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Study of the Electric Field-Induced Low Temperature Phase in Pb(Mg1/3Nb2/3)O3: Titanium Influence
O. Bidault, E. Husson, P. Gaucher
To cite this version:
O. Bidault, E. Husson, P. Gaucher. Study of the Electric Field-Induced Low Temperature Phase in Pb(Mg1/3Nb2/3)O3: Titanium Influence. Journal de Physique III, EDP Sciences, 1997, 7 (6), pp.1163-1172. �10.1051/jp3:1997182�. �jpa-00249639�
J. Phys. III Fkance 7 (1997) 1163-1172 JUNE 1997, PAGE 1163
Study of the Electric Field-Induced Low Temperature Phase in
Pb(Mgi/3Nb2/3)03: Titanium Influence
O. Bidault (~>*), E. Husson (~,~>**) and P. Gaucher (~)
(~) LPMM, ESEM, rue Ldonard de Vinci, 45072 Orldans Cedex 02, France (~) CRPHT-CNRS, 45071 Orldans Cedex 02, France
(~) Thomson CSF-LCR, Domame de Corbeville, 91404 Orsay Cedex, France
(Received I July 1996, revised 26 September 1996, accepted 13 Noi~ember 1996)
PACS 77 80.-e Ferroelectricity and antiferroelectricity PACS.77.22.Ej Polarization and depolarization PACS 64.60.-I General studies of phase transitions
Abstract. The temperature dependence of the dielectric permittivity under a dc electric
field is studied in ii z)Pb(Mgi/3Nb2/3)O~-~PbTi03 ceramics (0 < z < 01) The size of the ordered regions, which is the key parameter to understand the dielectric response of such
materials, is suggested to be reduced by Ti doping. A ferroelectric transition can be induced by
a field E from the average cubic phase. Whereas PMN is a relaxor undergoing a macroscopic
phase transition only if E > 4 kV cm~~, materials with z
= 0.1 exhibit a spontaneous cubic
to rhombohedral transition at 280 K on cooling. Moreover, m all the studied samples, the Ti temperature deduced from the Vogel-Fulcher relation is found to be very close to the poling temperature under high field.
R4sum4. La variation en tempdrature de la constante didlectrique de cdramiques de com-
position 11 ~)Pb(Mgi/3Nb2/3)O~-zPbTi03 (0 < ~ < 0,1) est dtudi#e alors qu'un champ dc est appliqud h l'dchantillon. La taille des rdgious ordonndes, paramktre-dd pour comprendre la
rdponse du matdriau, est rdduite par l'introduction de titane Une transition de phase ferrodlec- trique peut 4tre induite sous champ h partir de la phase cubique en moyenne. Si PMN est un
relaxeur, ne transitant qu'h condition d'avoir E > 4 kV cm~~, la transition cubique -+ rhomboA- drique devient spontande pdur PMN
: 10 il Ti (h 280 K au refroidissement). De plus, pour tous les dchantillons, la tempdrature Ti, ddduite de la relation de Vogel-Fulcher est systdmatiquement
trouvde voisine de celle d'apparition de la phase polaire sous champ.
1. Introduction
The ferroelectric relaxor material lead magnesium niobate, Pb(Mgj /3Nb~/3)03 (or PMN), has been extensively studied due to its peculiar physical properties. The dielectric permittivity
exhibits a rounded and frequency dependent maximum in the vicinity of room temperature (*)Present address: LPUB/Matdriaux pour l'optique, Univ de Bourgogne, BP 400, 21011 Dijon Cedex, France
(**) Author for correspondence
Les #ditions de Physique 1997
without the occurrence of any macroscopic phase transition. Indeed, the PMN mean structure remains cubic down to at least 5 K. In fact, it is now well established that polar nanoregions of rhombohedral symmetry develop upon cooling from about 600 K ill without the building
up of a long range ferroelectric order.
