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Pressure-temperature phase diagrams for several modified lead zirconate ceramics
J. F’6ys. @ Pm
Ckm.
Sdi& Vd. 39. pp. I16L1161 Press Lid.. 1978. Ptinkd in Geal
0022-369717811101-1
I63!$02.00/0
Brilain
PRESSURE-TEMPERATURE PHASE DIAGRAMS FOR SEVERAL MODIFIED LEAD ZIRCONATE CERAMICS? I. J. FRITZ and J. D. KECK Sandia Laboratories.Albuquerque, NM lI7l IS. U.S.A. (Received 9 January
1978; accepted 1 April 1978)
Aisstract-Using dielectric constant and loss measurementswe have determined the pressure-temperaturephase diagramsof four ferroelectric ceramics based on lead zirconate (PbZrOs). The materials chosenfor study all have pressure-inducedfenoelectric to antiferroelectric transitions at -0.3 GPa. In all four materials, five phases were identilkd by comparing our results with studies, previously published by other workers, of the phase behavior of modifiedPbZrO, materialsas a function of compositionand temperature.The observedphasesare the ideal cubic perovskite paraelectric phase, two rhombohedral ferroelectric phases, and two antiferroelectric phases, one orthorhombic and one pseudotetragonal. Comparison is made with previous temperaturecomposition studies of modified PbZrQ materials.
1. WrRoDucTloN
known and important example is the lead zirconate-lead
which crystallize in perovskite-type structures are among the most interesting and extensively studied of solid state mater&[ I]. Much of the fundamental interest in these materials has centered around the variety of structures and of displacive structural phase transitions they exhibit. From example, transitions involving relative displacements of certaio sublattices and others involving essentially rigid rotations of the comer linked octahedra (of oxygen, fluorine, chlorine, etc.) can both be found in many of these substances. Perovskite materials are also of technological importance, as shown, e.g. by the widespread use of piezoelectric ceramics based on barium titanate and on lead zirconate titanate [2]. The materials of interest in the present investigation are mixed oxide perovskites based on lead zirconate (PbZrO3. L.ead zirconate has been extensively studied. A recent paper by Burns and Scott [3] lists references for much of the early work in this area. At temperatures above -230°C PbZr03 has the paraelectric ideal cubic perovskite structure, which we will denote hereafter as P. Below -230°C and over a narrow temperature range of -12-26°C Burns and Scott find that the structure is rhombohedral and ferroelectric. denoted here as FR2.At lower temperatures PbZrOs is in the so-called orthorhombic antifetmclectric phase[4], which we denote as & Structural considerations indicate that A0 is actually a polar phase, but attempts at measuring the polarization have not been successful[3,5]. A fourth structural phase is observed in many mod&d PbZrOx compositions, and it has a free energy nearly equal to those of & and FRI. tetragonal This phase is called generally antiferroelectric[6], although it may only be pseudotetragonal. This phase is referred to as AT in this paper. Several investigations have been made into the properties of compositionally modified PbZrOa. A wellSubstances
Work supported by the U.S. Department of Energy, under Contract No. AT(2%1)789. IPCS Vd. 39 No. IL-B
or PZT system [2,6,7], the properties of which were tirst investigated by Shirane et al. [7]. Studies of the temperature-composition phase diagram of the PZT system have shown that the substitution of Ti for Zr in PbZrOx leads to a stabilization of the AT phase (over a narrow range of composition and temperature) and of two rhombohedral ferroelectric phases (at higher Ti concentrations). The ferroelectric phases are the Fn, phase mentioned above and a phase denoted as Fn, which is related to Fn, through a phase transformation involving a unit cell doubling(8,9]. The effects of various other dopants on PbZtQ have also been studied. For example, it is known that the rhombohedral ferroelectric phases can be stabilized by the substitution of Ba for Pb[lO] or of Nb for Zr[ll], whereas antiferroelectricity is favored by the substitution of La for Pb[2] or of Sn for Zr[ I I]. It has been known for some time that the app#cation of hydrostatic pressure to modified PbZrO, tehds to favor antiferroelectricity over ferroelectricity. Pressureinduced ferroelectric to antiferroelectric phase transitions have been described by Berlincourt et aL[l2,13] for PZT materials with -5% Ti and also for Sn doped compositions. These pressure-induced transitions’ are of technological importance, as they are the basis for the operation of shock-wave actuated pulsed power supplies(l4, IS]. Despite the importance of tiessure effects in modified PbZrOa materials, little work has been done in characterizing the properties of various compositions as a function of temperature pressure. Berlincourt cl al. [ 121 published a pressure-temperature phase diagram for a PZT 95/S composition, but, as we will show below, this diagram appears to be incomplete, as the AC,phase was not observed. Phase diagrams for several other compositions have been given in unpub lished reports [ 161. In this paper we present results of pressure-temperature phase studies done on four mod&d PbZrOs ceramics. Compositions were chosen whose ferroelectric titanate
1163
I. J. FRITZ and J. D. KECK
1164
to antiferroelectric transitions occur at moderate pressures (-0.3GPa). The various phase transitions were detected by dielectric constant and dielectric loss measurements made on unpoled samples.
