Jahn-Teller Distortion in Bimetallic Oxalates

TL;DR

This paper investigates how Jahn-Teller distortions influence the magnetic and structural properties of Fe(II)Fe(III) bimetallic oxalates, revealing temperature-dependent symmetry breaking and the role of spin-orbit coupling.

Contribution

It provides a theoretical and computational analysis of Jahn-Teller effects in bimetallic oxalates, predicting re-entrant distortions and their impact on magnetic compensation.

Findings

Jahn-Teller distortion breaks C3 symmetry around the ferrimagnetic transition.

Low-temperature JT distortion disappears in compounds with magnetic compensation.

First-principles calculations estimate the distortion size and its dependence on organic cation ordering.

Abstract

A C$_3$-symmetric crystal-field potential in the Fe(II)Fe(III) bimetallic oxalates splits the L=2 Fe(II) multiplet into two doublets and a singlet. In compounds that exhibit magnetic compensation, one of the doublets was predicted to lie lowest in energy and carry a non-quenched orbital angular momentum $\pm \ld $, where $\ld $ exceeds a threshold value. In a range of $\ld $, a Jahn-Teller (JT) distortion increases the energy splitting of the low-lying doublet and breaks the C$_3$ symmetry of the bimetallic planes around the ferrimagnetic transition temperature. At low temperatures, the JT distortion disappears in compounds that display magnetic compensation due to the competition with the spin-orbit coupling. A comparison with recent measurements provides strong evidence for this re-entrant, low-temperature JT transition and a prediction for the normal, high-temperature JT transition. The size of the JT distortion is estimated using first-principles calculations, which suggest that the long-range ordering of smaller, non-C$_3$-symmetric organic cations can eliminate magnetic compensation.