More generally, the complex perovskites Pb(B'~B[_~)03 can be viewed as having compo-
sitional fluctuations on a microscopic scale, because more than one type of ion can occupy the same crystallographic position. As a result, the B-site occupation of the AB03 Perovskite
can be fully random (disordered materials) or characterized by a coherent long-range order, with the intermediate case of nanoscale ordered regions [2]. In some materials with a B-site
ratio of I:I, like Pb(Sci
/~, Tai/2)03 (PST), the degree of order in the B-site can be controlled by suitable thermal annealing [3,4] allowing to modulate the phase transition from a normal ferroelectric (for a fully-ordered sample) to a diffuse one. This establishes a clear link between the electrical properties of the material and the degree of ordering, i.e. the size of the ordered
domains.
In PMN, high resolution electron microscopy images [5-7] evidence also the presence of ordered regions, characterized by a regular one-by-one alternation of the Mg~+ and Nb~+
cations and spread inside a Nb-rich matrix. It is worthwhile to note that in PMN the ordered
regions are electrically (negatively) charged. Thus the nanometer scale ordering seems to result from a balance between electrical and elastic strain energy (due to a great size difference
between the Mg and Nb cations). As a consequence, a long-time annealing has no influence
on the ordered domain size (about 2.5 nm). Meanwhile substitution on cationic sites may induces a change in the degree of ordering. In the case of an unchanged Mg/Nb ratio (1:2),
the compensation for an heterovalent ions doping is linked to the formation of vacancies, so
that the electroneutrality is preserved. On the A (Pb~+ )-site, donor doping with La~+ [6j can compensate for the local charge imbalance and thus enhances the degree of ordering, whereas acceptor doping, like Na+, favours its decrease. On the other hand, an isovalent substitution
can alsb influence ordering. This was recently observed in the (Pbi-~Ba~)(Mgi/3Nb2/3)03
solid solution [8] in which chemical ordering changes regularly up to z = I. Indeed, the Ba- based material exhibits a 1:2 type ordering and achieves a complete long-range order with a
tripled unit cell, in distinction to the doubled unit cell of short-range ordered PMN.
Another example is the solid solution of Pb(Mgi/3Nb2/3)03 and PbTi03 (PMN:PT), re- sulting from a Ti~+ substitution on the B-site according to the chemical formula Pb[(Mgi/~
Nb2/3)1-~Ti~]03 with 0 < z < I. Complete solid solutions between PMN and PT can be formed, with a rhombohedral/tetragonal morphotropic phase boundary near 35 mol i~ PT [9].
In the same time, the cationic distribution changes from partially ordered on a nanomet- ric scale to fully random: distinct superstructure can no more be observed by TEM [2j for
x > 0.35. Moreover, this composition is found to separate relaxor behavior from normal ferro- electric properties [10]. Another Ti content has to be noted: x
= 0.I, which is the minimum value needed for a spontaneous ferroelectric phase transition to take place at low temperature (280 K) [11]. For lower values of a~, the samples keep a mean cubic structure on cooling down to 5 K. Due to the random fields resulting from the composition fluctuations, the polar clusters,
which appear in the Nb-rich matrix, never reach a sufficient degree of growth to induce a strong correlation between neighbouring clusters needed for a long-range ferroelectric order; the ma- terial undergoes a transition from a paraelectric to a nonergodic low-temperature phase [12j.
However, a transition into a macroscopic rhombohedral phase can be induced by applying a
dc field, E, as evidenced in a single PMN crystal by X-ray study [13] or dielectric measure~
ments [14]. Thus the properties of the (I z) PMN-z PT system depend strongly on the Ti
content: without any dc field, the low temperature phase changes in nature by increasing the
z value from glass-like (a~ < 0.I) to ferroelectric (rhombohedral for z < 0.35, then tetragonal).
