Divergent electrostriction at ferroelectric phase transitions: example of strain-induced ferroelectiricty in KTaO3

Finite temperature dielectric properties of KTaO$_3$ from first principles and machine learning: Phonon spectra, Barrett law, strain engineering and electrostriction

“…We demonstrate the advantages of this methodology on the case of KTaO 3 close to the quantum critical point, showing that it captures the saturation of the soft mode and dielectric constant following Barret's law. We then explore the stability of the paraelectric state under changes of the volume due to isotropic strain as well as epitaxial strain, both at 0 K and 300 K. Recently, the potentially giant electrostriction in this material has caught a renewed attention [24,25]. We confirm these previous reports and further calculate the temperature dependence of the electrostrictive coefficients.…”

“…We find that the stability range of the paraelectric phase at 0 K is indeed extremely narrow, spanning from around -0.15 % compressive to around 0.1% tensile strain (See Fig 5a and b). This is slightly smaller than the values for the stability range reported from ground state DFT calculations, where Tyunina et al report 2 % [38] using LDA and Tanner et al [24] 0.4% with PBEsol. This is however consistent with the fact that we obtain a slightly larger lattice parameter and slightly lower frequencies compared to these works.…”

“…In the previous section we have shown that our approach of QSCAILD combined with machine-learnt interatomic potentials is able to quite accurately reproduce the quantum behavior of the soft mode frequency and of the dielectric constant at ambient pressure and low temperatures, despite some discrepancies due to the high sensitivity of the dielectric constant close to the quantum critical point. We will now explore the pressure-and temperature-dependent behavior of the dielectric properties around this critical point, towards the transition to a ferroelectric phase, which can be introduced by both by the application of pressure [37] and epitaxial strain [13,24,38].…”

“…It allows to control both polarization and anisotropy axes in thin film ferroelectrics [41]. In KTaO 3 , epitaxial strain has been predicted from first principles calculations to induce a ferroelectric phase transition [24,38], however these studies were done without considering the influence of temperature. The application of in-plane strain breaks the cubic symmetry of bulk KTaO 3 , and thus leads to a splitting of the three-fold degenerate soft mode branches at Γ, to a doubly degenerate branch in-plane with in-plane polarizations, and a branch with out-of-plane polarization.…”

“…The experimental reports of electrostrictive properties in literature in the past have mostly been relying on the Q-tensor, which was found to be relatively constant in the vicinity of ferroelectric phase transitions [37,42]. However, recently Tanner et al [24] have pointed out that the electrostrictive tensor relevant for mechanical applications is actually M rather than Q -the tensor that links the strain to the applied electric field. This tensor diverges at the ferroelectric phase transitions since it is proportional to the square of the dielectric susceptibility χ kk = kk − 1:…”