Structure-dependent ferroelectricity of niobium clusters (NbN, N=2-52)

TL;DR

This study uses density-functional theory and global search methods to explore how the structure of niobium clusters influences their ferroelectric properties, revealing size-dependent electric dipole moments and an even-odd oscillation pattern.

Contribution

It introduces a comprehensive analysis of NbN clusters' structures and ferroelectricity, highlighting the size-dependent behavior and proposing an inverse coordination number model.

Findings

Most NbN clusters exhibit a non-zero electric dipole moment.

Larger clusters (N>=25) tend to have amorphous structures.

An even-odd oscillation in EDM values is observed for N>=38.

Abstract

The ground-state structures and ferroelectric properties of NbN (N=2-52) have been investigated by a combination of density-functional theory (DFT) in the generalized gradient approximation (GGA) and an unbiased global search with the guided simulated annealing. It is found that the electric dipole moment (EDM) exists in the most of NbN and varies considerably with their sizes. And the larger NbN (N>=25) prefer the amorphous packing. Most importantly, our numerical EDM values of NbN (N>=38) exhibit an extraordinary even-odd oscillation, which is well consistent with the experimental observation, showing a close relationship with the geometrical structures of NbN. Finally, an inverse coordination number (ICN) function is proposed to account for the structural relation of the EDM values, especially their even-odd oscillations starting from Nb38.