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

Abstract

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.

Keywords:
Relaxor ferroelectrics; Curie temperature; polar nanoregions; random electric fields; acoustic emission

1. 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 saturation¹.

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 Pb²⁺ ions off-centered shift, PNRs are initially pinned to random electric fields, RFs, in turn arising due to difference in B-sites ions charges². 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 E¹.

An appearance of V-shape is well documented in pure canonical RFE of PMN and its non-canonical solution with FE of PbTiO₃, PMN-xPT². 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-xPT¹,³,⁴. While T_m dependence exhibits a V-shape, T_c exhibits a quasi-linear dependence on E³. The T_c(E) quasi-linear 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-shape³, 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-0.30PT crystals 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-xLa_x(Zr_yTi_1-y)_1-x/4O₃ with x=0.09 and y=0.65, denoted as 9/65/35 PLZT, ceramics⁶. 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.

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).

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.

2. Experimental Details

The 9/65/35 PLZT ceramic samples were prepared via the conventional solid-state route, according to the formula Pb_1-xLa_x(Zr_yTi_1-y)_1-x/4O₃, which provides charge-compensating vacancies (for the Pb²⁺/La³⁺ substitution) in the Zr/Ti sublattice. The specific composition was Pb_0.91La_0.09(Zr_0.65Ti_0.35)_0.9775Δ_0.0225O₃, where Δ represents B-site vacancies. Analytical-grade starting oxide powders were thoroughly mixed by ball-milling and calcined at 850^oC for 4h, then sintered in two stages: at 1200 ^oC for 2h and at 1300^oC, while packed with PbZrO₃+5 wt% excess ZrO₂ 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% 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).

Figure 2
The XRD diffraction spectra (a), and cross-section SEM image (b) of 9/65/35 PLZT ceramics.

Samples with sizes of 4x4x0.45 mm³ 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 elsewhere⁷. 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, $\dot{N}$ (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.

3. 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 6·10³ at T_m in good agreement with that measured in Ref⁸. The sharp bursts of $\dot{N}$ of AE point out the value of T_m ≈ 82.6°C precisely enough in good agreement with that measured in Ref⁸, 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 $\dot{N}$ similar to that detected in canonical RFEs Pb(Mg_1/3Ta_2/3)O₃⁹.

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

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 Ref⁶. 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-0.30PT⁵.The E_th is found to be 1.15 kV/cm, that is approximately the same as in both PMN-0.24PT¹ and PFN-0.02PT¹⁰. Because the E_th is equivalent to RFs in the RFEs, it is means that the 0.09 mol. of La³⁺ ions in 9/65/35 PLZT creates the same strength of RFs that the 0.24 mol. of Ti⁴⁺ ions in PMN-0.24PT and the 0.02 mol. of Ti⁴⁺ ions in PFN-0.02PT (Table 1).

Table 1
Comparison of the Eths in PMN-xPT and x/65/35 PLZT relaxor ferroelectrics.

Compound	x	Substituted site	Eth (kV/cm)	Source
PMN-xPT	0.24	B	1.12	[1]
PFN-xPT	0.02	B	~ 1	[10]
x/65/35 PLZT	0.09	A	1.15	Present

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 ions¹¹. 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 Ti⁴⁺ ions substitute the Nb⁵⁺ 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 Ti⁴⁺ in both PMN-xPT and PFN-xPT is obviously caused by difference in valences of Mg²⁺ versus Fe³⁺ in their B'-sites, respectively.

In x/65/35 PLZT, La³⁺ ions substitute Pb²⁺ 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 Pb²⁺ ions for La³⁺ ions is accompanied by arising of both A- and B-sites charge-compensating vacancies¹², which are another source of RFs that enhances the RFs summary strengths additionally. Also an incorporation of La³⁺, possessing the isotropic outermost electron shell, disturbs the stereochemically active lone-pair electron shell of Pb²⁺ that leads to arising the corresponding electrostrictive strains similar to those detected using in situ Raman spectroscopy in Pb_1−xLa_xSc_(1+x)/2Ta_(1−x)/2O₃ relaxor ferroelectric crystals¹³, 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 Pb²⁺ ions for La³⁺ ions in 9/65/35 PLZT creates the same strength of RFs that the substitution of Nb⁵⁺ ions for Ti⁴⁺ ions creates in both PMN-0.24PT and PFN-0.02PT due to three mechanisms existing in x/65/35 PLZT instead of single mechanism in both PMN-xPT and PFN-xPT compounds.

4. 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 La³⁺ ions in 9/65/35 PLZT creates the same strength of random electric fields that the 0.24 and 0.02 mol. of Ti⁴⁺ ions creates in both PMN-0.24PT and PFN-0.02PT due to three mechanisms existing in 9/65/35 PLZT instead of single mechanism in both PMN-0.24PT and PFN-0.02PT.

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Publication Dates

• Publication in this collection
  14 Apr 2023
• Date of issue
  2023

History

• Received
  07 Feb 2022
• Reviewed
  14 July 2022
• Accepted
  01 Aug 2022