2.ExPERIMENTAL
The compositions of the four materials studied and the techniques used for preparing the ceramic specimens are as follows. The first material is a 95/S PZT composition modified with -2% Nb to improve the electrical characteristics [ 171.This material will be denoted as PZT formula chemical is its 95/5-2Nb; Pbo.99Nbo.02(Zro.9~T~.~~)~.~O~. The second material is the same as the first, with 14% of the Zr replaced by Sn. This chemical formula has the material thus Pbo.99Nbo.oz[(Zro.86Sno.14)o.9~Tio.oslo.9803. We denote this composition as PSZT 86/14-5-2Nb, following Berlincourt et al. [ 161.The third material is a La doped PZT with the chemical formula Pbo.aLao.02(Zro.9zTi.~)Os which we denote as PZT 92/8-2La. The fourth material is doped with both La and Ba. It has the chemical formula Pbo.92Bao.osLao.02(Zro.94Tio.os)Os and is denoted herein as PZT 94/6-2La-5Ba. The first two samples (PZT 95/5-2Nb and PSZT 86/W 5-2Nb) were prepared by milling the oxides, calcining at 1WC in a rotary calciner, dry pressing and atmospheric sintering at 1345°C for 6hr. The other two materials (PZT 92/8-2La and PZT 94/6-2La-5Ba) were chemically prepared by coprecipitating ZrGl from zirconium tetran-butoxide and Ti02 from titanium tetra-n-butoxide as discussed by Haertiing and Land[l8]. Lead was added as lead oxide, lanthanum was added as lanthenum acetate, and barium was added as barium acetate. Mixing was done with alumina balls in a polyethylene mill jar. The powder was calcined at 950°C for 6 hr and hot pressed at 1280°C and 2000 psi for 1 hr. Test specimens were sliced from the various slugs with a diamond wheel saw and plated with fired silver electrodes. Dielectric constant and loss data were obtained by means of an automatic capacitance bridge (HewlettPackard model 4270A) in conjunction with an automated data acquisition system (Hewlett-Packard model 3050A). Most measurements were made at a frequency of 1 kHz on sample plates with nominal dimensions of 0.5 x 5 x 5 mm3. Hydrostatic pressure was generated by a standard Bridgman press utilizing a 1: 1 mixture of n-pentane and i-pentane as the pressure medium, and the magnitude of the pressure was measured to 1% accuracy by a calibrated manganin gauge. The samples were heated by a coil inside the pressure vessel and temperature was monitored by a chromel-alumel thermocouple mounted next to the sample. 3. RESULTS Some typical data scans, obtained on the PZT 95/S2Nb sample, are shown in Fig. 1. In this figure the dielectric constant l is plotted as a function of pressure for three different temperatures. Data are shown for increasing pressure only. At 23°C there is a drop in c
1 100
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.-__. 4~
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2-
“\
23?
-L> +.
‘-.-.
--2.%__._
0 0
0.3 PRESSURE
0.6 (Gh)
0.9
Fig. I. Typical dielectric constant vs pressure data for PZT 95/S2Nb.
near 0.3 GPa. It is known from separate dielectric charge loss measurements on poled specimens that this is the pressure at which a transition from a ferroelectric to a nonferroelectric state occurs. At 125°C two transitions are induced with pressure, one at -0.4GPa and the other at -0.95 GPa. At 180°C the material is near the Curie temperature, which is -230°C at atmospheric pressure and decreases with increasing pressure. This causes the fairly large value of e and its initial increase with pressure shown by the upper curve in Fig. 1. At - 0.4 GPa a transition to a nonferroelectric state occurs, causing the sharp drop in l above - 0.4 GPa shown by the data. Data scans similar to those shown in Fig. 1 and including points taken as a function of decreasing pressure were obtained for all four samples studied. Scans were also taken as a function of temperature at nearly constant pressure (actually, at constant ram force, so that the pressure varied slightly). Using the results from ah these scans it is possible to trace out the boundaries between the various phases in the pressure-temperature (pT) plane. As a final step it is necessary to actually identify the various phases. There are two ways in which these identifications can be easily made without recourse to X-ray or neutron diffraction data. The first is to realize that there are certain systematics always observed in modified PbZrGx materials at zero pressure. Thus, ferroelectric compositions have the sequence of phases P + F&-) FR, with decreasing temperature, and antiferroelectric compositions have the sequence P + AT + A0 or P --, A0 with decreasing temperature. Furthermore we know that pressure favors the antiferroelectric phases over the ferroelectric ones. The second method of identifying the various phases is by the characteristic shapes of the dielectric anomalies at the various transitions. These characteristic shapes do not vary drastically from one material to another. For example the FRI-FR2 transition is accompanied by an anomaly in the dielectric loss but there is no anomaly in E at this transition[l2]. The A=c+Ao transition is broad, somewhat sluggish, and has a large hysteresis [ 161.
