Unusual soft mode dynamics in ferroelectric PbTiO3 nanowire under different mechanical boundary conditions

Abstract

First-principles-based atomistic simulations are used to investigate equilibrium phases and soft mode dynamics in ultrathin ferroelectric PbTiO3 nanowire with poor surface charge compensation subjected to a wide range of mechanical boundary conditions. The presence of the depolarizing field along the nanowire's transverse directions leads to the appearance of a unique high-frequency hard phonon mode that can be used to characterize electrical boundary conditions. This mode is insensitive to the mechanical load. Hydrostatic pressure was found to significantly influence the Curie point and ferroelectric soft modes in the nanowire. Uniaxial stress applied either along axial or transverse nanowire's direction is capable of inducing polydomain flux-closure phases that have a unique “dynamical” fingerprint. In such phases, the modes that originate from the soft modes of bulk PbTiO3 become hard, which could open a way to potential identification of such nanodomain phases. In all cases, uniaxial stress significantly increases the Curie temperature.

All-Mechanical Polarization Control and Anomalous (Electro)Mechanical Responses in Ferroelectric Nanowires

“…In fact, our computational results suggest that at the point of the phase switching the discontinuous changes in the polarization and strain are likely to lead to the divergence of both coefficients. Note that the same FC phase was reported as the equilibrium phase of the nanowire under the uniaxial compression …”

“…Piezoelectric and ferroelectric nanowires often exhibit properties that are not accessible in bulk. Some examples include anomalous coupling of light into the ferroelectric nanowire, enhanced Curie temperature, hardening of the soft modes, enhanced energy conversion and storage, unusual phase transitions, phases, and morphotropic phase boundary …”

“…At the same time, the possibility to induce polar to nonpolar vortex-like phase switching in nanowires by application of external stress has been so far overlooked. In ref it was predicted that PbTiO 3 nanowires with poor surface charge compensation develop a flux-closure (FC) phase (or vortex-like phase) in the presence of uniaxial compressive stress, while stress-free nanowires exhibit a polar (P) phase with axial polarization direction, which points to the potential of such nanowires to switch phases. The aims of this work are (i) to predict the possibility to reversibly switch between the P and nonpolar FC phases in PbTiO 3 nanowires by application of uniaxial compression; (ii) to report the anomalous electromechanical and mechanical responses associated with such stress-driven transformation; (iii) to explore the high-frequency dynamics of this stress-driven phase switching; and (iv) to discuss the potential applications of this phenomenon in nanogenerators and other nanodevices.…”

“…14 Piezoelectric and ferroelectric nanowires often exhibit properties that are not accessible in bulk. Some examples include anomalous coupling of light into the ferroelectric nanowire, 15 enhanced Curie temperature, 16 hardening of the soft modes, 17 enhanced energy conversion and storage, 18 unusual phase transitions, 19 phases, 20 and morphotropic phase boundary. 21 Interestingly, the unusual morphotropic phase boundary in Pb(Zr 0.5 Ti 0.5 )O 3 nanowires was predicted to exhibit outstanding values of piezoelectric coefficient d 31 (up to 4900 pC/ N) and of dielectric susceptibility (up to 40 000).…”

“…From annealing simulations we found that under chosen electrical boundary conditions the nanowire develops the polarization along the axial direction below the computational Curie temperature of 540 K which is in agreement with the previous studies. 17,28,34 The direction of polarization is confined to the axial direction in this case due to a large depolarizing field associated with other polarization directions. We begin by considering evolution of the axial polarization and strain under quasi-static application of the uniaxial stress.…”