Synthesis and Characterization of the Cobalt-Iron and Nickel-Iron Prussian Blue Analogues in a Silica Matrix

There has been a considerable interest in the fabrication and characterization of molecular magnets. A complex that has garnered a great deal of attention is Prussian blue, Fe(III)4[Fe(II)(CN)6]3, and its transition metal cyanide analogues. Molecular magnets of these Prussian blue complexes have been studied extensively, due to their interesting magnetic properties. For this research the sol gel process was used to fabricate some of these complexes in a silica matrix known as a xerogel. Specifically the two Prussian blue analogues that were studied are KxCo(II)y[Fe(III)(CN)6]z and KxNi(II)y[Fe(III)(CN)6]z. These two analogues were incorporated in to a transparent porous silica matrix using the sol-gel process. Transparent glasses containing the complexes were produced with total metal (Fe(III) and Co(II) or Ni(II)) concentrations between 0.01 and 0.06; and between 0.01 and 0.1 mol % respectively. It was found that for the cobalt – iron xerogel samples nanoparticles on the order of 8 – 10 nm were formed. In the nickel – iron xerogel samples it was discovered that nanorods (15 nm by 40 nm) and nanodots (40 nm diameter) were formed during the sol gel process and are concentration dependent in that the lower concentrations were more likely to form the nanodots. It is believed that the particles for both types of Prussian blue analogues are formed by arresting the precipitation of the complexes when the gelation of the sol gel occurs. The size of these particles allows for the samples to be magnetically characterized as superparamagnets. The blocking temperatures that are exhibited by these complexes are 10 and 15.5 Kelvin respectively for the cobalt and nickel – iron Prussian blue analogues. Both analogues showed frequency dependence in the AC susceptibility measurements furthering the characterization as a superparamagnet. The cobalt – iron xerogel samples displayed a previously determined photomagnetic effect that is attributable to the presence of cobalt (III) – iron (II) defect sites in the lattice of the complex. This photomagnetic effect is controlled by the introduction of anhydrous ammonia to the samples, which causes more defect sites to form. When more defect sites are present the photomagnetic effect is increased.