Morphotropic Relaxor Boundary in a Relaxor System Showing Enhancement of Electrostrain and Dielectric Permittivity

In ferroelectric and relaxor-ferroelectric materials, piezoelectric and dielectric properties are significantly enhanced at the morphotropic phase boundary (MPB), a boundary between different ferroelectric phases with different macroscopic symmetries. By contrast, in relaxor systems, such an MPB does not exist because relaxors of different compositions possess the same macroscopic symmetry. Here, we report the existence of a morphotropic relaxor boundary (MRB) in the single phase relaxor region of a ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}{\mathrm{NbO}}_3\text{−}x{\mathrm{BaTiO}}_3$ system, which is a composition-induced boundary between two relaxors with different local polar symmetries (tetragonal versus rhombohedral) but with the same macroscopic cubic symmetry. At the MRB the electrostrain increases by $\sim 3$ times and the permittivity increases by $\sim 1.5$ times over a wide temperature range of more than 100 K, as compared with off-MRB compositions. Our Letter demonstrates that the MRB may become an effective mechanism to enhance the dielectric and electrostrictive properties of relaxors.

Figure 1

(a) Morphotropic relaxor boundary in the relaxor region of $\mathrm{KNN}\text{−}x\mathrm{BT}$ ($0.2<x<0.9$). The MRB region (i.e., the coexistence of tetragonal and rhombohedral local symmetries regions or PNRs) is determined by composition-dependent Raman spectra (red open dots), CBED, HREM images and temperature-dependent Raman spectra (red solid dots) in Figs. 2, 3, 4 and Supplemental Material Fig. S7. (Insets) PNRs of the two relaxors with local tetragonal symmetry ($T$ relaxor) and local rhombohedral symmetry ($R$ relaxor), respectively; the arrow represents the local polarization ($\mathit{P}$) in each type of PNR. (b) Determination of Burns temperature $T_B$ line and freezing temperature $T_f$ line in the phase diagram by frequency dispersion of dielectric permittivity versus temperature curves for various relaxor compositions $x=0.2$, 0.45, 0.5, and 0.9. Freezing temperature $T_f$ is determined by the Vogel-Fulcher equation, and the Burns temperature $T_B$ is determined by the start of deviation from the Curie-Weiss law, respectively.

Figure 2

Composition-dependent Raman spectra from (a) 150 to $350{\mathrm{cm}}^{-1}$ and (b) 450 to $700{\mathrm{cm}}^{-1}$. ${\mathrm{FE}}_T$, ${\mathrm{RE}}_T$, ${\mathrm{RE}}_R$, and ${\mathrm{FE}}_R$ represent the ferroelectric phase with $T$ symmetry, relaxor with local $T$ symmetry, relaxor with local $R$ symmetry, and ferroelectric phase with $R$ symmetry, respectively.

Figure 3

Local probes of CBED and HREM provide direct evidence for a composition-induced local symmetry boundary (i.e., the MRB) in the KNN-BT relaxor system. (a) Tetragonal local symmetry at $x=0.30$, (b) a mixture of tetragonal and rhombohedral local symmetry at $x=0.45$, (c) rhombohedral local symmetry at $x=0.60$. The experiment was carried out with [001] incidence beam at room temperature. (Inset) The FFT of the selected area in the HREM image. CBED patterns demonstrate the fourfold symmetry of $T$ structure (P4mm) and twofold symmetry of $R$ structure (R3m) along [001] direction.

Figure 4

Temperature-dependent Raman spectra for the MRB composition ($x=0.45$).

Figure 5

Property enhancement of the MRB composition in comparison with the off-MRB compositions. (a)–(c) Composition dependence of small-field dielectric (at ${10}^4\mathrm{Hz}$), high-field dielectric, and electrostriction properties at room temperature (298 K). The dielectric permittivity at $50\mathrm{kV}/\mathrm{cm}$ (b) is calculated through the slope of the $P\text{−}E$ loop (insets). (d)–(f) Temperature dependence of small-field dielectric, high-field dielectric, and electrostriction properties of the MRB and off-MRB composition ($x=0.5$).