Reemergence of a Polar Instability at High Pressure in KNbO_{3}

Ferroelectric instabilities in perovskites are known to be suppressed by a moderate hydrostatic pressure. The prediction of their re-entrance in a much higher pressure regime is well accepted theoretically, but a conclusive experimental confirmation is still missing. Here, we show its occurrence in a classical but comparatively underlooked ferroelectric perovskite KNbO_{3}. 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 polar cation displacements emerge locally but have to combine with tilts of the oxygen octahedra that are also developing under pressure. This results in a macroscopically centrosymmetric phase with an incommensurate modulation that reflects the tight competition between the polar and tilt instabilities. Soft modes associated with the tilts and the modulation, along with persistent order-disorder signatures, are clearly observed, suggesting that local disorder plays an important role in mediating this competition.