Intrinsic ionic screening of the ferroelectric polarization of KTP

TL;DR

This study explores how mobile K+ ions in ferroelectric KTiOPO4 influence its polarization through ionic screening, significantly enhancing nonlinear optical effects like Second Harmonic Generation.

Contribution

It introduces an ionic space charge model explaining polarization screening in ferroelectric KTiOPO4, supported by experimental evidence and comparison with non-ionic ferroelectrics.

Findings

Ionic accumulation near surfaces enhances Second Harmonic Generation.

The effect is absent in non-ionic ferroelectrics like BaTiO3.

Temperature dependence and domain wall depletion support the ionic screening model.

Abstract

Mobile charges and lattice polarization interact in ferroelectric materials because of the Coulomb interaction between the mobile free charges and the fixed lattice dipoles. We have investigated this mutual screening in KTiOPO4, a ferroelectric/superionic single crystal in which the mobile charges are K+ ions. The ionic accumulation close to the crystal surfaces leads to orders of magnitude increase of the Second Harmonic Generation. This ionic space charge model is supported by the absence of such an effect in non-ionic conductor but ferroelectric BaTiO3, by its temperature dependence in KTiOPO4 and by its broad depletion at domain walls.