Tuning the magnetic properties of prussian blue analogues: size control and the effects of external stimuli

Abstract

The hetero-bimetallic mixed valence metal polycyanides (Prussian Blue analogues (PBAs)) with formula AxMII[M’III(CN)6]y (where A= alkali-metal cation and M, M’= transition-metal cations respectively) are archetypal examples of molecule-based magnets, exhibiting a wealth of exotic electronic and magnetic behaviours. Similar intriguing electronic and magnetic properties are anticipated in nanoscale structures employing the PBA molecular building block. Herein investigations of multifunctional molecular magnets based on the PBA building block, with varying dimensionalities, are reported. Synthesis and characterisation of a novel family of mixed ferri-ferromagnets, RbNizMn(1-z)[Fe(CN)6] (0<z<1), illustrates how relatively minor changes in chemical composition can induce significant changes in the magnetic properties. Selected members of this series show photo-induced electronic transitions and pressureinduced magnetic pole inversions. The rare phenomenon of a double magnetic pole inversion is observed when FeII is introduced as a third MII ion. PBAs incorporating the Cu2+ ion are studied with the aim of establishing the influence of Jahn-Teller distortions on the photo-induced electron transfer which has been observed in a number of PBAs. X-ray powder diffraction and absorption measurements of CsCu[Fe(CN)6] under laser illumination reveal an unusual and unexpected behaviour, with photo-irradiation initiating a simultaneous reduction of both transition metal centres, which is proposed to occur via a chemical reaction. Superparamagnetic nanoparticles based on the CuII-NC-CrIII and Mn-NC-CrIII moieties are prepared by polymer protected synthesis, a method based upon spatial confinement. Control over the particle size, and consequently magnetic properties, of the isolated polymer-capped nanoparticles is achieved by altering the polymer concentration. The relaxation dynamics of the polymer-capped nanoparticles are studied using AC susceptibility.