Reproducible Magnetic Fluctuations of Microwave Absorption in Zeolites

For polycrystalline specimens of zeolites treated to thermal oxidation we have revealed and studied the weak spectra of reproducible magnetic microwave absorption fluctuations at a frequency of 9.4 GHz. The investigation was carried out by the ESR method over the 77-500 K temperature range. The fluctuation spectra have features typical of mesoscopic phenomena observed previously in ESR spectra of high temperature superconductors and metals at temperatures below critical. The similar fluctuation spectra were obtained at 300 K for some oxides, oxidized silver and gold particles. The experiments performed are, perhaps, the first observation of the phenomenon at unusually high temperatures which has been observed only in superconducting systems. An investigation of properties of natural and synthetic zeolites which belong in aluminosilicates with a geometric rigid set of pores and channels is progressing largely because of their catalytic and adsorption activity; this latter is of interest for technical applications and yet to be explained physically in many aspects [1]. Zeolites exhibit certain unique properties, in particular, after the high-temperature (700-1000 K) treatment in O 2 atmosphere [2,3]. This paper reports the results of an experimental investigation of specific microwave absorption by ze-olites in the magnetic field. We have studied polycrystalline specimens of various zeolites, namely, ZSM-5, ZSM-12, and mordenite. When preparing the specimens we have previously performed an ion-exchange in aqueous solution of NH 4 Cl and then the zeolites were kept in O 2 atmosphere at 850 K for 1-2 hours. An ESR-300 3cm-range Bruker spectrometer was used in our study. The temperature varied within the 77-500 K range. 1. All the specimens registered in the normal fashion exhibit the familiar large-intense broad ESR signals caused by the paramagnetic F e 3+ ions which usually present as impurities in zeolites. Against a broad spectrum background, however, one can register quite weak absorption lines in an unusually large range of values for magnetic field-from a value close to zero to that attainable in this spectrometer ((8000 Oe). At a single magnetic field sweep, these lines are virtually lost to noises and their reliable registration requires a digital accumulation of ∼ 10 2 scans. The experiment with ZSM-5 zeolite specimens prepared specially (a content of the iron impurities was reduced by a factor of more than 10) showed that the intensity of weak lines is virtually unchanged in this case. It is felt that the origin of the unusual absorption weak lines …