Electric Polarization and Magnetic Properties of (NH$_4$)$_{1-x}$K$_x$I (x = 0.05-0.17)

TL;DR

This study discovers ferroelectric and pyroelectric phases in mixed ammonium-potassium iodide compounds, revealing how K+ ions induce polar order and suggesting new routes to ferroelectricity in ammonium-based materials.

Contribution

It identifies ferroelectric and pyroelectric phases in (NH4)1-xKxI compounds and explains how K+ ions disrupt symmetry to induce polarization, a novel pathway for ferroelectricity.

Findings

(NH4)0.95K0.05I is ferroelectric.

(NH4)0.87K0.13I and (NH4)0.83K0.17I are pyroelectric.

Order-disorder transition near 245 K.

Abstract

While all of the polymorphs of pure NH$_4$I and KI are non-polar, we identify that (NH$_4$)$_{0.95}$K$_{0.05}$I is ferroelectric and (NH$_4$)$_{0.87}$K$_{0.13}$I and (NH$_4$)$_{0.83}$K$_{0.17}$I are pyroelectric through measurements of their pyroelectric current and complex dielectric constant. The order to disorder phase transitions occur near 245 K. Magnetic susceptibility measurements indicate that the proton orbitals of the NH$_4$$^+$ continue to become ordered in the ground state in the (NH$_4$)$_{1-x}$K$_x$I system up to x <= 0.17. The polar phases are proposed to stem from K$^+$ ions disrupting the symmetry of proton-orbital-lattice interactions between the NH$_4$$^+$ and I$^-$ ions. Our work introduces a new pathway for the ordered phases of ammonium-based compounds to potentially become ferroelectric.