SUPERSTRUCTURE FORMATION AND MAGNETISM OF SYNTHETIC SELENIAN PYRRHOTITES OF FE-7(S1-YSEY)(8), Y-LESS-THAN-OR-EQUAL-TO-1 COMPOSITION

Synthetic pyrrhotites of composition Fe-7(S1-ySey)(8) were synthesized at 423 and 873 K and then quenched in cold water. It is shown from X- ray diffraction and indirectly from Superconducting Quantum Interferen ce Device (SQUID) magnetometry measurements that vacancy ordering give s superstructures (of 3C and 4C type) only close to the end-member com positions (y < 0.15; 0.85 < y); however, for intermediate compositions , short-range ordering still occurs as a result of vacancy repulsion. The characteristic spin reorientation, which takes place at 130 K in F e7Se8 (3C), shows spins turning form the ab-plane to the c-axis with l owering temperature; it is depressed in temperature when Se is substit uted for by S and inhibited for y <approximate to 0.6. At room tempera ture, all Mossbauer spectra of Fe-7(S1-ySey)(8) can be fairly well fit ted using four sextets related to different vacancy formations in the neighbourhoods of the Mossbauer atoms. Spectra recorded at low tempera ture show broad absorption profiles. The continuous broadening of the spectra with lowering temperature points to dynamic processes such as electron hopping. A simple model is proposed for the observed order-di sorder phenomena and the mineralogical implications are discussed.