Pressure-temperaturephase diagrams for several modified lead zirconate ceramics The two methods just discussed of identifying the phases give consistent results for the four materials we investigated. The resulting phase diagrams are given in Pigs. 2-5 and will be discussed in htm. Fiie 2 is the phase diagram for the PZT 95/S-2Nb sample. Solid lines are used to indicate boundaries obtained with increasing pressure, whereas dashed lines indicate the reverse transformation. Considerable hysteresis exists for all transitions except those involving the paraelecbic (P) phase. The transitions P-FRZ and P-A, do exhibit hysteresis regions of a few degrees, but no attempt was made to plot these. In Fig. 2 different plotting symbols are used for the different transitions, and these are listed in the caption. A notable feature of PZT 95/S-2Nb is that the boundaries among the four phases FR,, Fn2, AT and A0 merge in the region 0.30.4GPa and 50-100°C. Because of the hysteresis in the transitions, and because the transitions are spread out
250,
1165
0.8
1.2
1.6
PfESSURtlCRl
Fig. 4. Pressure-tern-e
phase diagram for PZT 92/&2L.a
Symbolssame as for Fig. 1.
I
250 PZI W6
L
-2La -58a
’
'-Jlal--+-s i'
P
PrifSSURE
IGPal
0
2. Pressure-temperature phase diagram for PZT 95/S2Nb. The transitions associated with the various plotting symbols are as follows: 0, F,t,++&; +, F,I,-AT; 0, F,t,++F,n;I, FR+AT; 0. Fnp+P; 0, P-AT; A, AT-&
PSZT 85114- 5 - 2Nb
I 0.4
0.8
1.2
1.6
PlifSSURfIGRI
Fu. 3. Pressure-tempemture
2Nb. Symbok
phase diagram for PSZT 86/14-S same as for Fii. I.
e-
__/-
0.4
0.8
1.2
_A--
1.6
PrEssulEIGPal Fig. 5.
Pressure-temperahue phase diagram for PZT 94/&2La5Ba Symbols same av for Fig. I.
over a range of pressure or temperature in the ceramic samples, it is difficult to determine the boundaries in the region where the four phases merge. The data points indicate the observed transitions, and the lines drawn in the Qture indicate the general directions of the phase boundaries. However, the detailed behavior in the region where the phases meet cannot be determined by the techniques used in this study. The diagram of Fig. 2 illustrates the general features observed for all four materials we have studied. At the highest temperatures the P phase is stable. With decreasing temperatures (at low pressure) the material transforms successively into FR2 and Fn,. The FRI++FR2 boundary increases in temperature with increasing pressure with a slope of -%deg/GPa. At higher pressures AT and A0 are the stable low temperature phases. The boundaries between the ferroekctric and antiferroekctric phases are extremely steep. This steepness is an. important factor in the utility of devices based on these materials, as such devices are often required to operate
1166
I.
J. Frurz and 1. D. KECK
over a wide range of temperatures. Comparing Figs. I and 2 we see that the transitions observed in Fig. I are FR, + A0 at 23°C. FR2+ AT -+A0 at 125°Cand Fn2+ AT at 180°C.
slope is - 250 deg/GPa compared to - 90 deg/GPa for PZT 95/S-2Nb. 4. DISCUSSION AND CONCLUSION
Figure 3 is the phase diagram for PSZT 86/M-5-2Nb. Work done in the past(2] on modified PbZrOl materiWe may think of this material as being derived from PZT als as a function of composition and temperature has clearly shown the sensitivity of the stability of the 95/5-2 Nb by replacing 14% of the Zr with Sn. A comparivarious phases to small changes in composition. The son of Figs. 2 and 3 shows that the phase diagrams of the two materials are similar, with four phases merging in a materials we have studied were chosen to have pressuresmall region of the pTplane for both compositions. The Sn induced ferroelectric to antiferroelectric transitions at doping has two major effects: it lowers the temperatures -0.3GPa at 23°C. Our results show that even within of most of the transitions by -4tPC, and it changes the such a limited class of materials the phase behavior is sign of the slope of the Fn2++AT boundary. Another strongly dependent on composition, as far as both the point to note in Fig. 3 is the scatter in the data points locations of the phase boundaries and their slopes are along the P-AT boundary. The dielectric anomaly at concerned. this transition becomes extremely small and spread out A comparison of Figs. 2-5 shows that the phase at high pressure for all four materials studied, so there is diagrams for the four modified PbZrOp compounds we always some uncertainty in determining the transition have studied are similar in that the phases appear in the points. However, the highest pressure point on the same order as pressure or temperature is varied. The P-AT boundary of Fig. 3 is out of line with the rest of observed order of the phases is similar to that observed the curve by an amount that seems to be outside of in temperature-composition studies of various modified experimental uncertainty. Berlincourt et of.