Jan 30, 2018 - Jo, S. Schaab, E. Sapper, L. A. Schmitt, H.-J. Kleebe, A. J. Bell, and J. Rödel, J. Appl. Phys. 110, 074106 (2011). 36F. Craciun, C. Galassi, and ...
Non-trivial behavior of the low temperature maximum of dielectric constant and location of the end critical point in Na0.5Bi0.5TiO3-0.06BaTiO3 lead free relaxor ferroelectrics crystals detected by acoustic emission Evgeniy Dul'kin, Jenia Tiagunova, Evgeny Mojaev, and Michael Roth
Citation: Journal of Applied Physics 123, 044103 (2018); View online: https://doi.org/10.1063/1.5009326 View Table of Contents: http://aip.scitation.org/toc/jap/123/4 Published by the American Institute of Physics
JOURNAL OF APPLIED PHYSICS 123, 044103 (2018)
Non-trivial behavior of the low temperature maximum of dielectric constant and location of the end critical point in Na0.5Bi0.5TiO3-0.06BaTiO3 lead free relaxor ferroelectrics crystals detected by acoustic emission Evgeniy Dul’kin, Jenia Tiagunova, Evgeny Mojaev, and Michael Roth Department of Applied Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
(Received 15 October 2017; accepted 14 January 2018; published online 30 January 2018) [001] lead free relaxor ferroelectrics crystals of Na0.5Bi0.5TiO3–0.06BaTiO3 were studied by means of dielectric and acoustic emission methods in the temperature range of 25–240 C and under a dc bias electric field up to 0.4 kV/cm. A temperature maximum of the dielectric constant was found near 170 C, as well as the acoustic emission bursts pointed out to both the depolarization temperature near 120 C and the temperature, corresponding to the maximum of dielectric constant, near 170 C. While the depolarization temperature increased linearly, the temperature of the dielectric constant maximum was shown to exhibit a V-shape behavior under an electric field: it initially decreases, reaches a sharp minimum at some small threshold electric field of 0.15 kV/cm, and then starts to increase similar to the Curie temperature of the normal ferroelectrics, as the field enhances. Acoustic emission bursts, accompanying the depolarization temperature, weakened with the enhancing field, whereas the ones accompanying the temperature of the dielectric constant maximum exhibited two maxima: near 0.1 kV/cm and near 0.3 kV/cm. The meaning of these two acoustic emission maxima is discussed. Published by AIP Publishing. https://doi.org/10.1063/1.5009326
I. INTRODUCTION
Pb-based relaxor ferroelectrics (RFEs) of Pb(B0 B00 )O3 perovskite-type attract great interest due to their fundamental intrinsic chemical chaos on the B-sites in contrast to the ordered ferroelectrics (FE),1 resulting in the appearance of both disordered and ordered regions, giving rise to quenched random electric fields due to the difference in both charges and radii between B-site cations. In contrast to them, the Pb-free RFEs, such as BaTiO3-based ones, result in the appearance of both disordered and ordered regions either on A- or B- or on both A- and B-sites, giving rise to the quenched random electric fields.2 The arising random fields are known to be a key moment in understanding the outstanding properties of RFEs.3 Coupling between the random fields and FE degrees of freedom creates polar nanoregions (PNRs), consisting of self-organizing A-site anions in Pbbased RFEs1 and B-site cations in Pb-free ones,2 chaotically dispersed within a cubic nonpolar paraelectric phase, breaking the long-range FE ordering, preventing the FE phase transition and are recognized as the cause of all the outstanding properties of RFEs.4 For example, the main characteristic property of RFEs, caused by the presence of PNRs, is a smeared maximum of their dielectric constant, E0 , at some temperature, Tm, which is known to be frequency dependent: namely, Tm shifts to higher temperatures with increasing measuring frequency.1,2 PNRs, being the reorientable dipoles, interact among themselves as well as with the random fields, leading to the smearing of the maximum of E0 at Tm.5 Upon cooling, the PNRs become pinned by random fields and freeze at freezing temperature, Tf, into the nonergodic state, similar to some kind of a glass-like phase.6 0021-8979/2018/123(4)/044103/5/$30.00
