Theoretical study of ferroelectric potassium nitrate

TL;DR

This paper provides a theoretical analysis of the polarization reversal mechanism in ferroelectric potassium nitrate, extending previous first-principles calculations with a two-parameter model and exploring pressure effects on its energy landscape.

Contribution

It introduces a detailed two-parameter model for KNO$_3$ that accurately reproduces first-principles results and maps the phase diagram including triple-well potentials.

Findings

The model accurately reproduces first-principles energy and polarization curves.

Pressure induces a discontinuity in the energy curve at zero polarization.

Phase diagram reveals regions with complex triple-well potentials.

Abstract

We present a detailed study of the structural behavior and polarization reversal mechanism in phase III of KNO$_3$, an unusual ferroelectric material in which the nitrate groups rotate during polarization reversal. This material was one of several studied in a previous work [O. Di\'eguez and D. Vanderbilt, Phys. Rev. Lett. {\bf 96}, 056401 (2006)] where methods were described for computing curves of energy versus electric polarization. In the present work we extend and systematize the previous first-principles calculations on KNO$_3$, and analyze in detail a two-parameter model in which the energy of the system is written as a low-order expansion in the polarization and the nitrate group orientation. We confirm that this model reproduces the first-principles results for KNO$_3$ very well and construct its parameter-space phase diagram, describing regions where unusual triple-well potentials appear. We also present first-principles calculations of KNO$_3$ under pressure, finding that its energy-versus-polarization curves change character by developing a first-derivative discontinuity at zero polarization.