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Transformation of amorphous iron(III)-hydroxide into alpha-Fe2O3 in the solid state

The transformation of amorphous iron(III)-hydroxide into alpha-Fe2O3 is common to many solid state synthesis of Fe-bearing materials. In order to obtain more information about the nature of this reaction, a series of amorphous iron(III)-hydroxide samples were subjected to thermal treatment between 195 and 325 oC in air. XRD showed only amorphous fraction up to 220 oC, a few percents was detected at 245 oC and futher, alpha-Fe2O3 fraction gradually increased with increase of the temperature. Mössbauer spectrum of the sample, produced at 220 oC, showed average quadrupole doublet at RT, whereas hyperfine-field distribution were observed at 78 and 12 K due to different chemical envinornments or clusters for Fe3+ ions. Sample, obtained at 245 oC, showed superposition of quadrupole doublet and sextet with maximum probablitity (MP) values 502 kOe, QS = - 0.21 mm s-1 and ?Fe = 0.33 mm s-1 corresponding to poorly crystalline hematite. At 78 K,two sextet components were detected. The fitted parameter values for the hematite component were 527, - 0.13 and 0.49, suggesting the coexistence of antiferomagnetic (AF) and weakly ferromagnetic (WF) spin states, the later being dominant. The iner sextet showed a multi-modal apperiance as in previous sample. At 12 K, two ill-resolved sextets were observed . The hematite has MP field of 535 kOe with QS = - 0.03 and IS = 0.47 mms-1. AF and WF states were present with a higher contribution of AF state as compared to 78 K. Inner more broad sextet showed MP field of 505 kOe with QS = 0.00 and IS = 0.45 mm s-1. The ratio sextet : doublet was 0.22 : 0.78 in good agreement with those at higher temperatures. Sample, obtained at 325 oC, showed a relatively narrow sextet corresponding to hematite. At 78 K the field is 538 kOe, QS = 0.37 mm s-1 and IS = 0.49 mm s-1 indicating that the WF contribution to the spectrum if any, must be very small.

Amorphous iron(III)-hydroxide. Mossbauer spectroscopy. alpha-Fe2O3. solid state transformation

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