Non-trivial Behavior of Temperature of Dielectric Constant Maximum in (Pb/La)(Zr/Ti)O3 9/65/35 Relaxor Ferroelectric Ceramics Detected by Acoustic Emission

9/65/35 PLZT relaxor was studied under a bias electric field using the acoustic emission method. It was established that the temperature of smeared dielectric constant maximum exhibits the V-shape, lying fully within the ergodic phase, as well as that the threshold electric field is found to be approximately the same as in both PMN-0.24PT and PFN-0.02PT relaxors. A reason of the latter phenomena is discussed from the viewpoint of incorporated ions properties.


Introduction
During last twenty years the non-trivial behavior of temperature of smeared dielectric constant, ε′, maximum, T m , induced by dc bias electric field, E, was recognized to be an intrinsic feature of the relaxor ferroelectrics, RFEs, like PMN, along with its frequency dependence.This non-trivial behavior of T m , or V-shape effect, is that the T m remains practically the same or even decreases as E enhances up to a certain threshold E th , above which T m increases and gradually tends to saturation 1 .
The polar nanoregions, PNRs, characteristic for RFEs and absent in ferroelectrics, FEs, are the reason of the V-shape effect.For example in PMN, while arising due to Pb 2+ ions off-centered shift, PNRs are initially pinned to random electric fields, RFs, in turn arising due to difference in B-sites ions charges 2 .When applying E the PNRs start to switch along E causing the decrease of T m .As E attains E th , all the PNRs orient along E and T m becomes minimum.As E further enhances the RFEs behave as the FEs and the T m increases similar to Curie temperature, T c , in dependence on E 1 .
An appearance of V-shape is well documented in pure canonical RFE of PMN and its non-canonical solution with FE of PbTiO 3 , PMN-xPT 2 .On cooling the PNRs cause maximum of ε′ in T m and then freeze in glass phase below freezing temperature, T f , transforming the RFE from ergodic phase, ER, to non-ergodic, NER, one.When applying E the PMN remains in NER phase until the E enhances up to critical value, E c , and then transforms to FE phase, T f transforms to T c and PNRs transform to macroscopic domains.
The behavior of both T m and T c in dependence on E, T -E phase diagram, was studied well enough in both PMN and PMN-xPT 1,3,4 .While T m dependence exhibits a V-shape, T c exhibits a quasi-linear dependence on E 3 .The T c (E) quasilinear curve separates the FE phase from the ER one, whereas the T m (E) V-shape lies within the ER one when E<E th .Both T m (E) and T c (E) dependences merge near E th in a single line separating the FE and ER phases when E>E th .It is need to stress that the piezomodule maximum occurs in the vicinity of V-shape 3 , that might be used in applications.Meanwhile, recently it was shown that the T m (E) V-shape fully lies within the ER phase in 0.70PMN-0.30PTcrystals when applying the acoustic emission (AE) 5 .This is due to sharp bursts of AE in comparison with smeared dielectric maximum.
Besides, the T-E phase diagram was thoroughly studied in Pb 1-x La x (Zr y Ti 1-y ) 1-x/4 O 3 with x=0.09 and y=0.65, denoted as 9/65/35 PLZT, ceramics 6 .This compound is similar to pure canonical PMN one: the NER phase exists up to E c ≈4.8 kV/cm, above which the FE phase forms (Figure 1).The T c (E) quasi-linear curve separates the FE phase from the ER one, too, but the T m (E) V-shape was not studied at all.
The goal of the present work is to study the dependence T m (E) to establish whether the T m V-shape exists in 9/65/35 PLZT due to its importance for applications and, if yes, to compare it with those observed in other RFEs.

Experimental Details
The 9/65/35 PLZT ceramic samples were prepared via the conventional solid-state route, according to the formula Pb 1-x La x (Zr y Ti 1-y ) 1-x/4 O 3 , which provides charge-compensating vacancies (for the Pb 2+ /La 3+ substitution) in the Zr/Ti sublattice.The specific composition was Pb 0.91 La 0.09 (Zr 0.65 Ti 0.35 ) 0.9775 Δ 0.0225 O 3 , where Δ represents B-site vacancies.Analytical-grade starting oxide powders were thoroughly mixed by ball-milling and calcined at 850 o C for 4h, then sintered in two stages: at 1200 o C for 2h and at 1300 o C, while packed with PbZrO 3 +5 wt% excess ZrO 2 powder in alumina crucibles in order to maintain a constant PbO activity at the sintering temperature.The process was optimized to achieve samples with high density, high purity phase and good crystallization.The density was 97% *e-mail: evgeniy.dulkin@mail.huji.ac.il of the nominal value.A pure perovskite cubic structure was evidenced from x-ray diffraction spectra, with the unit cell size of 4.093 Å. Cross-section scanning electron microscopy evidenced a rather uniform microstructure with an average crystallite size of about 3.3 μm (Figure 2).
Samples with sizes of 4x4x0.45mm 3 cut from the same batch and annealed at 750°C for 30 min were used in the present work.
The AE technique is described in basic details elsewhere 7 .A ceramic plate sample with silver contacts is pasted with a silver epoxy to the polished side of a fused silica acoustic rod waveguide.A PZT-19 disk piezoelectric sensor is attached to the rear end of the waveguide.The sensor is electrically coupled to a 500 kHz band-pass low noise variable (up to 40 dB) preamplifier connected to a detector-amplifier (40 dB).A chromel-alumel thermocouple junction is glued to the waveguide near the sample by a special thermoconductor paste.The higher part of the acoustic waveguide with the pasted sample is mounted from below in a resistance element tube furnace.
The dielectric data are measured using a HP4263A LCR meter wired to the sample.The thermocouple, amplifier, and LCR meter outputs are interfaced with a PC for coupled readout.The measurement of both real part ε′ of the dielectric constant and the AE count rate, N  (s -1 ), are performed with the frequency of 10 kHz in the temperature range 50-125°C with a rate of about 1-3°C/min.When measuring the AE under a bias electric field E, a high voltage supply is wired to the sample along with ac electric field of 1 V in amplitude and the data are recorded by a fixed steps of E up to 5 kV/cm during the thermal cycling.
Before each cycle the sample is annealed at temperature of 125°C for 15 min and then measurements of both dielectric and AE data during cooling and heating under bias electric field, E (field heating after field cooling regime (FH-FC), are performed.