N°6 Ti~* INFLUENCE ON THE FIELD-INDUCED PHASE IN PMN 1165
09PMN-01
PMN,~)
10000 '$''
, , '
Ill '
,/,/ '
,', '
',' ' '
,j,', '
'$/ 'W
W 1',',
"'
iooo
50 100 150 200 250 300 350 400
T jK)
Fig. 1. Temperature dependence of the real part of the dielectric permittivity of a pure and a 10 mol it Ti doped PMN ceramics on a semi-log scale if =10~, 10~, 10~, 10~, 10~ and 10~ Hz). The
arrow indicates a spontaneous macroscopic phase transition in the Ti doped sample [11]
In the present study, we will investigate a small part of the phase diagram, the one containing
less than 10 mol il Ti, by means of impedance spectroscopy, since the dielectric response is
known to be dependent on the degree of chemical ordering [3,4]. The Ti influence on the permittivity can be seen in Figure I for a~
= 0 (PMN) and a~
= 0.I (data from Ref. [ll]), i.e.
for the two extreme Ti contents of the present work. The substitution induces, as noted many
times, a shift to higher temperatures and an increase of the dielectric maximum whereas the relaxor behavior (especially the frequency dispersion) persists. The weak shoulder observed for a~ = 0.I at 280 K independently of the measuring frequency corresponds to a spontaneous (E = 0 kV cm~~) macroscopic relaxor to ferroelectric phase transition. The purpose of this
investigation is to study the link between the microstructure and the appearance of the field- induced phase in PMN-PT ceramics with low Ti content.
2. Experimental Procedure
The (I -z)Pb(Mgi /3Nb2/3)O~ -zPbTi03 lx = 0, 0.0375, 0 05 and 0,10) samples were prepared
in the form of disks at Thomson- CSF /LCR according to the columbite method, in order to avoid the formation of a pyrochlore phase [15]. The faces of the discs were electroded using gold vapor deposition. The dielectric susceptibility measurements were performed with a Schlumberger
Solartron SI1260 impedance analyzer in a frequency range from 10° Hz up to 10~ Hz. The complex permittivity as a function of temperature and frequency was determined by measuring the capacitance and loss tangent. The sample temperature was monitored between 77 to 400 K with an accuracy better than 0.I K. A dc field (0 < E < 10 kV cm~~) was eventually applied during experiments. In order to complete the data, high frequency measurements
(10~ Hz < f < 10~ Hz) were performed with a HP 4191.
All the measurements were carried out on samples freshly annealed at a sufficiently high
temperature (390 K) to erase all the effects of previous treatments and especially to eliminate possible remanent polar regions. A preliminary heating and cooling cycle was also necessary
to ensure reproducibility. A typical temperature run (rate +I K min~~) started from 390 K.
The sample ~N.as cooled down to 77 K under a dc field (field cooling, FC) or under zero field
(ZFC). If the field is higher than a threshold value, Eth, the sample undergoes a field-induced phase transition at a temperature called T~~. It is dependent both on the E value and on the Ti content, as explained in the next section When the low temperature was reached and before heating, the dc field was either maintained (field heating after field cooling, FH/FC), or
switched off (zero field heating after field cooling, ZFH /FC). In both cases, a depoling process
takes place either near Tpc (for a FH/FC cycle) or at Tdo (ZFH/FC). This last temperature is known to be independent of the initial poling field strength [14]. All the phase transition temperatures are also frequency independent (for more details upon the different possible cycles
and the (E, T) phase diagram in a pure PMN single crystal, see Refs. [12,14,16j). The thermal
depoling currents were also measured as a function of temperature on the ZFH/FC protocol with a Keithley 617 electrometer. The heating rate was fixed at 2 K min~~.
3. Results and Discussion
3.I. PURE PMN CERAMICS. Before discussing the titanium influence on the dielectric response of PMN-based ceramics, the main characteristic aspects of the field-induced phase
will be presented for pure PMN.
During a Zero Field Cooling, dispersion is observed on the low-temperature side of the dielectric peak (Fig. I). The permittivity maximum occurs at Trrax which depends on the measurement frequency f. This dependence can be fitted using the well-known Vogel-Fulcher
law:
f lo ~XP l~j~ ~~ j~ ~~)
max f
where lo, Ea and Tf are adjustable parameters. Usually, following the dipolar glass model [17j, this relationship is considered as a sign of the freezing of the system at Tf, due to the
development of correlation between dipolar moments. In fact, a recent data analysis in the
frequency space [18] reveals the occurrence of two distinct relaxation branches, called LF and HF for the one appearing at lower and the other at higher frequencies at a given temperature.