[12] have in- PbZrOX ceramics. vestigated the phase behavior of Sn doped PZT An important result of our work is that the AT-AC, compositions, and they find a new high temperature transition occurs in all compositions studied. The only phase, denoted by them as MCC (multiple cell cubic), for previously published pT phase diagram of a modified sufficiently large Sn concentrations. It may be possible PbZrO, composition is that given by Berlincourt et that we are inducing the MCC phase with pressure in a[.[121 for a composition which is almost exactly the PSZT 86/14-5-2Nb. However, the transactions are not same as our PZT 95/S-2Nb sample. That diagram does sharp enough to allow a detailed study to be made. not show the A0 phase. For the materials we have The phase diagram of PZT 92/8-2La is shown in Fig. 4. studied, the Ar++Ao boundary is depressed in temperaThis diagram has several distinctive features compared ture for the materials with La and Ba doping. At 23°C the to Figs. 2 and 3. The most remarkable feature is the succession of phases with pressure increasing is Fn, + depression in temperature of the Ao++AT boundary. At AT + A0 for PZT 92/8-2La and Fnz+ FRI + AT + A0 room temperature the pressure induced depoling of an for PZT 94/6-2La-SBa. So far as we know there have initially poled sample is due to the FRI --)AT transition been no studies of the effects of the various transitions in rather than to the FRI + A0 transition as is the case for modified PbZrO, materials on shock-wave propagation PZT 95/S-2Nb and PSZT 84/16-5-2Nb. Another charac- other than those related to the FnI + A0 transition [IS]. teristic is that the region of indifference for the pressureIn summary, we have determined the pT phase induced transitions extends to negative pressures. Thus, diagram for several modified PbZtQ compositions a sample that is pressurized into an antiferroelectric state which have pressure-induced ferroelectric to antiferremains in that state when pressure is released. To bring roelectric transitions near 0.3 GPa. The data indicate the the material into a ferroelectric state after pressure cycl- high sensitivity of the phase behavior to small changes in ing, the samples were heated to above the Curie point composition. In particular, and of possible importance and then cooled at atmospheric pressure. One feature of for applications of these materials, we have shown that Fig. 4 is unusual, and that is the positive slope drawn for the pressure induced ferroelectric to antiferroelectric the PwFR2 boundary. This slope is negative for all the transition may involve either the AT or A0 phase, dependother material studied and probably should be so for ing on composition and temperature. PZT 92/8-2La. Unfortunately we do not have enough data in the narrow pressure region of the PcrFn2 tran- Acknowledgements-We thank R. H. Dungan for supplying the samples of PZT 95/S-2Nb and PSZT %/M-S-2Nb. and P. D. sition to actually determine the slope of the boundary. The curve drawn in Fig. 4 is meant to serve merely as a Wilcox for helpful discussions. The technical assistance of E. M. Edge and V. R. Yarberry is gratefully acknowledged. guide to the eye showing the general trend of the observed data points. A more detailed study would most likely reveal the expected negative slope. -cFS Figure 5 is the phase diagram for PZT W/6-2L.a-5Ba. I. Galasso F. S., Srrucrure, Properties and Preparation of Perooskire-Type Compounds. Pergamon Press. London Compared to the PZT doped only with La, the transition (1%9). temperatures are further depressed. The largest effect is 2. Jatfe ft., Cook W. R. Jr. and Jatfe H.. Piezoelectn’c Ceramics. on the FRI++FRI transition temperature, which is Academic Press, London (1971). lowered by -95” at atmospheric pressure. The La-Ba 3. Scott B. A. and Bums G., 1. Am. Ceram. Sot. 55,331 (1972). doping also increases the slope of this transition 4. Jona F., Shirane G., Maxzi F. and Pepinsky R.. Phys. Rev. to!%849 (1957). compared to PZT 95/5-2Nb. For PZT 94/6-2La-SBa this
Pressure-temperature phase diagrams for several modified lead zirconate ceramics 5. Fesenko 0. E. and Smotrakov V. G., Ferwtectrics
12,211
(1976). 6. Sawaguchi E., 1. Phys. Sac. 3apan 8,615 (1953). 7. Shirane G. and Takeda A.. I. Pk. Sac. Japan 7, 5 ft952); Shiraw G., Suzuki K. and Takeda A.. 3. Phis. Sac. Japan 7, I2 (1952). 8. Bamctt H. M., J. Appl. Phys. 33, 1606(1962).
9. Michel C., Moreau J.-M., Achenbach G. D., Gcrson R. and James W. 1.. Salid Stare Commun. 7,865 (1%9). 10. Shine G. and Hoshino S., Acra Crysr. 7,203 (1954). 11. Krainih N. N., Sou. Phys.-Tech. Phys. 3,493 (1958).
1167
12. Berlincourt D., Krueger H. H. A. and Jaffe B.. 1. Phys. Chem. Solids Zs. 659 (1964). 13. Berlincourt D., Ja!ie H., Kroeger H. H. A. and Jaffe B., Appl. Phys. lkt!. 3,90 (1963). 14. Neiison F. W., Bull. Am. Phys. Sac. 2,302 (1957). 15. Lysne P. C. and Percival C. hi.. 1. Appl. Phys. 46, 1519
(1975). 16. Berlincourt D. A. and Krueger H. H. A.. Annual Ptvgress Rep., Sandia Cutp., P.O. Sf-96B9A(1963). 17. Gerson R. and Jaffc. H., 1. Phys. Chem. Solids 24,979 (1963). 18. Haertling G. H. and Land C. E., Fenvelectrics 3,269 (1972).
