Origin of the ESR spectrum in the Prussian Blue analogue RbMn[Fe(CN)6]*H2O

TL;DR

This study investigates the origin of ESR signals in a Prussian Blue analogue, revealing that the observed ESR features are due to cubic Mn2+ clusters and defect-related states, with unusual spin relaxation behavior linked to charge transfer transitions.

Contribution

It provides a detailed ESR analysis of RbMn[Fe(CN)6]*H2O, identifying the origin of ESR signals and the effects of charge transfer on spin relaxation, which was not previously understood.

Findings

ESR signals originate from cubic Mn2+ clusters around vacancies

Superparamagnetic behavior observed below 50 K in powders

Unexpected strong spin relaxation above charge transfer transition

Abstract

We present an ESR study at excitation frequencies of 9.4 GHz and 222.4 GHz of powders and single crystals of a Prussian Blue analogue (PBA), RbMn[Fe(CN)6]*H2O in which Fe and Mn undergoes a charge transfer transition between 175 and 300 K. The ESR of PBA powders, also reported by Pregelj et al. (JMMM, 316, E680 (2007)) is assigned to cubic magnetic clusters of Mn2+ ions surrounding Fe(CN)6 vacancies. The clusters are well isolated from the bulk and are superparamagnetic below 50 K. In single crystals various defects with lower symmetry are also observed. Spin-lattice relaxation broadens the bulk ESR beyond observability. This strong spin relaxation is unexpected above the charge transfer transition and is attributed to a mixing of the Mn3+ - Fe2+ state into the prevalent Mn2+ - Fe3+ state.