2D to 1D magnetic interactions evolution in Cu$_2$F$_{5-x}$ through electron doping by fluoride non-stoichiometry

TL;DR

This study uses ab-initio calculations to show that fluoride vacancies in Cu$_2$F$_{5-x}$ induce a transition from 2D to 1D magnetic interactions by altering copper spin-states and creating linear chains of Cu ions.

Contribution

It reveals how fluoride non-stoichiometry causes a spin-state transformation and dimensional crossover in magnetic interactions in Cu$_2$F$_{5-x}$, supported by a proposed microscopic mechanism.

Findings

Fluoride vacancies lead to a change from 2D to 1D magnetic interactions.

Copper ions' spin-states transform from mixed to S=1/2 and S=0.

Formation of linear Cu chains with enhanced magnetic anisotropy.

Abstract

The copper fluoride Cu$_2$F$_5$ is a compound with 2D-magnetic exchange interactions between the Cu ions in the $S=1$ and $S=\frac{1}{2}$ spin-states. Using ab-initio calculations, we predict that the existence of 5% vacancies in the fluoride sublattice of Cu$_2$F$_5$ results in the drastic transformation of the spin-state of all copper ions and the final spin-states are $S=\frac{1}{2}$ and $S=0$. Consequently, the anisotropy of magnetic interactions increases, and the 1D linear chains of the Cu $d^9$, $S=\frac{1}{2}$ ions appear. We also propose a microscopic mechanism of such exchange interaction transformation via CuO$_6$ octahedra elongation.