Superparamagnetic particles in ZSM-5–type ferrisilicates

Abstract

As-synthesized, low iron content, ferrisilicates of ZSM-5-type contain well-separated Fe(III) ions in a tetrahedral environment and display paramagnetic behavior. After hydrothermal treatment, the iron ions are partially extracted from the framework, generating nanosize iron oxide or oxyhydroxide ferrimagnetic particles. This process has been studied by transmission electron microscopy (TEM), Mössbauer spectroscopy, magnetic ac susceptibility (χac), and field dependent magnetization, on samples containing up to 6.7 wt.% Fe. The experiments evidence the growth of nonaggregated particles, with a typical size around 3 nm, presumably located at the surface of the ferrisilicate crystallites. From a thorough granulometric analysis involving TEM and χacdata, it is concluded that, in the range from 1.5 to 4.6 wt.% Fe, the particle size distributions are significantly independent of the iron content.