Ckm.
Sdi& Vd. 39. pp. I16L1161 Press Lid.. 1978. Ptinkd in Geal
0022-369717811101-1
I63!$02.00/0
Brilain
PRESSURE-TEMPERATURE PHASE DIAGRAMS FOR SEVERAL MODIFIED LEAD ZIRCONATE CERAMICS? I. J. FRITZ and J. D. KECK Sandia Laboratories.Albuquerque, NM lI7l IS. U.S.A. (Received 9 January
1978; accepted 1 April 1978)
Aisstract-Using dielectric constant and loss measurementswe have determined the pressure-temperaturephase diagramsof four ferroelectric ceramics based on lead zirconate (PbZrOs). The materials chosenfor study all have pressure-inducedfenoelectric to antiferroelectric transitions at -0.3 GPa. In all four materials, five phases were identilkd by comparing our results with studies, previously published by other workers, of the phase behavior of modifiedPbZrO, materialsas a function of compositionand temperature.The observedphasesare the ideal cubic perovskite paraelectric phase, two rhombohedral ferroelectric phases, and two antiferroelectric phases, one orthorhombic and one pseudotetragonal. Comparison is made with previous temperaturecomposition studies of modified PbZrQ materials.
1. WrRoDucTloN
known and important example is the lead zirconate-lead
which crystallize in perovskite-type structures are among the most interesting and extensively studied of solid state mater&[ I]. Much of the fundamental interest in these materials has centered around the variety of structures and of displacive structural phase transitions they exhibit. From example, transitions involving relative displacements of certaio sublattices and others involving essentially rigid rotations of the comer linked octahedra (of oxygen, fluorine, chlorine, etc.) can both be found in many of these substances. Perovskite materials are also of technological importance, as shown, e.g. by the widespread use of piezoelectric ceramics based on barium titanate and on lead zirconate titanate [2]. The materials of interest in the present investigation are mixed oxide perovskites based on lead zirconate (PbZrO3. L.ead zirconate has been extensively studied. A recent paper by Burns and Scott [3] lists references for much of the early work in this area. At temperatures above -230°C PbZr03 has the paraelectric ideal cubic perovskite structure, which we will denote hereafter as P. Below -230°C and over a narrow temperature range of -12-26°C Burns and Scott find that the structure is rhombohedral and ferroelectric. denoted here as FR2.At lower temperatures PbZrOs is in the so-called orthorhombic antifetmclectric phase[4], which we denote as & Structural considerations indicate that A0 is actually a polar phase, but attempts at measuring the polarization have not been successful[3,5]. A fourth structural phase is observed in many mod&d PbZrOx compositions, and it has a free energy nearly equal to those of & and FRI. tetragonal This phase is called generally antiferroelectric[6], although it may only be pseudotetragonal. This phase is referred to as AT in this paper. Several investigations have been made into the properties of compositionally modified PbZrOa. A wellSubstances
Work supported by the U.S. Department of Energy, under Contract No. AT(2%1)789. IPCS Vd. 39 No. IL-B
or PZT system [2,6,7], the properties of which were tirst investigated by Shirane et al. [7]. Studies of the temperature-composition phase diagram of the PZT system have shown that the substitution of Ti for Zr in PbZrOx leads to a stabilization of the AT phase (over a narrow range of composition and temperature) and of two rhombohedral ferroelectric phases (at higher Ti concentrations). The ferroelectric phases are the Fn, phase mentioned above and a phase denoted as Fn, which is related to Fn, through a phase transformation involving a unit cell doubling(8,9]. The effects of various other dopants on PbZtQ have also been studied. For example, it is known that the rhombohedral ferroelectric phases can be stabilized by the substitution of Ba for Pb[lO] or of Nb for Zr[ll], whereas antiferroelectricity is favored by the substitution of La for Pb[2] or of Sn for Zr[ I I]. It has been known for some time that the app#cation of hydrostatic pressure to modified PbZrO, tehds to favor antiferroelectricity over ferroelectricity. Pressureinduced ferroelectric to antiferroelectric phase transitions have been described by Berlincourt et aL[l2,13] for PZT materials with -5% Ti and also for Sn doped compositions. These pressure-induced transitions’ are of technological importance, as they are the basis for the operation of shock-wave actuated pulsed power supplies(l4, IS]. Despite the importance of tiessure effects in modified PbZrOa materials, little work has been done in characterizing the properties of various compositions as a function of temperature pressure. Berlincourt cl al. [ 121 published a pressure-temperature phase diagram for a PZT 95/S composition, but, as we will show below, this diagram appears to be incomplete, as the AC,phase was not observed. Phase diagrams for several other compositions have been given in unpub lished reports [ 161. In this paper we present results of pressure-temperature phase studies done on four mod&d PbZrOs ceramics. Compositions were chosen whose ferroelectric titanate
1163
I. J. FRITZ and J. D. KECK
1164
to antiferroelectric transitions occur at moderate pressures (-0.3GPa). The various phase transitions were detected by dielectric constant and dielectric loss measurements made on unpoled samples.
