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arXiv · 2508.06399

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

Mohamad Baker Shoker·Sitaram Ramakrishnan·Boris Croes·Olivier Cregut·Nicolas Beyer·Kokou Dorkenoo·Pierre Rodière·Björn Wehinger·Gaston Garbarino·Mohamed Mezouar·Marine Verseils·Pierre Fertey·Salia Cherifi-Hertel·Pierre Bouvier·Mael Guennou
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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.

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Mohamad Baker Shoker, Sitaram Ramakrishnan, Boris Croes, Olivier Cregut, Nicolas Beyer, Kokou Dorkenoo, Pierre Rodière, Björn Wehinger, Gaston Garbarino, Mohamed Mezouar, Marine Verseils, Pierre Fertey, Salia Cherifi-Hertel, Pierre Bouvier, Mael Guennou. 2025-08-08. An underdog story: Re-emergence of a polar instability at high pressure in KNbO3. https://doi.org/10.1103/vwp9-tr7b

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