Mossbauer Resonance Studies

Above the Neel temperature, the paramagnetic spectrum shows two doublets corresponding to both sites and the ratio Fe(0)/Fe(T) is also close to 1.0 (Fig. 13 a). 

Usually, the fit of paramagnetic spectra is slightly inaccurate because of the non-Lorentzian profile of the widened octahedral peaks. This effect can be due to the existence of different octahedral sites or, more likely, to the random occupancy of these sites by Ti and Fe atoms. Due to the fact that the Fe(0)/Fe(T) ratio remains constant (= 1) in this range of composition, we can conclude that Fe3+ is equally distributed among the octahedral and tetrahedral sites. According to our structural model Ti4+ occupies octahedral sites exclusively. 

In this section we will assume that the latter conclusion can be extended to the whole composition range, namely that the Ti4+ ions are never fourfold coordinated, even at high concentrations (low y). This hypothesis seems quite reasonable, because Ti4+ is very rarely found in tetrahedral coordination in oxides (the only known oxides with C. N. 4 are strongly basic titanates such as Na4Ti04, K4Ti04 or Ba2 Ti0451,52 ). 

As will be seen below, the Neel temperature of these phases is very high (TN 700 K). The Mossbauer spectra obtained at room temperature show two Zeeman patterns, once again corresponding to trivalent iron in octahedral and tetrahedral sites. Mossbauer data are reported in Table 2 and conclusions similar to those for the CaTi1 2yFe2y03 y can be drawn. For ordered structures (for instance y = 0.40 and y = 0.33, Fig. 14), the Fe(O)/ Fe(T) ratio is in perfect agreement with the ratio deduced from the predicted structures. 

For smaller values of y S 0.25, despite the deviation from (m the existence of tetrahedra gives evidence of short-range order. Thus, an unexpected tendency of the anionic defects to couple in the neighbourhood of ions such as Fe3+, inducing a change from C. N. 6 to 4 is found. A model taking account of this unusual segregation phenomenon is proposed in Chap. 7. 

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Domeneghetti MC, Steffen G (1992) Ml, M2 site populations and distortion parameters in synthetic Mg-Fe orthopyroxenes from Mossbauer spectra and X-Ray stmcture refinements. Phys Chem Min 19 298-306 Drickamer HG, Vaughn RW, Champion AR (1969) High-pressure Mossbauer resonance studies with iron-57. Accounts Chem Res 2 40-47... 

Mossbauer resonance of Zn to study the influence of the gravitational field on electromagnetic radiation. A Ga ZnO source (4.2 K) was used at a distance of 1 m from an enriched ZnO absorber (4.2 K). A red shift of the photons by about 5% of the width of the resonance line was observed. The corresponding shift with Fe as Mossbauer isotope would be only 0.01%. The result is in accordance with Einstein s equivalence principle. Further gravitational red shift experiments using the 93.3 keV Mossbauer resonance of Zn were performed later employing a superconducting quantum interference device-based displacement sensor to detect the tiny Doppler motion of the source [66, 67]. 

Another study of the temperature dependence of the 6.2 keV Mossbauer resonance of Ta has been carried out by Salomon et al. [197] for sources of WAV metal and W/Ta metal in the temperature range from 15 to 457 K. In more recent investigations, Salomon et al. [198] have extended such studies of the temperature behavior of the 6.2 keV Mossbauer transition of Ta in tantalum metal to temperatures up to 2,300 K which has been the highest temperature range for any Mossbauer study so far. 

Table 7.8 Summary of results obtained for the four Os Mossbauer transitions studied. The absorber thickness d refers to the amount of the resonant isotope per unit area. The estimates of the effective absorber thickness t are based on Debye-Waller factors / for an assumed Debye temperature of 0 = 400 K. For comparison with the full experimental line widths at half maximum, Texp, we give the minimum observable width = 2 S/t as calculated from lifetime data.

Ding, X.-Q., Bominaar, E.L., Bill, E., Winkler, H., Traitwein, A.X., Driieke, S., Chaudhuri, P., and Wieghardt, K. 1990. Mossbauer and electron paramagnetic resonance study of the double exchange and Heisenberg-exchange interactions in a novel binuclear Fe(II/III) delocalized-valence compound. Journal of Chemical Physics 92 178-186. 

More recently, NH FeF was studied by susceptibility and Mossbauer resonance measurements14. From an expansion series, a value of -13 K was obtained for J/k and TN = 135 K from Mossbauer data. The law of variation of the hyperfine field with temperature has the form... 

Schulz CE, Rutter R, Sage JT et al (1984) Mossbauer and electron paramagnetic resonance studies of horseradish peroxidase and its catalytic intermediates. Biochemistry 23 4743 1754... 

Samples were studied after drying (100°C), after calcination in air to 540°C, and after hydrothermal treatment with 95% steam/5% Np mixtures at 730°C. Calcination and steaming were performed in quartz reactors. For electron paramagnetic resonance studies samples were sealed off in 2 mm quartz tubes luminescence studies were performed with the same sealed off tubes. Samples sealed in Teflon cells were used for Mossbauer studies. 

Rancourt DG, Klingelhofer G (1994) Possibility of a Mossbauer resonant-electron microscope. Fourth Seeheim Workshop on Mossbauer Spectroscopy, p 129 Rancourt DG, Mcdondd AM, Lalonde AE, Ping JY (1993) Mossbauer absorber thicknesses for accurate site populations in Fe-bearing minerals. Am Mineral 78 1-7 Riedel E, Karl R (1980) Mossbauer studies of thiospinels. 1. The system FeCr2S4-FeRh2S4. J Sol State Chem 35 77-82... 

Mossbauer hyperfine spectra are useful in the determination of nuclear parameters, especially those of the excited states. Their significance stems from the fact that the structure of the nucleus is still poorly understood. Comparison of the parameters as measured with the values estimated from theory is used to discover the validity or inadequacy of the nuclear model. The rare-earth elements are popular for this type of work because of the proliferation of Mossbauer resonances, making it feasible to study the effects of successive proton or neutron addition over a range of nuclei. Although theory and experiment are sometimes in accord, gross differences are not unusual. 

At the Fe4S4 " " oxidation level, the Fc4S4 cluster is in a mixture of S= 1/2 and S= 3/2 states. " Electron paramagnetic resonance studies, magnetism, Mossbauer, and MCD studies have led to this conclusion, although the details of this spin crossover/admixture are not clear. ... 

Unfortunately, the requirements of recoil-free emission and resonant absorption and transmission through the absorber limit the useable energy range of the Mossbauer effect 7-ray to approximately 10-100 keV. Further, in order to obtain rather sharp absorption lines and a reasonable spectral resolution, the mean lifetime of the Mossbauer 7-ray precursor state should be between 1 ns and 100 ns. Further, the Mossbauer nuclide must have a sufficiently high isotopic abundance in the element to yield a usable signal-to-noise ratio over a reasonable acquisition time. Finally, the radioactive source containing the Mossbauer 7-ray precursor state must be easily prepared and have a mean lifetime of several weeks to be practical. These various requirements limit the number of nuclides available for typical Mossbauer spectral studies. 

In 1956 and 1957, young physicist R. Mossbauer has performed the experiments concerning the scattering of the 129 keV y-ray of Ir by Ir and discovered an increase in scattering at low temperatures. Results obtained and his interpretations were published in 1958 [1-3], which is the beginning of the Mossbauer effects study and its development as the Mossbauer spectroscopy. The Nobel Prize for physics 1961 was awarded to him [4]. Mossbauer spectroscopy is the recoilless emission and the recoilless resonant absorption of the y-ray by the nucleus. After... 

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