A Geometric Pathway for Tuning Ferroelectric Properties via Polar State Reconfiguration

Abstract

We report the discovery of a geometric pathway for tuning ferroelectric properties through thermally driven reconfiguration between coexisting polar states in Li-substituted NaNbO3. Using first-principles density functional theory calculation and 7Li solid-state nuclear magnetic resonance spectroscopy measurement, we reveal that Li substitution creates two distinct polar configurations whose transformation under annealing enhances the Curie temperature and induces piezoelectric hardening. Our findings establish a geometrically-driven polar state reconfiguration mechanism, providing a general design principle for ferroics whereby macroscopic functional properties can be engineered via lattice geometry.

Figures (5)

• Figure 1: (a) DFT-simulated ferroelectric phase of pure NaNbO$_3$ with $Pmc2_1$ symmetry. Relaxed structures when Li cation occupies the (b) Na(1) or (c) Na(2) positions of NaNbO$_3$. (d) Interpolated structural variation of the Li-substituted NaNbO$_3$ between non-polar and polar states. (e) NEB-simulated transformation between two polar states. (f) NEB-simulated polarization switching in different polar states.
• Figure 2: Illustration of the geometric constraint from octahedral tilting to the displacements of A-site cations. “Acute” and “obtuse” are defined according to the angle between two tilting BO$_6$ octahedra.
• Figure 3: Inverse octahedral tilting during the transformation from the Li@Na(1) polar state to the Li@Na(2) polar state. The “t.p.” is the abbreviation of transition path, obtained from the NEB simulation in Fig. 1(e).
• Figure 4: Temperature-dependent (a) dielectric permittivity $\varepsilon_r$, (b) loss $\tan\delta$, (c) piezoelectric constant $d_{33}$, and (d) mechanical quality factor $Q_m$ of Li-substituted NaNbO$_3$ ceramic samples before and after annealing. $^{7}$Li NMR spectra of (e) unpoled and unannealed powder sample, (f) ceramic sample poled and annealed for 8 hours, and (g) ceramic sample poled and annealed for 16 hours.
• Figure 5: $^7$Li MAS NMR full spectra of (a) unpoled and unannealed powder sample, (b) ceramic sample poled and annealed for 8 hours, and (c) ceramic sample poled and annealed for 16 hours. Grey dashed lines are drawn above the fitting curves of Li@Na(1) and Li@Na(2) to guide the eye.