Lanthanum substituted lead zirconate-titanate (PLZT) ceramics of composition 11.1/55/45 have been studied by measuring high electric field properties. Dc bias, dielectric and P-E hysteresis loop measurements have been employed to construct an E (electric field) versus T (temperature) phase diagram. At lower temperatures and small electric fields an antiferroelectric phase with tetragonal symmetry has been found. Applying high electric fields gives rise to a field-induced phase transition from the antiferroelectric to the ferroelectric state, however, without X-ray detectable change in crystal symmetry

Burggraaf, A.J.

Thieme, C.L.H.

Wolters, M.

English

University of Twente Research Information

Mat. Res. Bull. Vol. II, pp. 315-3ZZ, 1976. Pergamon Press, Inc. Printed in the United States. THE ELECTRIC FIELD-INDUCED ANTIFERROELECTRIC TO FERROELECTRIC PHASE TRANSITION IN SOME (Pb,La) Zr0.55Ti0.4503 CERAMICS M. Wolters, C.L.H. Thieme and A.J. Burggraaf Twente University of Technology, Department of Chemical Engi- neering, Laboratory of Inorganic Chemistry and Materials Science P.O. Box 217, Enschede, the Netherlands (Received January 12. 1976; Communicated by G. H. Jonker) ABSTRACT Lanthanum substituted lead zirconate-titanate (PLZT) ceramics of composition 11.1/55/45 have been studied by measuring high electric field properties. Dc bias, dielectric and P-E hysteresis loop measurements have been employed to construct an E (electric field) ver- sus T (temperature) phase diagram. At lower tempera- tures and small electric fields an antiferroelectric phase with tetragonal symmetry has been found. Applying high electric fields gives rise to a field-induced phase transition from the antiferroelectric to the fer- roelectric state, however, without X-ray detectable change in crystal symmetry. Introduction In the perovskite system (Pb,La) (Zr,Ti)O 3, usually called PLZT, interesting electrical and electrooptical properties have been found. Especially PLZT with a Zr to Ti ratio of 65 to 35, abbreviated as PLZT x/65/35 (x=La content in at.%) offers possi- bilities for useful applications Iii. The phase relations and dielectric properties of PLZT x/65/35 have been investigated intensively 12,3,4]. For investigations of diffuse phase transitions in PLZT ceramics Wolters and Burg- graaf 15,61 have chosen PLZT with a Zr to Ti ratio of 55 to 45 (PLZT x/55/45). These compositions have been chosen to avoid ad- ditional phase transitions like the rhombohedral to tetragonal ~ hase transition (see the phase diagram for PLZT by Heartling 71).This phase transition can disturb the ferroelectric (FE) to paraelectric (PE) transition, which is under study here. To know more about existing phases and phase transitions it looks useful to study high-field dielectric properties. It is already known that crystal structure and physical properties of PLZT 315 316 M. WOLTERS, et al Vol. II, No. 3 ceramics are strongly dependent on high electric fields 12,4,8,91. Here, we especially want to know if high electric fields induce a FE phase or only align the (micro) domains in the already exis- ting FE phase. The investigations reported here, have been res- tricted mainly to the composition PLZT 11.1/55/45. Properties of other PLZT x/55/45 (with 8.1 ~ x $ 14.1) ceramics at high elec- tric fields have been studied too, but will not be reported ex- tensively here. Experiments For these experiments samples with a density better than 96% and a grain size larger than 5 ~m have been used. The preparation and characterisation have been described in ref. 5. To study high field dielectric properties, two kinds of measu- rements have been performed. By means of a modified Sawyer-Tower circuit P (polarization) - E (electric field) hysteresis loops were obtained at a frequency of 0.05 Hz. Using a General Radio Impedance Bridge 1650A, with dc bias fields between 0 and 12 kV/ cm, capacitance and dielectric losses were measured. Results In the next sections the expressions "thermally depoled" and "ac depoled" will be frequently used. A material is thermally de- poled by heating it to a sufficiently high temperature, followed 22- / V " I J E(kV/cm) FIG. 1 P-E hysteresis loops for thermally depoled PLZT I].I/55/45 ceramics at dif- ferent temperatures. by slow cooling in short-circuited condition. The material can be ac T=30°C depoled by continuously reducing the ac field, which initially has been equal to the maximum field used in hysteresis loop measure- ments. For PLZT 11.1/55/45 the P-E hys- T=53.5°Cteresis loop changes with tempera- ture as shown in fig. l.a-d At low temperatures a rather square loop is observed (fig. la) which chan- ges in a "double loop" (fig. ib,c) I at increasing