# The Role of Anharmonicity in (Anti-)Ferroelectric Alkali Niobates

## TL;DR

This paper explores how anharmonicity affects the antiferroelectric and ferroelectric properties of alkali niobates.

## Contribution

The study provides experimental evidence for anharmonic contributions in alkali niobates, offering a new design path for (anti-)ferroelectric materials.

## Key findings

- Anharmonic contributions of the A-site atoms differ in strength among LiNbO3, KNbO3, and NaNbO3.
- Electron density analysis reveals distinct bonding behaviors linked to ferroelectric and antiferroelectric properties.
- Anharmonicity is identified as a key factor distinguishing ferroelectric and antiferroelectric perovskites.

## Abstract

NaNbO3 (NN), known for the complexity of its phase transition sequence, is antiferroelectric (AFE) at room temperature, while both LiNbO3 (LN) and KNbO3 (KN) are ferroelectric (FE). The origin of ferroelectricity in ABO3 perovskites is believed to lie in the B-O hybridization, but the origin of antiferroelectricity remains unclear. Recent ab initio studies have shown that the same B-O hybridization is necessary in AFE and proposed an additional, anharmonic contribution to the potential of the A-site atom as the crucial difference between FE and AFE perovskites. We used structure factors obtained from X-ray diffraction experiments in combination with the Maximum Entropy Method to obtain electron densities for LN, KN, and NN and identify differences in their bonding behavior. We present experimental evidence for anharmonic A-site contributions of varying strength in alkali niobates, pointing at a new path for the design of (anti-)ferroelectric materials.