2.ExPERIMENTAL
The compositions of the four materials studied and the techniques used for preparing the ceramic specimens are as follows. The first material is a 95/S PZT composition modified with -2% Nb to improve the electrical characteristics [ 171.This material will be denoted as PZT formula chemical is its 95/5-2Nb; Pbo.99Nbo.02(Zro.9~T~.~~)~.~O~. The second material is the same as the first, with 14% of the Zr replaced by Sn. This chemical formula has the material thus Pbo.99Nbo.oz[(Zro.86Sno.14)o.9~Tio.oslo.9803. We denote this composition as PSZT 86/14-5-2Nb, following Berlincourt et al. [ 161.The third material is a La doped PZT with the chemical formula Pbo.aLao.02(Zro.9zTi.~)Os which we denote as PZT 92/8-2La. The fourth material is doped with both La and Ba. It has the chemical formula Pbo.92Bao.osLao.02(Zro.94Tio.os)Os and is denoted herein as PZT 94/6-2La-5Ba. The first two samples (PZT 95/5-2Nb and PSZT 86/W 5-2Nb) were prepared by milling the oxides, calcining at 1WC in a rotary calciner, dry pressing and atmospheric sintering at 1345°C for 6hr. The other two materials (PZT 92/8-2La and PZT 94/6-2La-5Ba) were chemically prepared by coprecipitating ZrGl from zirconium tetran-butoxide and Ti02 from titanium tetra-n-butoxide as discussed by Haertiing and Land[l8]. Lead was added as lead oxide, lanthanum was added as lanthenum acetate, and barium was added as barium acetate. Mixing was done with alumina balls in a polyethylene mill jar. The powder was calcined at 950°C for 6 hr and hot pressed at 1280°C and 2000 psi for 1 hr. Test specimens were sliced from the various slugs with a diamond wheel saw and plated with fired silver electrodes. Dielectric constant and loss data were obtained by means of an automatic capacitance bridge (HewlettPackard model 4270A) in conjunction with an automated data acquisition system (Hewlett-Packard model 3050A). Most measurements were made at a frequency of 1 kHz on sample plates with nominal dimensions of 0.5 x 5 x 5 mm3. Hydrostatic pressure was generated by a standard Bridgman press utilizing a 1: 1 mixture of n-pentane and i-pentane as the pressure medium, and the magnitude of the pressure was measured to 1% accuracy by a calibrated manganin gauge. The samples were heated by a coil inside the pressure vessel and temperature was monitored by a chromel-alumel thermocouple mounted next to the sample. 3. RESULTS Some typical data scans, obtained on the PZT 95/S2Nb sample, are shown in Fig. 1. In this figure the dielectric constant l is plotted as a function of pressure for three different temperatures. Data are shown for increasing pressure only. At 23°C there is a drop in c
1 100
I ‘. 6.
.-__. 4~
\ .-.-.-...
2-
“\
23?
-L> +.
‘-.-.
--2.%__._
0 0
0.3 PRESSURE
0.6 (Gh)
0.9
Fig. I. Typical dielectric constant vs pressure data for PZT 95/S2Nb.
near 0.3 GPa. It is known from separate dielectric charge loss measurements on poled specimens that this is the pressure at which a transition from a ferroelectric to a nonferroelectric state occurs. At 125°C two transitions are induced with pressure, one at -0.4GPa and the other at -0.95 GPa. At 180°C the material is near the Curie temperature, which is -230°C at atmospheric pressure and decreases with increasing pressure. This causes the fairly large value of e and its initial increase with pressure shown by the upper curve in Fig. 1. At - 0.4 GPa a transition to a nonferroelectric state occurs, causing the sharp drop in l above - 0.4 GPa shown by the data. Data scans similar to those shown in Fig. 1 and including points taken as a function of decreasing pressure were obtained for all four samples studied. Scans were also taken as a function of temperature at nearly constant pressure (actually, at constant ram force, so that the pressure varied slightly). Using the results from ah these scans it is possible to trace out the boundaries between the various phases in the pressure-temperature (pT) plane. As a final step it is necessary to actually identify the various phases. There are two ways in which these identifications can be easily made without recourse to X-ray or neutron diffraction data. The first is to realize that there are certain systematics always observed in modified PbZrGx materials at zero pressure. Thus, ferroelectric compositions have the sequence of phases P + F&-) FR, with decreasing temperature, and antiferroelectric compositions have the sequence P + AT + A0 or P --, A0 with decreasing temperature. Furthermore we know that pressure favors the antiferroelectric phases over the ferroelectric ones. The second method of identifying the various phases is by the characteristic shapes of the dielectric anomalies at the various transitions. These characteristic shapes do not vary drastically from one material to another. For example the FRI-FR2 transition is accompanied by an anomaly in the dielectric loss but there is no anomaly in E at this transition[l2]. The A=c+Ao transition is broad, somewhat sluggish, and has a large hysteresis [ 161.
