Tuning the Ferroelectric Polarization in $AA^{\prime}$MnWO$_6$ Double Perovskites through $A$ Cation Substitution

TL;DR

This study uses first-principles calculations to explore how substituting different $A$ cations in $AA^{\u2019}$MnWO$_6$ double perovskites can significantly enhance ferroelectric polarization while maintaining magnetic order, guiding the design of multiferroic materials.

Contribution

It provides crystal-chemistry guidelines for increasing electric polarization in $AA^{}$MnWO$_6$ perovskites via $A$-site substitution, based on structural and electronic analysis.

Findings

Polarization can be increased by up to 150% with ionic size mismatch.

Purely electronic remnant polarization exists without displacive modes.

Structural distortions correlate with polarization contributions.

Abstract

Recent experimental and theoretical work has shown that the double perovskite NaLaMnWO$_6$ exhibits antiferromagnetic ordering owing to the Mn $d$ states, and computational studies further predict it to exhibit a spontaneous electric polarization due to an improper mechanism for ferroelectricity [King \textit{et al., Phys.\ Rev.\ B}, 2009, \textbf{79}, 224428; Fukushima \textit{et al., Phys.\ Chem.\ Chem.\ Phys.}, 2011, \textbf{13}, 12186], which make it a candidate multiferroic material. Using first-principles density functional calculations, we investigate nine isostructural and isovalent $AA^{\prime}$MnWO$_6$ double perovskites ($A$=Na, K, and Rb; $A^{\prime}$=La, Nd, and Y) with the aim of articulating crystal-chemistry guidelines describing how to enhance the magnitude of the electric polarization through chemical substitution of the $A$-site while retaining long-range magnetic order. We find that the electric polarization can be enhanced by up to 150\% in compounds which maximize the difference in the ionic size of the $A$ and $A^{\prime}$ cations. By examining the tolerance factors, bond valences, and structural distortions (described by symmetry-adapted modes) of the nine compounds, we identify the atomic scale features that are strongly correlated with the ionic and electronic contributions to the electric polarization. We also find that each compound exhibits a purely electronic remnant polarization, even in the absence of a displacive polar mode. The analysis and design strategies presented here can be further extended to additional members of this family ($B$=Fe, Co, etc.), and the improper ferroelectric nature of the mechanism allows for the decoupling of magnetic and ferroelectric properties and the targeted design of novel multiferroics.