An underdog story: Re-emergence of a polar instability at high pressure in KNbO3

TL;DR

This study provides experimental evidence of a high-pressure polar instability in the lead-free perovskite KNbO3, revealing a re-emergence of ferroelectric behavior through incommensurate structural modulations up to 63 GPa.

Contribution

It demonstrates the reappearance of ferroelectric instability at high pressure in KNbO3, supported by multiple spectroscopic techniques and structural analysis.

Findings

Incommensurate modulation indicates polar instability at high pressure

Soft modes associated with tilts and modulations are observed

Persistent order-disorder signatures confirm the instability

Abstract

Ferroelectricity in perovskites is known to be suppressed by a moderate hydrostatic pressure. The notion that a polar instability should reappear in a higher pressure regime is well accepted theoretically but experiments have failed so far to provide a conclusive evidence for it. Here, we investigate a classical but comparatively underlooked ferroelectric perovskite KNbO3. We use single crystal X-ray diffraction, infrared and Raman spectroscopy and second-harmonic generation to explore the phase transition sequence at high pressures up to 63 GPa. We show that the ferroelectric instability manifests itself in the emergence of an incommensurate modulation of the perovskite structure that combines cation displacements and tilts of the oxygen octahedra. Soft modes associated to the tilts and the modulation are clearly observed along with persistent order-disorder signatures. This demonstrates the presence of the high-pressure polar instability in a lead-free perovskite in spite of the centrosymmetric character of all observed high-pressure phases.