Powder- and single crystal M�ssbauer spectroscopy on synthetic fayalite

Abstract

Abstract. M6ssbauer measurements on synthetic iron orthosilicate Fe2SiO 4 (fayalite) were carried out in the antiferromagnetic spin state below TN-~65K. The M6ssbauer parameters isomer shift c~, inner magnetic field H(0), angle O between H(0) and the z-component of the electric field gradient (efg), quadrupole splitting QS and asymmetry parameter r/were determined as a function of temperature, These parameters could be attributed to the two crystallographic sites MI and M2.The smaller isomer shift on M1 with respect to M2 displays the more covalent character of the Fe-O bond on M1, which is supported by previous neutron diffraction experiments./-/(0) shows a Brillouin-type behaviour with different fields on the two crystallographic sites (stronger on M1) and a small discontinuity at T= 23 K which corresponds with previous magnetic measurements. The quadrupole splitting is equal on both sites within error bars, in agreement with previous theoretical results and in contradiction to previous M6ssbauer refinements published elsewhere.Single crystal M6ssbauer spectroscopy revealed the orientation of the crystallographic c-axis (Pnma) with respect to the axes of the efg. A second parameter set which also holds true for the powder refinement yields the angle between c and H(0) on M1 to be temperature-dependent in quantitative agreement with previous neutron diffraction results and solves an old discrepancy discussed elsewhere. On M2, the angle between c and H(0) is found to be much smaller than 90 ~ in contradiction to symmetry requirements.

Drifting inwards in protoplanetary discs I Sticking of chondritic dust at increasing temperatures

“…Owing to the distribution of iron contents in the olivines existing in the sample, the lines of these octets are broadened and therefore reproduced using a distribution of hyperfine magnetic fields. Fits based on the Mössbauer parameters for AFM olivines by Lottermoser et al (1995) lead to a good agreement to the measurement. Furthermore, the paramagnetic olivine doublet decreases in spectral area.…”

Structural and magnetic properties of iron species/SiO 2 nanocomposites obtained by sol‐gel methods

“…The central singlet with an isomer shift of about 0.51 mm/s corresponds to a paramagnetic Fe 3+ specie diffused into the silica matrix. The asymmetric intense internal sextet (IS = 1.28 mm/s, QS = 2.55 mm/s and B hf = 34 T) is assigned to Fe 2 SiO 4 [11,12]. It is worth mentioning that Fe 2 SiO 4 is an antiferromagnet below the transition temperature to the paramagnetic state, T t = 66 K [11,12].…”

“…It is worth mentioning that Fe 2 SiO 4 is an antiferromagnet below the transition temperature to the paramagnetic state, T t = 66 K [11,12]. Two iron positions giving rise to two magnetic patterns with quite different hyperfine fields (about 32 T and 12 T, respectively) are generally reported for the low temperature Mössbauer spectra of this compound [11,12]. However, in the present case, the fit quality of the Möss-bauer spectrum gave strong indications for only one spectral component assigned to fayalite.…”

“…The asymmetric intense internal sextet (IS = 1.28 mm/s, QS = 2.55 mm/s and B hf = 34 T) is assigned to Fe 2 SiO 4 [11,12]. It is worth mentioning that Fe 2 SiO 4 is an antiferromagnet below the transition temperature to the paramagnetic state, T t = 66 K [11,12]. Two iron positions giving rise to two magnetic patterns with quite different hyperfine fields (about 32 T and 12 T, respectively) are generally reported for the low temperature Mössbauer spectra of this compound [11,12].…”

Complex diagnostics of ordinary chondrites Markovka, Polujamki, Sayh al Uhaymir 001, Dhofar 020, and Jiddat al Harasis 055 by X‐ray techniques and Mössbauer spectroscopy

“…The olivines are known to be a solid solution of fayalite (Fe 2 SiO 4 ) and forsterite (Mg 2 SiO 4 ). Fayalites are known to have an antiferromagnetic phase transition at 65 K (Hafner et al, 1990;Lottermoser et al, 1995). Thus, the D1 doublet corresponds to the olivine with intermediate Fe 2+ content with the magnetic phase transition temperature below 15 K. The line broadening of D3 and D4 doublets, which correspond to pyroxene at 15 K, also takes place in the vicinity of magnetic phase transition.…”