Thermotropic phase boundaries in classic ferroelectrics

Abstract

High-performance piezoelectrics are lead-based solid solutions that exhibit a so-called morphotropic phase boundary, which separates two competing phases as a function of chemical composition; as a consequence, an intermediate low-symmetry phase with a strong piezoelectric effect arises. In search for environmentally sustainable lead-free alternatives that exhibit analogous characteristics, we use a network of competing domains to create similar conditions across thermal inter-ferroelectric transitions in simple, lead-free ferroelectrics such as BaTiO 3 and KNbO 3 . Here we report the experimental observation of thermotropic phase boundaries in these classic ferroelectrics, through direct imaging of low-symmetry intermediate phases that exhibit large enhancements in the existing nonlinear optical and piezoelectric property coefficients. Furthermore, the symmetry lowering in these phases allows for new property coefficients that exceed all the existing coefficients in both parent phases. Discovering the thermotropic nature of thermal phase transitions in simple ferroelectrics thus presents unique opportunities for the design of 'green' high-performance materials.

Composition driven structural instability in perovskite ferroelectrics

“…It then becomes especially important to improve the piezoelectricity by the polarization extension mechanism. In PMNT (x=0.36 or 0.37), the variation of d 33 is much less than that of d 15 . 17 The d * 33 along [001], which is enhanced by the MPB driven extensional instability, remains relatively stable in the temperature range from 300 K to 380 K, whereas the value of d * 33 along [101] direction falls in half.…”

“…4(a)]. As the temperature or composition approaches sufficiently close to the orthorhombic phase, the term containing d 15 becomes dominant, leading to a distortion of the d * 33 surface. The maximum d * 33 develops along a direction other than [001].…”

“…Some more typical examples are the tetragonal PZT with all compositions and orthorhombic Mn-doped (Li,Na,K)(Nb,Ta)O 3 single crystals where the longitudinal piezoelectric coefficient d * 33 exhibits its maximum along polar axis. 10,24 In such cases, the shear d 15 does not dominate and the ease of polarization extension is of particular importance to achieve high piezoelectricity. Another reason we should emphasize is that it is promising to develop piezoelectric materials with both enhanced piezoelectricity and enhanced thermal stability through the MPB driven extensional instability.…”

“…13,14 While MPB systems separating two polar phases are quite common, there are only few known MPBs separating the polar and nonpolar phases at which enhanced functional response has been demonstrated. 14,15 In this work, we compare the structural instability near the polytrophic and morphotropic phase boundaries (PPB and MPB) and reveal that the MPB can drive both rotational and extensional instability in perovskite ferroelectrics. We demonstrate that the polarization extension strength can be enhanced not only at the polar-nonpolar phase boundary but also at the MPB separating two polar phases.…”

“…11,16 Thus in the tetragonal phase, these four parameters, ε 11 , ε 33 , d 15 and d 33 are critical to evaluate the piezoelectric performance. Consider first the tetragonal BaTiO 3 near the PPB, which exhibits spontaneous polarization along [001] direction.…”