temperature. At still T=60°C higher temperatures a so-called "slim-loop" is found (fig. id), which gradually approaches linear P(~C/cm 2) dielectric behaviour. The remanent polarization Pr and the coercive field Ecobtained from T=75°C these loops as function of tempera- ture are shown in fig. 2. At a cer- tain temperature Tp we notice a sharp decrease in Pr" In a rather large temperature interval above Tp a finite remanent polarization Pr is still observed which decreases slowly with increasing temperature. Only in a temperature interval of a few degrees above Tp this decrease Vol. 11, No. 3 FERROELECTRIC PHASE TRANSITION 317 of Pr with increasing temperature is fairly large. The charac- teristic temperature Tp can also be deduced from E c versus tempe- rature. At Tp the shape of the hysteresis loops changes from square to double loops. In the same fig. 1 the initial P-E curves are also shown, starting from the thermally depoled state. A discontinuous change will be noticed at a certain electric field, called the critical field Ecu. Fig. 3 shows the variation of both E c and Ecr as a function±of temperature. It shows a d_~ecrease of E c and an i__nncrease Pr (~c~: 200- 100- i ol 2O \ \ \ Ec "x \ 4b ~ @3 Tf~C) B (kV Icm) T -5.0 .25 FIG. 2 Remanent polarization Pr and coercive field E c as function of temperature for PLZT 11.1/ 55/45. Ec.E~I (k~m)l / 6.0 4.0 2.0 , ~ ' e b ' sb TeC) - F I G . 3 Coercive field E and criti- c cal field Ecr versus tempera- ture for PLZT ] I. 1/55/45. of Ecr with i__nncreasing tempera- ture. Furthermore two parts of Ecr as a function of temperature can be distinguished with a break- point around Tp. Thermally depoled material be- low T_ gives P-E hysteresis loops of th~ type shown in fig. 4a. With electric fields smaller than Ecr only a small remanent polari- zation is found. However, with electric fields higher than Ecr normal square loops with high Pr are observed. So the hysteresis loop characteristics of thermally depoled material change suddenly at Ecr. Ac depoled material below Tp shows square loops both at electric fields smaller and E (kV/cm) Z~ j I 4a 4b P(pC/cm 2 ) FIG. 4 P-E hysteresis loops for thermal- ly depoled (4a) and ac depoled (4b) PLZT 11.1/55/45 at 40°C. (1: E < E ; 2: E > E ). max cr max cr 318 M. WOLTERS, et al Vol. ii, No. 3 higher than Ecr (fig. 4b). At temperatures above Tp no differen- ces in the P-E hysteresis loops between thermally Nnd ac depoled materials have been found. In fig. 5 the results of dielectric measurements on ac depoled samples are shown. The temperature at which e' is maximum will be called T'_ while T b is the temperature corresponding to the maximum value'of e". The application of a bias field causes a shift of T b to higher temperature, whereas T c' is only weakly in- fluenced (compare Table I). For thermally depoled material a small decrease in T b (and T' ) has been found for increasing dc c bias electric fields smaller than Ecr (Table I), while T b in- creases for increasing fields larger than Ecr (Table I, poled ma- terials). At temperatures below Tb, e' and e" for ac depoled ma- terials and for poled materials are considerably smaller than those for thermally depoled materials. For temperatures above T b there is a little difference between thermally and ac depoled ma- terials. Once again it must be remarked that there are no or only little differences in dielectric behavior between the poled and the ac depoled condition, whereas thermally depoled material shows a rather different dielectric behaviour. X-ray experiments show no detectable differences in crystal symmetry or unit cell dimensions of the ac depoled and thermally depoled materials. Nor have there been found any superstructure reflections. The crystal structure has been indexed as tetragonal with a rather small deviation from the cubic symmetry 15]. For PLZT 11.1/55/45 there has been found a c/a ratio of 1.005 (~ 0.001). 15O00~ E' t 1OOOO 5000- \ j , 5O IO0 o o 15o E" t -1000 -5OO FIG. 5 Dielectric permittivity e' and loss ~" (10 kHz) versus temperature for ac depoled PLZT 11.1/55/45. (I: no bias field; 2: with dc bias field of 9.43 kV/ cm; : E'; ---: e") TABLE 1 Variation of characteristic dielec- tric parameters (at 10 kHz) by ap- plying dc bias fields to thermally depoled, ac depoled and poled PLZT 11.1/55/45 ceramics. Tb(°C) ic blas(kV/cm) e max Tc'(°C) C'max Thermally depoled 0 57 750 87 12500 1.32 56 850 84 12900 A.c. depoled 0 56 810 84 12800 1.32 56 860 84 12900 3.77 62 1130 82 13800 9.43 73 620 84 12800 Poled 0 55 850 85 12600 1.32 58 850 88 13150 9.43 74 575 87 13000 Discussion In this section a restriction has been made to phenomena oc- curring mainly in the temperature region below Tp, in which