "Electrical and optical properties of single crystals of ferroelectrics with a diffused phase transition", FizTverd.TeJa. vol3, p.841 (1961); [Soviet Phvs. - Solid State. vol3, p.613 (1961)].

16) CASmolenskii, VA,Isupov, AlAgranovskaya and S.N.Popov, "Ferroelectrics with d&use phase transition", Fiz.TverdTela. vol.2, p.2906 (1960); [Soviet Phvs. - Solid State, vol.2, p.2584 (196l)J.

17) N.P.Khuchua, "Microwave investigation of some ferro- and antiferroelectrics of perovslate type", in Proceedings of the International Meeting on Ferrolectrics. Prague, 1966, vol2, pp.161-170.

18) J.-P.Rivera and H.Schmid, "Piezoelectric measurements of Ni-I boracite by the techniques of admittance circle and motional capacitance", Ferroelectrics. vol.42, pp.35- 46 (1982).

19) AKJonscher, Dielectric relaxation in solids. Chelsea Dielectric Press, London, 1983.

20) J.-P.Rivera, Z.G.Ye and H.Schmid (to be published).

1990 IEEE 7th International Symposium on Applications of Ferroelectrics

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