Also, another characteristic property of RFEs, caused by the presence of PNRs, too, is now widely accepted: a dependence of the Tm on a small dc external bias electric field, E, the non-trivial behavior or the so-called V-shape. The V-shape effect is observed in Pb-based RFEs, such as PbMg1/3Nb2/3O3–0.1PbTiO3 ceramics,7 PbMg1/3Nb2/3O3–x PbTiO3 (0 x 0.35) crystals,8,9 PbScl/2Tal/2O3 crystals,10 PbScl/2Tal/2O3 doped with La and Ba crystals,11 PbSc0.5 Nb0.5O3 doped with La, Ba and Sr,12 PbFe0.5Ta0.5O3 ceramics,13 and crystals,14 Pb(Mg1/3Ta2/3)O3 crystals,15 as well as in BaTiO3-based RFEs of Ba0.6Sr0.4TiO3.16 It is well documented that the Tm initially decreases, reaches a minimum at some small threshold bias electric field, Eth, and then starts to increase similarly as Tc for the second-order phase transitions in the ordered FEs, as E increases. The origin of such nonlinear behavior of Tm is concluded to be a competition between both the random fields and E when affecting the PNRs.8–11 Necessary to stress, applying E induces a line of critical end points (CPs) of the liquid-vapor type where the paraelectric to FE transitions terminate in the compositiontemperature x-T phase diagram in Pb-based RFEs such PbMg1/3Nb2/3O3 as PbMg1/3Nb2/3O3–0.295PbTiO3,17 –0.13PbTiO3,18 and pure PbMg1/3Nb2/3O3,19 and, more interesting, the maxima values of piezomodulus have been measured in the vicinity of this line. Such a huge piezomodulus in Pb-based RFEs was proven to be due to easy polarization from the rhombohedral phase to the tetragonal phase through the intermediate monoclinic phases MA, MB, or MC under E.17,19 Later, the CP was detected using a pyroelectric response in PbMg1/3Nb2/3O3–0.2PbTiO3,20 and an acoustic
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emission (AE) in both Pb-based of PbMg1/3Nb2/3 O3–0.33PbTiO3,9 and Pb(Mg1/3Ta2/3)O3,15 and Pb-free of Ba0.60Sr0.40TiO3,16 RFEs. Note that a justification for application of AE to detect the CP is caused by an approximately proportional correspondence between the strains and its count rate.21–23 Indeed, all the CPs, determined in PbMg1/3 Nb2/3O3–xPbTiO3 by a maximum of piezomodulus,17–19 pyrocurrent,20 and AE,9 are localized around the Tm(E) dependence minimum, near the Eth. Along with the Pb-free RFE of Ba0.60Sr0.40TiO3, the RFE of Na0.5Bi0.5TiO3 and its solid solution with BaTiO3, namely, Na0.5Bi0.5TiO3–0.06BaTiO3 (NBT-6BT) is wellknown to be a good candidate for many applications due to its great piezomodulus.24 However, unlike all the above mentioned RFEs, NBT contains a mixture of different phases, leading to several phase transitions in a wide temperature range,25–27 and so NBT-xBT compounds possess two temperature maxima of the dielectric constant instead of one. Indeed, in NBT-6BT, E0 exhibits two maxima: the lowtemperature one, referred to as a “hump” or local maximum, occurs at Tlm within approximately 120–130 C and the hightemperature one occurs at Tm within approximately 150–320 C.28–44 While the Tlm exhibits a strong frequency dispersion,28–44 characteristic for RFEs,1,2 Tm exhibits an absence of dispersion,28–34,37–43 as the presence of slight dispersion.35,36,44 The absence of characteristic strong dispersion of Tm in the major part of NBT-6BT, studied in both crystals and ceramics, manifests that this Tm does not correspond to one of the Pb-based RFEs, but is presumably shown to be caused by the high-temperature P4bm $ P4/mbm phase transition,44 whereas this slight dispersion is clearly provided by the interaction between the P4bm PNRs.35 Meanwhile, in NBT-6BT, previously it was commonly accepted that the frequency dispersive dielectric “hump” at Tlm corresponds to the depolarization temperature, Td,32,41,42 above which the piezomodulus sharply falls down.30 However, later it was noted that this Tlm lies on some tens of degrees higher than Td,28,30,33,34,44 and finally it was emphasized that the Tlm is not the Td.34 Traditionally it was thought that the FE $ antiferroelectric (AFE) phase transition took place at Td for NBT-5BT at least.45 However a scrupulous observation of the domain structure of NBT-6BT ceramics shown that the two-phase mixture (complex domains) with volumes of rhombohedral R3c FE domains (100 nm) embedded in the matrix of tetragonal P4bm AFE nanodomains (
