Doping and substitution methods are predominantly employed in the synthesis of ceramics to achieve their desired functional properties. We studied the behavior of excessive dopants in addition to an existing stoichiometric composition using a high-throughput continuous compositional spread sputtering method. We paid attention to the possible formation of thermodynamically unstable phases by the addition of an excessive amount of dopants. We showed that even when dopants were added as an additive, they dissolved into the existing lattice due to the benefit of the entropy of mixing. Regardless of excessiveness, all added elements incorporated into the lattice, stabilized by the tolerance factor. We also demonstrated our findings exemplarily with lead iron niobate to induce magnetic properties alongside inherent ferroelectricity (M_S = 10 emu/cm³, P_S = 16 μC/cm²). We compare the results from CCS with those from the non-additive solid-state method, leading to a conclusion that the benefit from the entropy of mixing allows foreign elements to substitute for the elements initially residing in the lattice to a degree in compliance with the Goldschmidt tolerance factor. This observation was confirmed by a density functional theory calculation. We anticipate that our study could necessitate intensive research on achieving desired composition through industry-friendly processing.

Relaxor-PbTiO₃ ferroelectric single crystals have drawn attention aiming at high-end piezoelectric applications thanks to their excellent piezoelectric properties. Like all the other ferroelectrics, relaxor-PbTiO₃ single crystals can only be piezoelectrically active upon being electrically poled. However, this poled state is thermally unstable, limiting their uses because of their relatively low depolarization temperature. Here, we show that a non-destructible permanent poled state can be realized in relaxor-PbTiO₃ single crystals by forming a 0–3 composite in the presence of charged mobile point defects. We demonstrate this on solid-state grown 0.71 Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃ single crystals doped with Mn (Mn-PMNT) as a donor with well-aligned and dispersed boron-rich MgO-based inclusions (MBIs). Mn-PMNT_MBI sharing [001] axis with arrayed MBIs were spontaneously polarized during cooling across the Curie temperature without an external electric field. The piezoelectric coefficient and dielectric permittivity of self-poled Mn-PMNT_MBI crystals were as large as 90% of that achieved by a direct-current poling treatment at room temperature, and such poled state was reproducible against repeated thermal cycles. We expect that the poling-free high-performance piezoelectric relaxor-PbTiO₃ single crystals offer an avenue for piezoelectric-based devices by removing the working temperature limit as one of the inherent fundamental limitations.

Piezoelectricity offers an electromechanical coupling that is widely utilized in transducer applications. There has been a consistent demand for transparent piezoelectric materials for optoelectrical applications. Therefore, despite the inherent tradeoff between the transparency and the piezoelectricity, numerous strategies have been explored to develop the transparent piezoelectric materials. Nonetheless, the most transparent piezoelectric materials developed to date is either a single crystal or materials that achieve transparency via hot-press sintering, limiting its industrial applicability. Therefore, we introduce a novel piezoelectric material that ensures transparency through co-doping and pressureless sintering of polycrystalline ceramics. In this study, we employed a compositional optimization approach to enhance the synergistic effect between the transparency and the piezoelectric properties of 0.71Pb(Mg_1/3Nb_2/3)O₃–0.29PbTiO₃ (PMN–0.29PT) ceramics. By utilizing the tape casting process for mass production and large-area manufacturing, our Pb_0.913La_0.0145Sm_0.0145(Mg_1/3Nb_2/3)_0.71Ti_0.29O₃ (TP2.9) ceramics exhibited over 60% transparency and large piezoelectric coefficient (d₃₃) of 1104 pC/N. This material holds considerable promise for a wide range of industrial applications in both the optical and electronic domains.

Among the unresolved issues in the study of relaxor ferroelectrics is the role of freezing temperature, across which the dynamics of polarization reversal in relaxor ferroelectrics changes. The presence of this freezing temperature is best manifested by the appearance of a double polarization hysteresis loop just above the freezing temperature. Given that the polarization pinching evolving into a double hysteresis starts well below the freezing temperature, there exists a transient temperature regime between the nonergodic and the ergodic relaxor states. To clarify the role of the freezing temperature on the pinching, the polarization reversal near the freezing temperature of relaxor (Pb_1-xLa_x)(Zr_1-yT_y)_1-x/4O₃ (PLZT) was monitored using three in situ electric field methods: electrocaloric effect, neutron diffraction, and transmission electron microscopy. We demonstrate that the pinching results from a two-step process, 1) domain detexturization in the ferroelectric state and 2) miniaturization of domains. This observation explains the recently reported gap between the depolarization temperature T_d and the ferroelectric-to-relaxor transition temperature T_F-R in lead-free relaxors. We further show that T_d and T_F-R, which have long been considered identical in lead-based relaxors, are not the same. The current study suggests that the mismatch between T_d and T_F-R is an inherent feature in both lead-based and lead-free relaxor ferroelectrics.