tem- perature region square P-E hysteresis loops have been found. Vol. 1 I, No. 3 FERROELECTRIC PHASE TRANSITION 319 The characteristic temperature T b is almost equal to Tp (at E = 0) and is probably a transition temperature. At this tempera- ture a strong decrease in ferroelectric properties (Pr, Ec) as well as a maximum in dielectric losses occur. At this tempera- ture preliminary work has shown a rather sudden decrease in unit cell dimensions too. The temperature of the maximum in the die- lectric constant T~ , however, occurs at a much higher tempera- ture. The discontinuous change in the first run of P-E hysteresis loops (fig. I) indicates an electric field-induced transition. Another indication for this transition is the change in the shape of the hysteresis loop for thermally depoled material by passing through Ecr (fig. 4a). At room temperature we have also noticed a marked decrease of e' and e" by passing through Ecr in dielec- tric measurements performed under increasing dc bias fields (to be published). This discontinuous change in the initial P-E curve does not represent the alignment of domains as normally found in ferro- electrics. For Ecr (characterising the discontinuous change) increases at increasing temperature (fig. 3), whereas domain alignment will occur at decreasing critical fields with i__nncrea- sing temperatures. This phenomenon of easier domain orientation is reflected in the temperature dependence of E c. In thermally depoled materials little indications for ferro- electricity have been found (a very low Pr, illustrated in fig. 4a). A ferroelectric domain structure has not been found too. On the other hand there are indications that thermally depoled material has antiferroelectric (AFE) features. In AFE materials the transition temperature to the PE state, here probably T b, should decrease at increasing dc bias field, as the latter has a higher dielectric constant. This corresponds with our results for PLZT 11.1/55/45 (table I and fig. 3). Ac depoled materials and poled materials clearly show FE fea- tures like square hysteresis loops (fig. 4b) and an increase in the transition temperature to PE with increasing dc bias fields 181, which is ±llustrated in fig. 5, table I and fig. 3. So in thermally depoled material (at temperatures below Tp) electric fields higher than Ecr will induce a phase transition from AFE to FE. The reverse transition will occur only by heating the material above T b and slow cooling below T b. So at temperatures below T b and at fields lower than Ecr a metastable FE phase can exist. The preceeding considerations make it possible to construct from fig. 3 an E-T phase diagram for (thermally depoled) PLZT 11.1/55/45, which is presented in fig. 6. The phase transi- tion AFE to PE is given by the transition temperature T b versus dc bias field (table I), which gives a "rather weak decrease of T b with increasing field. The transition FE to PE is indicated by the high temperature part of Ecr versus T above the break- point in fig. 3. This FE to PE phase transition can also be in- dicated by the change of T b at higher dc bias fields (table I, poled materials), which shows a comparable temperature dependence, although the absolute values are different. At last the AFE to FE transition is defined by the lower temperature part of Ecr versus temperature in fig. 3. A thermodynamical description of phase transitions in AFE materials has already been given by Okada I 101 . For 3Z0 M. WOLTERS, et al Vol. Ii, No. 3 lO0- E(kV~:r~ 5.C FE / AFE 1 % T~:2) FIG. 6 // // / / / / / / / I I / / PE sb ~o The electric field (E) - temperature (T) phase diagram for PLZT ]].]/55/45. a first order transition from the AFE to the FE state he stated that Ecr is decreasing slowly when the temperature is raised. Whereas for the induced first order phase transition from PE to FE Okada predicted a strong increase of Ecr with increasing temperature. This pre- dicted behavior for Ecr is observed in our experiments for the PE to FE tran- sition (fig. 3). However, we did not find a small decrease of Ecr (below Tb), but just the opposite viz. a small in- crease of Ecr with increasing tempera- ture. It must be remarked that theoretical- ly all phase transitions, e.g. AFE to PE, FE to PE and AFE to FE can be broadened. Indeed there are in- dications that for instance the FE to PE transition is a diffused one. For above T b (the phase transition tempera- ture) the remanent polarization has still a finite value, which slowly decreases with increasing temperatures to zero. For PLZT 10.5/55/45 the same kind of properties as for PLZT 11.1/55/45 have been found. For thermally depoled materials with La concentrations < 10.2 at.