Result and Discussion
Figure 3 presents the plot of real part of dielectric constant, ε′, in dependence on temperature, T, in absence of bias electric field, E. ε′ exhibits a smeared maximum, characteristic for RFEs, and attains the value of about 6•10 3 at T m in good agreement with that measured in Ref 8 .The sharp bursts of N  of AE point out the value of T m ≈ 82.6°C precisely enough in good agreement with that measured in Ref 8 , too, using the dielectric method.Because the 9/65/35 PLZT belongs to canonical RFEs, no thermal hysteresis is detected in T m due to absence of any phase transition because ε′ maximum is caused by the interactions between the PNRs only, causing the sharp bursts of N  similar to that detected in canonical RFEs Pb(Mg 1/3 Ta 2/3 )O 3 9. Figure 1 presents the T-E phase diagram of behavior of T m in 9/65/35 PLZT as well as the behavior of T c in the same compound reconstructed from Ref 6 .It is clearly seen that the T m , detected by AE, in dependence on E exhibits a V-shape, as it would be expected, lying fully within ER phrase as previously observed in PMN-0.30PT 5 .The E th is found to be 1.15 kV/cm, that is approximately the same as in both PMN-0.24PT 1 and PFN-0.02PT 10 .Because the E th is equivalent to RFs in the RFEs, it is means that the 0.09 mol. of La 3+ ions in 9/65/35 PLZT creates the same strength of  RFs that the 0.24 mol. of Ti 4+ ions in PMN-0.24PT and the 0.02 mol. of Ti 4+ ions in PFN-0.02PT(Table 1).
It is well known that in some RFEs an incorporation of the dopants essentially influences on RFs strength due to their differences in valences in comparison with hosting ions 11 .Let us now consider the mechanisms of RFs creation in both x/65/35 PLZT and PMN-xPT and PFN-xPT compounds.In both PMN-xPT and PFN-xPT the Ti 4+ ions substitute the Nb 5+ ions in B"-sites due to practical equality of their radii (r Ti =0.605Å, r Nb =0.64Å), that weakens the RFs strengths.No other mechanisms exist to influence the RFs strengths in both PMN-xPT and PFN-xPT when substituting the B-sites ions.The equality of RFs strengths created by 0.24 and 0.02 mol. of Ti 4+ in both PMN-xPT and PFN-xPT is obviously caused by difference in valences of Mg 2+ versus Fe 3+ in their B'-sites, respectively.
In x/65/35 PLZT, La 3+ ions substitute Pb 2+ ions in A-sites due to approximate equality of their radii (r La =1.36Å, r Pb =1.49Å), that enhances the RFs strengths.On the other hand, substitution of Pb 2+ ions for La 3+ ions is accompanied by arising of both A-and B-sites charge-compensating vacancies 12 , which are another source of RFs that enhances the RFs summary strengths additionally.Also an incorporation of La 3+ , possessing the isotropic outermost electron shell, disturbs the stereochemically active lone-pair electron shell of Pb 2+ that leads to arising the corresponding electrostrictive strains similar to those detected using in situ Raman spectroscopy in Pb 1−x La x Sc (1+x)/2 Ta (1−x)/2 O 3 relaxor ferroelectric crystals 13 , which in turn create the weak RFs.So there are three mechanisms which influence the RFs strengths in x/65/35 PLZT.
Thus, we have concluded that the substitution of Pb 2+ ions for La 3+ ions in 9/65/35 PLZT creates the same strength of RFs that the substitution of Nb 5+ ions for Ti 4+ ions creates in

Conclusion
In summary, we have studied the 9/65/35 PLZT ceramics under a bias electric field using the acoustic emission method.We have plotted the T-E phase diagram and established that the temperature of smeared dielectric constant maximum exhibits the V-shape, lying fully within the ergodic phase.We have measured the threshold field to be 1.15 kV/cm that is approximately the same that in both PMN-0.24PT and PFN-0.02PT.We have concluded that the 0.09 mol. of La 3+ ions in 9/65/35 PLZT creates the same strength of random electric fields that the 0.24 and 0.02 mol. of Ti 4+ ions creates in both PMN-0.24PT and PFN-0.02PTdue to three mechanisms existing in 9/65/35 PLZT instead of single mechanism in both PMN-0.24PT and PFN-0.02PT.

Figure 1 .
Figure 1.A plot of the T-E phase diagram of 9/65/35 PLZT ceramics: constructed from Ref. 6 (lower curve) and detected in the present work (higher curve).

Figure 3 .
Figure 3.A plot of the real part ε′ of the dielectric constant and the AE count rate, N  (s -1 ), of 9/65/35 PLZT ceramics.

Table 1 .
Comparison of the Eths in PMN-xPT and x/65/35 PLZT relaxor ferroelectrics.PMN-0.24PT and PFN-0.02PTdue to three mechanisms existing in x/65/35 PLZT instead of single mechanism in both PMN-xPT and PFN-xPT compounds. both