VARIABLE-TEMPERATURE MOSSBAUER-SPECTROSCOPY OF NANO-SIZED MAGHEMITE AND AL-SUBSTITUTED MAGHEMITES

Synthetic aluminum-substituted maghemite samples, gamma-(Fe1-xAlx)(2)O -3, have been prepared by thermal decomposition of Al-lepidocrocite (g amma-Fe1-xAlxOOH), with x = 0, 0.04, 0.06, 0.14 and 0.18. The particle s are needle-shaped and the mean crystallite diameter along the [311] crystallographic direction was found to be between 2.0 and 5.0 nm. Mos sbauer spectra were collected at 6 K and from 80 K up to 475 K at step s of 25 K. In a wide range of temperatures the spectra of the non-subs tituted sample consist of a superposition of a broad sextet and a supe rparamagnetic doublet, whereas for the Al-maghemites this range is muc h smaller. From the temperature variation of the fractional doublet ar ea two different parameters were defined: the temperature correspondin g to a 50/50 doublet-sextet spectrum (T-1/2), and the temperature belo w which The doublet ceases to exist (T-0). These two parameters (T-1/2 and T-0) decrease from 390 K and 92 K (Al-free sample), to 118 K and 64 K (4 mole % Al) and to 100 K and 48 K (18 mole % Al), respectively. The average hyperfine fields at 6 K undergo a steep drop in going fro m the Al-free sample ((H) over bar(hf), = 506 kOe) to the sample with 4 mole % Al ((H) over bar(hf) = 498 kOe), but for higher substitutions the effect is much smaller. The A- and B-site quadrupole splittings, obtained from the data between 220 K and 475 K, were found as: Delta E (Q,A) = 0.86 +/- 0.04 mm/s and Delta E(Q,B) = 0.65 +/- 0.04 mm/s for t he 4 mole % Al sample. The characteristic Mossbauer temperature, deter mined from the temperature dependence of the average isomer shift, was found to be in the range of 500-600 K.