Pressure-temperaturephase diagrams for several modified lead zirconate ceramics The two methods just discussed of identifying the phases give consistent results for the four materials we investigated. The resulting phase diagrams are given in Pigs. 2-5 and will be discussed in htm. Fiie 2 is the phase diagram for the PZT 95/S-2Nb sample. Solid lines are used to indicate boundaries obtained with increasing pressure, whereas dashed lines indicate the reverse transformation. Considerable hysteresis exists for all transitions except those involving the paraelecbic (P) phase. The transitions P-FRZ and P-A, do exhibit hysteresis regions of a few degrees, but no attempt was made to plot these. In Fig. 2 different plotting symbols are used for the different transitions, and these are listed in the caption. A notable feature of PZT 95/S-2Nb is that the boundaries among the four phases FR,, Fn2, AT and A0 merge in the region 0.30.4GPa and 50-100°C. Because of the hysteresis in the transitions, and because the transitions are spread out
250,
1165
0.8
1.2
1.6
PfESSURtlCRl
Fig. 4. Pressure-tern-e
phase diagram for PZT 92/&2L.a
Symbolssame as for Fig. 1.
I
250 PZI W6
L
-2La -58a
’
'-Jlal--+-s i'
P
PrifSSURE
IGPal
0
2. Pressure-temperature phase diagram for PZT 95/S2Nb. The transitions associated with the various plotting symbols are as follows: 0, F,t,++&; +, F,I,-AT; 0, F,t,++F,n;I, FR+AT; 0. Fnp+P; 0, P-AT; A, AT-&
PSZT 85114- 5 - 2Nb
I 0.4
0.8
1.2
1.6
PlifSSURfIGRI
Fu. 3. Pressure-tempemture
2Nb. Symbok
phase diagram for PSZT 86/14-S same as for Fii. I.
e-
__/-
0.4
0.8
1.2
_A--
1.6
PrEssulEIGPal Fig. 5.
Pressure-temperahue phase diagram for PZT 94/&2La5Ba Symbols same av for Fig. I.
over a range of pressure or temperature in the ceramic samples, it is difficult to determine the boundaries in the region where the four phases merge. The data points indicate the observed transitions, and the lines drawn in the Qture indicate the general directions of the phase boundaries. However, the detailed behavior in the region where the phases meet cannot be determined by the techniques used in this study. The diagram of Fig. 2 illustrates the general features observed for all four materials we have studied. At the highest temperatures the P phase is stable. With decreasing temperatures (at low pressure) the material transforms successively into FR2 and Fn,. The FRI++FR2 boundary increases in temperature with increasing pressure with a slope of -%deg/GPa. At higher pressures AT and A0 are the stable low temperature phases. The boundaries between the ferroekctric and antiferroekctric phases are extremely steep. This steepness is an. important factor in the utility of devices based on these materials, as such devices are often required to operate
1166
I.
J. Frurz and 1. D. KECK
over a wide range of temperatures. Comparing Figs. I and 2 we see that the transitions observed in Fig. I are FR, + A0 at 23°C. FR2+ AT -+A0 at 125°Cand Fn2+ AT at 180°C.
slope is - 250 deg/GPa compared to - 90 deg/GPa for PZT 95/S-2Nb. 4. DISCUSSION AND CONCLUSION
Figure 3 is the phase diagram for PSZT 86/M-5-2Nb. Work done in the past(2] on modified PbZrOl materiWe may think of this material as being derived from PZT als as a function of composition and temperature has clearly shown the sensitivity of the stability of the 95/5-2 Nb by replacing 14% of the Zr with Sn. A comparivarious phases to small changes in composition. The son of Figs. 2 and 3 shows that the phase diagrams of the two materials are similar, with four phases merging in a materials we have studied were chosen to have pressuresmall region of the pTplane for both compositions. The Sn induced ferroelectric to antiferroelectric transitions at doping has two major effects: it lowers the temperatures -0.3GPa at 23°C. Our results show that even within of most of the transitions by -4tPC, and it changes the such a limited class of materials the phase behavior is sign of the slope of the Fn2++AT boundary. Another strongly dependent on composition, as far as both the point to note in Fig. 3 is the scatter in the data points locations of the phase boundaries and their slopes are along the P-AT boundary. The dielectric anomaly at concerned. this transition becomes extremely small and spread out A comparison of Figs. 2-5 shows that the phase at high pressure for all four materials studied, so there is diagrams for the four modified PbZrOp compounds we always some uncertainty in determining the transition have studied are similar in that the phases appear in the points. However, the highest pressure point on the same order as pressure or temperature is varied. The P-AT boundary of Fig. 3 is out of line with the rest of observed order of the phases is similar to that observed the curve by an amount that seems to be outside of in temperature-composition studies of various modified experimental uncertainty. Berlincourt et of.