A typical frequency spectrum is displayed in Figure 2. On these curves, one can distinguish
the e2 asymmetric maximum due to the two relaxational processes fitted using a superposition
of two Cole-Cole type equations:
where em is the high frequency value of the dielectric susceptibility. The he parameter is connected to the relaxation amplitude and a is used to fit the broadening around the most
probable relaxation time T for each relaxational process. A conductivity term (a(T)/w) is also added to improve the quality of the fits, especially at low frequencies. If one excludes any extrinsic (e.g. electrodes) contribution for the HF relaxation [19j, a microscopic model based on correlated chains of off-centered Nb cations may be an explanation On the other
hand, the dispersion observed in Figure I and linked to the relaxor properties is connected with the LF relaxation, which can be detected in the e*(w) spectrum only below Trra~. As expected, the fitted LF relaxation frequency value if = 1/2~Tj is found to slow down regularly
with decreasing temperature until it disappears from our experimental frequency window (see Fig. 3). Its temperature variation can be well described by the Vogel-Fulcher law with an
adjustable Ti temperature of 228 K.
N°6 Ti~+ INFLUENCE ON THE FIELD-INDUCED PHASE IN PMN 1167
Solartron SI 1260 HP 419 lA
2000 w"
isoo
iooo
soo
0 °
10° 10~ 10~ 10~ 10~
f(Hz)
Fig. 2. Imaginary part of the dielectric susceptibility of a PMN ceramic versus frequency in the vicinity of the dielectric maximum (286 K). The lines are the best fit using a Cole-Cole model for the two relaxations and a superposed conductivity term
lo?
E = 9 kV/cm io~
E = 0 kV/cm 10~
Iit
j~3 E = 7.5 kV/cm
io2 228 K
10~
150 200 250 300 350
T (K)
Fig. 3. Temperature dependence of the relaxation frequency of a PMN ceramic determined from a Cole-Cole type equation for ZFC and FC (7.S and 9 kV cm~~). The straight lines are interpolations
to guide the eye in the case of FC. The best fit using the Vogel-Fulcher relationship is given for ZFC.
The application during cooling of a dc field, E, higher than the threshold value
(Eth = 4 kV cm~l) gives rise to a shoulder detected both on ElIT) and e2IT) at Tpc it 225 K for 7.5 and 9 kV cm~l). This dielectric anomaly is connected to the onset of a macroscopic phase of rhombohedral symmetry instead of a glassy one. The e*(w) curves recorded under dc field can still be fitted using Cole-Cole relation [20]. All the deduced f values are reported on a
logarithmic scale as a function of temperature in Figure 3. When a field (E > Eth) is applied,
the f(T) variation no more follows the Vogel-Fulcher relationship but exhibits a marked critical
100
3.75% Ti Tp~
80
,,' [ ~
/ ~
60
uJ~ ,,w~ ~~
~2
FWZFC FWFC
loo 120 140 160 180 200 220
T (K)
Fig. 4. Dielectric response (ei and e2) of a 3,7Sil Ti doped sample recorded during FH, either after FC (solid line) or after ZFH (cross) (E
= 7 S kV cm~~, f
= 63 Hz)
behavior in the vicinity of the phase transition temperature, ~v.here f reaches a minimum value like in a normal ferroelectric. Thus, in pure PMN, critical behavior linked to the field-induced phase transition (relaxor ~ ferroelectric phase) is clearly demonstrated and occurs close to the
temperature Ti deduced from the Vogel-Fulcher analysis.
