Magnetic Studies of Paramagnetic Clusters Encapsulated within the Sodalite Cage
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ABSTRACT The magnetic properties of alkali-doped zeolites have been investigated using magnetic resonance spectroscopy. Although paramagnetic centres have been detected, the predominant species, in many cases, is spin-paired.
INTRODUCTION Zeolites are a well defined class of crystalline aluminosilicates, with structures based on tetrahedral networks which encompass channels and cavities of molecular dimensions. The general formula for the composition of a zeolite is: Mx/n[(A10 2)x(Si0 2 )y]'mH20 where cations M of valence n neutralise the negative charges on the aluminosilicate framework. The framework is made up from comer sharing Si0 4 and A10 4 tetrahedra, with the cations found within the cavities and channels of the framework co-ordinated to the framework oxygens or water molecules. The framework structures of three zeolites, based on a common sodalite cage are shown in figure 1. The idea of using the zeolite as a host in which to dissolve alkali metals is not a new one 1 . On contact with the dehydrated zeolite, the incoming alkali atoms are spontaneously ionised by the intense electric fields within the host matrix. This approach focuses on the deliberate modification of the host material to generate new solid-state compounds that have interesting electronic and magnetic properties 2 . We have studied a number of zeolite-based systems, containing the sodalite cage as a basic building block, in an attempt to understand and perhaps control the electronic properties of these types of materials.
EXPERIMENTAL The syntheses of all of these powdered materials was carried out through the controlled reaction of the dehydrated host with alkali metal. The reactions were carried out between 100 and 2500C, in a sealed, evacuated quartz tube for between 12 to 48 hours, as described elsewhere3 . The electronic properties of these materials were probed using ESR (Electron Spin Resonance) Spectroscopy. ESR studies were performed using a Bruker ESP300 Spectrometer, operating at XBand frequencies (9.5GHz) between 4K and room temperature. To quantify the ESR measurements, the ESR signal from each sample was compared to that of a known CuSO 4 .5H2 0 standard in a dual-mode cavity. Errors in this technique are estimated at ±20%. Preliminary Solid-State NMR measurements have also been performed on some samples.
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Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
RESULTS With regard to the composition of these materials, the following nomenclature will be adopted. The addition of x-mole equivalents of metal (M) to a host zeolite (Z) containing cations (N) results in the formation of a product labelled as MJN-Z. On addition of sodium metal to sodalite, the sample undergoes a remarkable colour change. The white host first turns blue, then purple and finally black as the concentration of reacted metal increases 4 . The ESR spectra, recorded at room temperature, of samples of sodalite containing x mole-equivalents of sodium are shown in figure 2 (Table I). A similar set of spectra, recorded on samples of meta
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