[12] have in- PbZrOX ceramics. vestigated the phase behavior of Sn doped PZT An important result of our work is that the AT-AC, compositions, and they find a new high temperature transition occurs in all compositions studied. The only phase, denoted by them as MCC (multiple cell cubic), for previously published pT phase diagram of a modified sufficiently large Sn concentrations. It may be possible PbZrO, composition is that given by Berlincourt et that we are inducing the MCC phase with pressure in a[.[121 for a composition which is almost exactly the PSZT 86/14-5-2Nb. However, the transactions are not same as our PZT 95/S-2Nb sample. That diagram does sharp enough to allow a detailed study to be made. not show the A0 phase. For the materials we have The phase diagram of PZT 92/8-2La is shown in Fig. 4. studied, the Ar++Ao boundary is depressed in temperaThis diagram has several distinctive features compared ture for the materials with La and Ba doping. At 23°C the to Figs. 2 and 3. The most remarkable feature is the succession of phases with pressure increasing is Fn, + depression in temperature of the Ao++AT boundary. At AT + A0 for PZT 92/8-2La and Fnz+ FRI + AT + A0 room temperature the pressure induced depoling of an for PZT 94/6-2La-SBa. So far as we know there have initially poled sample is due to the FRI --)AT transition been no studies of the effects of the various transitions in rather than to the FRI + A0 transition as is the case for modified PbZrO, materials on shock-wave propagation PZT 95/S-2Nb and PSZT 84/16-5-2Nb. Another charac- other than those related to the FnI + A0 transition [IS]. teristic is that the region of indifference for the pressureIn summary, we have determined the pT phase induced transitions extends to negative pressures. Thus, diagram for several modified PbZtQ compositions a sample that is pressurized into an antiferroelectric state which have pressure-induced ferroelectric to antiferremains in that state when pressure is released. To bring roelectric transitions near 0.3 GPa. The data indicate the the material into a ferroelectric state after pressure cycl- high sensitivity of the phase behavior to small changes in ing, the samples were heated to above the Curie point composition. In particular, and of possible importance and then cooled at atmospheric pressure. One feature of for applications of these materials, we have shown that Fig. 4 is unusual, and that is the positive slope drawn for the pressure induced ferroelectric to antiferroelectric the PwFR2 boundary. This slope is negative for all the transition may involve either the AT or A0 phase, dependother material studied and probably should be so for ing on composition and temperature. PZT 92/8-2La. Unfortunately we do not have enough data in the narrow pressure region of the PcrFn2 tran- Acknowledgements-We thank R. H. Dungan for supplying the samples of PZT 95/S-2Nb and PSZT %/M-S-2Nb. and P. D. sition to actually determine the slope of the boundary. The curve drawn in Fig. 4 is meant to serve merely as a Wilcox for helpful discussions. The technical assistance of E. M. Edge and V. R. Yarberry is gratefully acknowledged. guide to the eye showing the general trend of the observed data points. A more detailed study would most likely reveal the expected negative slope. -cFS Figure 5 is the phase diagram for PZT W/6-2L.a-5Ba. I. Galasso F. S., Srrucrure, Properties and Preparation of Perooskire-Type Compounds. Pergamon Press. London Compared to the PZT doped only with La, the transition (1%9). temperatures are further depressed. The largest effect is 2. Jatfe ft., Cook W. R. Jr. and Jatfe H.. Piezoelectn’c Ceramics. on the FRI++FRI transition temperature, which is Academic Press, London (1971). lowered by -95” at atmospheric pressure. The La-Ba 3. Scott B. A. and Bums G., 1. Am. Ceram. Sot. 55,331 (1972). doping also increases the slope of this transition 4. Jona F., Shirane G., Maxzi F. and Pepinsky R.. Phys. Rev. to!%849 (1957). compared to PZT 95/5-2Nb. For PZT 94/6-2La-SBa this
Pressure-temperature phase diagrams for several modified lead zirconate ceramics 5. Fesenko 0. E. and Smotrakov V. G., Ferwtectrics
12,211
(1976). 6. Sawaguchi E., 1. Phys. Sac. 3apan 8,615 (1953). 7. Shirane G. and Takeda A.. I. Pk. Sac. Japan 7, 5 ft952); Shiraw G., Suzuki K. and Takeda A.. 3. Phis. Sac. Japan 7, I2 (1952). 8. Bamctt H. M., J. Appl. Phys. 33, 1606(1962).
9. Michel C., Moreau J.-M., Achenbach G. D., Gcrson R. and James W. 1.. Salid Stare Commun. 7,865 (1%9). 10. Shine G. and Hoshino S., Acra Crysr. 7,203 (1954). 11. Krainih N. N., Sou. Phys.-Tech. Phys. 3,493 (1958).
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12. Berlincourt D., Krueger H. H. A. and Jaffe B.. 1. Phys. Chem. Solids Zs. 659 (1964). 13. Berlincourt D., Ja!ie H., Kroeger H. H. A. and Jaffe B., Appl. Phys. lkt!. 3,90 (1963). 14. Neiison F. W., Bull. Am. Phys. Sac. 2,302 (1957). 15. Lysne P. C. and Percival C. hi.. 1. Appl. Phys. 46, 1519
(1975). 16. Berlincourt D. A. and Krueger H. H. A.. Annual Ptvgress Rep., Sandia Cutp., P.O. Sf-96B9A(1963). 17. Gerson R. and Jaffc. H., 1. Phys. Chem. Solids 24,979 (1963). 18. Haertling G. H. and Land C. E., Fenvelectrics 3,269 (1972).