  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.

Strongly Correlated Electrons