Phase relations and modulated structures of pyrrhotite Fe_1-xS.

Abstract

The phase relations in the range of non-stoichiometric pyrrhotite, Fe^1-xS (0≤x≤0.125) have been studied in detail using the X-ray diffraction technique at high temperatures. During this time, some attractive phenomena have been found, e. g., there are nine closely related superstructures below 320°C and in that range of composition the super-cells are “non-integral” multiples of the NiAs-type fundamental cell. The size of the super-cells is variable corresponding to the change in T-X conditions.
Because the usual procedures of X-ray single crystal structure analysis are not generally appropriate to the non-integral superstructures, an attempt has recently been made to draw qualitative models of the vacancy distributions of the non-integral super-structures on the basis of a four- (or n-) dimensional description of the modulated structure.
Wave distribution of vacancies is a characteristic of the models for the NC- and NA-type superstructures. The waves were successfully observed in high resolution electron micrographs. Further refinement of the NC-type structure has also been successful by using the X-ray intensity data. We can thus understand that the structures shuld be described as four-dimensionally periodic and three-dimensionally nonperiodic structures. The structure variation of pyrrhotite, which is indicated by the displacements of the satellite reflections, depending on temperature and composition, is realized by the change in the period and the direction of the vacancy distribution wave.