engleski

Effect of heat treatment on structural changes and magnetic properties of Fe2O3-P2O5 glass

The emergence of nanocrystals in glass leads to changes in the properties thus opening new fields of the application and research of glass-ceramics. Iron phosphate glass exhibits the spin-glass behavior [1, 2] and represents a prototype glass with disordered spatially distributed magnetic moments. Since the iron ions in glass are present in two valence states (Fe2+ and Fe3+) [3, 4] changes in magnetic properties, during the change of valence and local surroundings of iron ions, are expected. However, details of the formation of crystalline phases in glass matrices and their influence on the various properties are hardly known. The purpose of the present study was to investigate the influence of induced crystallization on structural changes and magnetic properties in iron phosphate glass and glass- ceramics. Starting glass of the composition 40Fe2O3-60P2O5 (mol%) was prepared by conventional melt-quenching method. Heat treatments at 893 (A1), 923(A2) and 1073 K (A3) for 24 h were performed to induce crystallization. The temperatures are chosen according to thermal analysis (DTA). The structural changes and evolution of various crystalline phases in glass-ceramics have been traced by X-ray diffraction (XRD), Mössbauer (MS) and Raman spectroscopy (RS). Magnetic properties of crystallized samples were studied by means of vibrating sample magnetometer (VSM). This study shows that the evolution of crystallization processes can be followed by Mössbauer spectroscopy. Room temperature Mössbauer spectra reveal that the iron ions exist as Fe2+ and Fe3+ in both, starting glass and crystallized samples. The individual doublets are assigned to specific Fe2+ and Fe3+ sites in glass matrix as well as in various crystalline phases present in the crystallized samples. Hyperfine parameters of the Fe3+ ions in glass and crystallized samples are typical of distorted octahedrally coordinated Fe3+ ions. However, in sample heat treated at 1073 K, a small amount of the Fe3+ ions in tetrahedral coordination (FePO4 crystalline phase) is also present. Further, in starting glass the Fe2+ ions have distorted octahedral cordination. The transition of the Fe2+ coordination from glassy phase to the coordination in Fe3(P2O7)2 crystalline phase, which contains both Fe2+ and Fe3+ ions, is nicely shown by the appearance of the new doublet for Fe2+ ions in the Mössbauer spectra associated with the trigonal prism coordination. This transition is confirmed by the unusually large quadrupole splitting value, 4.34 mm/s. Overall Fe2+/Fetotal ratio slightly decreased with heat-treatment temperature increasement. Investigation of magnetic nature showed dominant antiferromagnetic interactions for all samples. Magnetic interactions beetwen iron ions strongly depend on the various crystalline phases present in glass-ceramics.

phosphate glasses; crystallization; Mossbauer spectroscopy

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