% and at temperatures below T b we did not observe an electrically induced phase transition. For these compositions we have only observed domain orientation in the already FE material at increasing dc electric fields. In thermally depoled PLZT 12.0/55/45 and PLZT 12.2/55/45 and at temperatures below T b indications for the existence of an AFE phase have been found too. Applying high electric fields to this material also results in a phase transition to the FE phase; removal of this field however gives the reversible transition from the FE phase to the AFE state without heating and cooling procedure as for PLZT 11.1/55/45. Still higher La concentrations, as e.g. PLZT 13/55/45,already show at room temperature linear behaviour between polarization and electric field and are consi- dered as PE. For PLZT ceramics with a Zr to Ti ratio near 55 to 45 an in- duced AFE to FE phase transition has not been reported in lite- rature. Though for PLZT ceramics with e high Zr content (e~. PLZT 10/80/20) this transition has already been observed 1 9 1 • Suggestions were made for the existence of an AFE phase in some PLZT x/65/35 ceramids 13,11,12 l , though no clear indications for such an AFE phase were given. Carl and Gei~en 131 rejected the AFE concept ~n the case of PLZT x/65/35 because the losses were considered too high. Indeed AFE materials having few (and rather large) domains should possess low losses. But for the PLZT compositions under consideration here the energy differen- ces between the FE AFE and PE phases are probably small (dif- fuse phase transitlons). So in the AFE phase a certain amount of FE phase can exist (metastable, fluctuations). This will raise the dielectric losses, in accordance with our experiments. Keve et al. 141 clearly found field-induced phase transitions in PLZT x/65/35 ceramics. The similarity between our observed Vol. II, No. 3 FERROELECTRIC PHASE TRANSITION 3ZI field-induced phase transitions and those of Keve et al. is the occurrence of a non-reversible phase transition and the occur- rence of ferroelectric features for poled and ac depoled mater rials. The difference is that no detectable crystal structure change has been observed at our field-induced phase transitions where both phases have tetragonal symmetry, whereas, Keve et al. found a cubic (non ferroelectric) structure for thermally de- poled material, which changed over to a tetragonal or orthorhom- bic structure at the field-induced transition. It is concluded that thermally depoled ceramic PLZT 11.1/55/45 possesses dominating AFE features at temperatures below the transition to the PE state. Applying electric fields gives a field-induced phase transition to a FE state. "Coexistence" of one stable with one or two metastable phases in a certain tem- perature-electric field region seems possible on the basis of quasi-static fluctuations (heterophase fluctuations). This will give rise to broadening of the phase transition. An explanation along this line has been given by Cook I131 for phase transi- tions in some metal systems. For ferroelectric materials this concept of heterophase fluctuations has already been used by Fritsberg 1141. Acknowledgement The present investigations have been carried out partly under the auspices of the Netherlands Foundation for Chemical Research (S.O.N.). References i. C.E. Land, Ferroelectrics ~, 45 (1974). 2. A.H. Meitzler and H.M. O'Bryan Jr., Proc. IEEE 61, 959 (1973). 3. K. Carl and K. Geisen, Proc. IEEE, 6-1, 967 (1973). 4. E.T. Keve and A.D. Annis, Ferroelectrics 5, 77 (1973). 5. M. Wolters and A.J. Burggraaf, Phys. Stat. Sol.(a) 24, 341(1974) 6. M. Wolters and A.J. Burggraaf, Mat. Res. Bull. i0, 417 (1975). 7. G.H. Heartling and C.E. Land, J.Amer. Ceram. Soc. 54, 1 (1971). 8. E.T. Keve and K.L. Bye, J. Appl. Phys. 46, 810 (1975). 9. W.A. Schulze, Dissertation (1973), Pennsylvania State Univer- sity. i0. K. Okada, J. Phys. Soc. Jap. 28, Supp. 58 (1970). Ii. J.T. Krause and H.M. O'Bryan Jr., J. Amer. Ceram. Soc. 55, 497 (1972). 12. A.H. Meitzler and H.M. O'Bryan Jr., J. Amer. Ceram. Soc. 55, 504 (1972). 13. H.E. Cook, J. AppI.. Cryst. 8, 132 (1975). 14. W.J. Fritsberg, Sov. Phys. Solid State iO, 304 (1968).

The electric field-induced antiferroelectric to ferroelectric phase transition in some (Pb,La)Zr0.55Ti0.45O3 ceramics

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The electric field-induced antiferroelectric to ferroelectric phase transition in some (Pb,La)Zr0.55Ti0.45O3 ceramics

Dissertation

Solid State iO,

https://doi.org/10.1016/0025-5408(76)90197-5

The electric field-induced antiferroelectric to ferroelectric phase transition in some (Pb,La)Zr0.55Ti0.45O3 ceramics

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