chemical physics of discharges - Argonne National Laboratory
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chemical physics of discharges - Argonne National Laboratory

chemical physics of discharges - Argonne National Laboratory chemical physics of discharges - Argonne National Laboratory

web.anl.gov
26.03.2013 Views

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L. 421 minutes, yielding c 600,000 counts and a relative standard deviation < 0.135. The velocity region scanned for each sauple was from -2.OC mm s-l to about + 4.00 mm s-'. Most measurements were made a rooi; temperature. A cryostat mde from I' S t y r O f a $as mounted on the moveable table 0: the instrument ;or low-terqerature I measurements. The sample was placed in this and submerged in liquid N2. The , spectrum of each coal sample was run twice at room temperature, befcke and after , the spectm was taken at liquid42 temperature. Spectral parameters were generally read from plotted spectra. The data from sample F(a) were fitted to a double Lorentz curve by a least-squares program using J an IB1 7C40 computer. Isomer shifts are reported (in m s-') with respect to the , ismer shift of sodium nitroprusside. Quadrupole splittings are reported (in mm s-') as the velocity differences between the minima of two associated absorption lines. I DATA / Room-temperature spectra for the nine coal samples are illustrated in Figure 1. J Each spectrum shows neither, either, or both of just two components: (1) a close doublet similar to those of pyrite and mrcasite, and (2) a wide doublet similar to those of many ferrous compounds. Table 3 lists the Mossbauer paramters, including the fractional peak absorptions, for what we will call respectively the pyrite and non-pyrite resonances. Isomer shifts and quadrupole splittings obtained / with our instrument on sane powdered iron campounds and minerals which were regarded as possibilities for the non-pyrite spectrum appear in Table 4. J / The parameters observed for the pyrite iron are: isomer shift (d) = +0.54 mm s-l; quadrupole splitting (A) = 0.58 m s-I. All of the coals having a non- ' pyrite absorption as shown in Figure 1 have: d = +l.38 mm s-'; A = 2.62 mm s-'. p' The computer analysis of sample F(a) indicated both lines in the spectrum had c widths (r) of 0.39 mm s-l; their intensity ratio is within 57; of unity. I i Liquid-N2 spectra showed the same absorption peaks as at room temperature, f with d = +0.65 m s-l and A = 0.58 mm s-l for pyrite and d = +1.47 mm s-' and > A = 2.78 mm s-l for the non-pyrite iron. INTERPRETATION i Y The isomer shift is a function of nuclear properties and the electron density at the absorbing nucleus relative to that of the source or a standard absorber, such as sodium nitroprusside (23,24). As the electron density at the iron nucleus, due essentially only to s electrons, increases, the isomer shift decreases '[algebraically. Thus, ferrous compounds have a more positive isomer shift than ferric compounds, as the additional 3d electron of the former increases the d shielding effect on the 3s electrons and thereby decreases their density at the f nucleus (18). For ferric iron, isomer shifts (relative to sodium nitroprusside) \ have been observed in the range 4.1 to +1.1 mm S'l, and for ferrous iron from -0.1 to +1.6 mm S-'. i; I Quadrupole splitting into a two-line spectrum occurs when the iron nucleus he field consists of two parts, (1) that f is in an asymmetric electric field. i produced by the electrons of the iron atom, including those shared with adjacent 1 atoms, and (2) that resulting from charges of the surrounding atoms; each part J can contribute to asymmetry at the nucleus. Ferric compounds shm quadrupole splittings from zero to about 2.3 nun s-l; here the asymmetry is caused chiefly by charges of the surrounding atoms. Ferrous compounds show larger values, frm zero Ferrous iron can exist in either a high-spin or a low-spin J 1 i I r I to - 3.3 mm s-I.

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