Temperature dependent ferroelectricity in strained KTaO3 with machine learned force field

TL;DR

This work investigates whether in-plane strain can stabilize ferroelectric order in the quantum paraelectric KTaO$_3$, and how quantum fluctuations compete with strain-induced polarization. The authors combine density functional theory (DFT) with the stochastic self-consistent harmonic approximation (SSCHA) and on-the-fly machine-learned force fields (MLFF) to capture anharmonic and nuclear quantum effects under in-plane strain up to 1% and temperatures up to 300 K. They find that uniaxial and biaxial strains break inversion symmetry and stabilize FE phases with maximal polarizations of about $0.233$ C/m$^2$ (uniaxial) and $0.293$ C/m$^2$ (biaxial), while the soft mode is stabilized by anharmonicity; the zero-temperature energy barrier under 1% strain is around $3.1$ meV/atom, comparable to BaTiO$_3$. Berry phase calculations map FE–PE phase diagrams as a function of strain and temperature, and the results indicate that quantum fluctuations can be overcome by strain to sustain FE order. The workflow demonstrates a scalable route to predict strain-engineered ferroelectricity in incipient ferroelectrics and may guide experimental efforts.

Abstract

Ferroelectric materials are a class of dielectrics that exhibit spontaneous polarization which can be reversed under an external electric field. The emergence of ferroelectric order in incipient ferroelectrics is a topic of considerable interest from both fundamental and applied perspectives. Among the various strategies explored, strain engineering has been proven to be a powerful method for tuning ferroelectric polarization in materials. In the case of KTaO3, first principles calculations have suggested that strain can drive a ferroelectric phase transition. In this study, we investigate the impact of in-plane uniaxial and biaxial strain, ranging from 0% to 1%, on pristine KTaO3 to explore its potential for ferroelectricity induction via inversion symmetry breaking. By integrating density functional theory calculations with the stochastic self-consistent harmonic approximation assisted by on the fly machine learned force field, we obtain accurate structural information and dynamical properties under varying strain conditions while incorporating higher-order anharmonic effects. Employing the Berry phase method, we obtained the ferroelectric polarization of the strained structures over the entire temperature range up to 300 K. Our findings provide valuable insights into the role of strain in stabilizing ferroelectricity in KTaO3, offering guidance for future experimental and theoretical studies on strain-engineered ferroelectric materials.

Figures (6)

• Figure 1: Qualitative phase diagram of the crossover among the ferroelectric (FE), paraelectric (PE) and quantum paraelectric (QPE) phases as the function of quantum tuning parameter and temperature
• Figure 2: Geometric structure of (a) pristine KTaO3(b) uniaxial strained KTaO3 and (c) biaxial strained KTaO3
• Figure 3: Variation of bond length O1-Ta for KTaO3 under uniaxial strain and temperatures, only data points corresponding to strained structures that exhibit distortion are included.
• Figure 4: Phonon spectrum of (a) 1% uniaxial strained KTaO3 and (b) 1% biaxial strained KTaO3 at 0 K without (black dash line) and with (red line) anharmonic effects.
• Figure 5: (a) Phase diagram of KTaO3 under uniaxial strain and at different temperatures and (b) Variation of soft mode in ferroelectric phase for 1% uniaxial strained KTaO3 across the temperature range of 0-300 K.