3.2. Ti DOPED SAMPLES. Figure 4 depicts the dielectric response of a titanium doped
sample recorded under 7.5 kV cm~~
on heating (FH). According as a field has been applied during cooling or not, an additional anomaly can be observed at low temperature, like in a
pure PMN ceramic. A ferroelectic phase can be field-induced below T~c (Fs 245 K) provided E is stronger enough (FC) and will persist on heating (FH/FC) up to approximately the same
temperature. On the other hand, if the field is applied after a field-free cooling (FH/ZFC), a peak appears at Tph (temperature of pooling on heating). Starting from the low temperature
frozen average cubic structure, a ferroelectric phase is then induced. This transition is easily observed because T~h « Trrax. Thus, the Ti doped and the pure samples exhibit qualitatively the same behaviour: a low temperature phase can be field-induced for E > Eth as evidenced in Figure 4.
As recalled in Introduction, the T14+ doping on B site is expected to modify both the chemical order and the dielectric behavior of PMN-based ceramics. Especially the decrease of the size of the ordered regions and the random field they induce favour the appearance of a long range ferroelectric order. Thus, the threshold field strength necessary for a macro- polarization induction should be decreased with titanium introduction. This can be seen in
Figure 5, which displays the dielectric losses verstts temperature for three different Ti contents:
0, 5 and 10il. The shoulder at T~c, corresponding to the transition from a relaxor state into
a ferroelectric one, appears as expected at lower field strengths as x increases (see Tab. I, column 2). Since the anomaly amplitude is weak on ceramics, the most convincing proof for a
phase transition is the depoling process observed during a ZFH, after FC, which gives rise to
a clearly detectable effect. Once established during a FC (E > lEth), the field-induced phase
N°6 Ti~+ INFLUENCE ON THE FIELD-INDUCED PHASE IN PMN 1169
0.12
0.10
(
~~c 0.08
~z
0.06 5%
10%Ti 0.04
0.02 0.00
50 100 150 200 250 300 350 400
T (K)
Fig. S. Dielectric losses (e21ei) measured at 10 kHz as the sample is cooled down under a field
equal to the threshold value- 4 kV cm~~ (PMN), 2 kV cm~~ (PMN: Sit Ti) and 0 kV cm~~ (PMN:
10i~ Ti). The arrows indicate the weak shoulders linked to the field-induced or spontaneous phase
transition.
Table I. Synthesis of the ea~perimental resttlts obtained for the fottr stttdied samples from
the dielectric measttrements: threshold field strength necessary to mdttce the phase transition, depofing f~do)/Poling /l~pc) temperatttres determined dttring ZFC/FC (E = 7.5 kV cnl~~) and the Tf temperatttre dedttced from the Vogel-fiillcher eqttation.
Eth (kV cm~~) Tdo (K) Tpc (K) Tf (K)
(E = 7.5 kV cm~~)
PMN 4.0 + 0.5 185 + 5 225 + 10 227 + 2
3.75% Ti 2.0 + 0.5 220 + 5 245 + 10 245 + 2
5il Ti 2.0 + 0.5 235 + 5 250 + 10 248 + 2
10il Ti 0 300 + 5 295 + 10 294 + 2
persists down to low temperatures after removing the electric field, but disappears on heating
at Tdo in the ZFH/FC cycle due to the decay of the ferroelectric state at that temperature.
Indeed, because of the thermal agitation and the decreasing of the magnitude of the local polar regions, a break-up of the long-range polar regions takes place. The polar clusters are thus able to switch between different orientation states and the rhombohedral symmetry turns from
a macroscopic into a nanometer scale. This transition is evidenced in Figure 6 which disjlays
the tan temperature variation for a 5il Ti doped ceramic. For field strengths applied during cooling lower than 2 kV cm~~, no detectable anomaly can be observed, whereas for 2.5, 4 and 7.5 kV cm~~, a peak due to the depoling process appears at about Tdo = 235 K, independently of the initial poling strength which only affects the anomaly amplitude. A same threshold
value of 2 kV cm~~
can be deduced from depoling current measurements. The dc current I(T)
curves recorded for different E values are shown for the same sample in Figure 7. In all cases,
even for E < Eth, a depoling peak is observed. Only its shape and amplitude change with
