Evolution of the Mössbauer recoilless fraction during mechanochemical activation of iron oxides

Abstract

The preparation and properties of ultrafine iron oxides continue to attract considerable interest and attention because of their importance in magnetic technologies and pigment applications [1–5]. Part of the interest comes from the fact that ball milling and related methods represent a novel alternative approach to the preparation of new materials and phases. Mossbauer spectroscopy represents a powerful method to characterize the sequence of magnetic phases formed during the ball milling process. The most important parameter in a Mossbauer effect experiment is the recoilless fraction f = exp(−k2〈x2〉), where k is the wavevector of the gamma rays and 〈x2〉 is the mean square vibrational amplitude of the resonant atom in the direction of observation. The only method available to date for the determination of the recoilless fraction relied on its temperature dependence and the determination of the Debye temperature from complicated equation plots. However, we have recently proposed a new method for the direct determination of the recoilless fraction by a single room-temperature transmission Mossbauer measurement. The method relies on a two-lattice comparative approach and made it possible to determine the recoilless fraction of various systems, from iron chlorides to nanoparticles [6, 7]. In the present paper we propose that the recoilless fraction represents an important parameter during the mechanochemical activation process. The magnetite precursor was milled in a hardened steel vial with six stainless-steel balls (type 440; four of 0.25 in. diameter and two of 0.5 in. diameter) in the SPEX mixer-mill for time periods ranging from 0–128 h. Room temperature transmission Mossbauer spectra were recorded using a constant acceleration spectrometer and a 50 mCi 57Co source diffused in a Rh matrix. For the recoilless fraction determination experiments, we produced physical mixtures of iron powder with the powder whose fraction was to be determined. Least squares fitting of the Mossbauer spectra was performed with the NORMOS program [8]. Fig. 1a–c shows the room-temperature Mossbauer spectra of the magnetite powder, after 0, 30 and 70 h of ball milling, respectively. The precursor in Fig. 1a was fitted by considering two sextets, corresponding to the tetrahedral and octahedral magnetic sublattices in the sample. The intermediate products in Fig. 2b were analyzed with five sextets and this analysis was consis-