Structural and valence state of iron ions in sodium-aluminum-phosphate glass

Abstract

(SAP) glass by means of electron paramagnetic resonance (EPR), M6ssbauer spectroscopy (MS), and nuclear magnetic resonance (NMR). Our results enabled us to determine the characteristics of the occupancy and distribution of iron ions over tetra- and octahedral positions as a function of the ratio of the concentration of iron and aluminum oxides, to determine the concentration of Fe 3+ and Fe 2+ ions and the number of phosphorus and aluminum atoms in the nearest-neighbor environment of the impurity iron ions, as well as to estimate the effect of the concentration of aluminum oxide on the average distance between the phosphorus atoms in the SAP glass. Experimental Conditions. To synthesize the SAP glasses, we prepared a charge consisting of powders of the pure reagents NaPO3, AI203, and NaNO 3. The charge was held at 1100~ in an alundum crucible for 4 h in air to obtain frits of three-component glass, corresponding to the following oxide composition (mass %): 25 Na20-(10-21)AIxO 3-(5363)P205. The frit was mixed with Fe203 powder and held at 1050~ for 6 h, after which the melt was allowed to cool in air to room temperature within 3 h. The samples of SAP glass contained from 0.4 to 6 mass % Fe203. The EPR, M6ssbauer, and NMR spectra were recorded using glass powders with grain size not exceeding 120/zm; all measurements were performed at room temperature. The EPR spectra of the Fe 3 + ions were recorded on a RI~- 1301 radio spectrometer, the 57Fe M6ssbauer spectra were recorded on a YAGRS-4 spectrometer with a 57Co ,,/-ray source in chromium with the signals recorded in a NTA-1024 multichannel analyzer, followed by least-squares analysis of the two doublets on a IBM PC/AT computer. The 27AI and 31p spectra were recorded on a Brucker (Germany) SKHR-90 pulsed Fourier spectrometer, using a "solid-echo" sequence in the high-power regime with external stabilization of the magnetic field. The chemical shifts in the 3tp NMR spectra were read from the signal from an 85 % solution of H3PO 4 used as an external standard. EPR Spectra of Fe 3+ Ions. Figure 1 displays the EPR spectrum of Fe 3 + ions in SAP glass, containing 6 mass % Fe203. For [Fe203] < 4 mass% the EPR spectrum consists of two lines with g-factors of 4.3 and 2, which are determined by the isolated Fe 3+ ions occupying, respectively, tetra-(FeO4) and octahedral positions (FeO6) in the glass structure [1]. The third (wider) line in the region g = 2 (dashed line in Fig. 1) is manifested in the spectrum only with Fe203 content in the glass exceeding 4 mass%. This line was identified with clusters of iron ions, which are characterized by ferromagnetic properties. Figure 2 displays the intensity (Iv_) of the EPR spectrum as a function of the iron oxide concentration in SAP glass with the following composition (mass%): 25 Na20-21AI203-53P xOs-xFe203 (x = 0.4-6 mass%). In the region [FexO 3] <4% In is a linear function of [Fe203]. At high iron oxide concentration the linear relation breaks down because of the sharp increase in the intensity of the line corresponding to iron-ion clusters, which eliminate the direct proportionality between the iron concentration in the clusters and the intensity of the EPR spectrum.