Studies of structures and phase transitions in pyrrhotite (Dissertation )
CHAPTER 1 GENERAL INTRODUCTION
The compounds in the Fe,.^S system (x=0-0.13), in mineralogy, are collectively called pyrrhotite, and the 1:1 stoichoimetric iron sulfide (FeS) with aVs Xa/3 x2 superstructure is particularly called troilite. Because of its important structural, electrical, magnetic and thermal properties, for over one century this system has been the subject of considerable interest for a vade range of disciplines.
The compounds in the Fe,.^S system (x=0-0.13), in mineralogy, are collectively called pyrrhotite, and the 1:1 stoichoimetric iron sulfide (FeS) with aVs Xa/3 x2 superstructure is particularly called troilite. Because of its important structural, electrical, magnetic and thermal properties, for over one century this system has been the subject of considerable interest for a vade range of disciplines.
The prototypic structure of this system is NiAstype, but a number of structure - and superstructure types resulting from the lowtemperature vacancy ordering and structural distortions have been reported in this narrow composition range between room temperature and 700K. The most abundant pyrrhotites in nature are the monoclinic (4C), troilite (2C) and the intermediate pyrrhotites. The latter tend to be more wide-spread and can be generally categorized as nC type, of which the 5C, 6C and 11C types are special cases (in this thesis A and C refer to axial lengths of NiAs-type cell, A=3.44A and C=5.70A).
Table 1 lists the superstructures reported for the natural pyrrhotites [1], Apart from these crystallographic modifications, at least two magnetic transitions and one electrical transition are known to be involved in this system below 600K: At 600K the order-disorder transition (the so called ;5-transition) for magnetic spins occurs, which, in fact, is a Curie transition for the ferrimagnetic phases and a Neel transition for the antiferromagnetic phases. This magnetic transition is insensitive to composition. Below T^. a second magnetic transition (called the Morin transition) occurs for which the magnetic spins rotate from the orientation lying in the a-b plane for T > Tp,, to the orientation parallel to c-axis for T < T^,. This change in spin orientation has been proved by neutron diffraction and the measurement of the anisotropy of the magnetic susceptibility as a fimction of temperature [2-4].
The temperature point for this transition is sensitively dependent on the composition. For FeS with the troilite structure type, T^ is found to be at 135°C, but theoretical and experiential investigations indicate that the transition temperature dramatically decreases with increasing iron deficiency [5-7].
On the other hand, the electrical properties also have a direct relationship with the compositions. At room temperature, troilite is considered to be a ferroelectric semiconductor, while Fe7Sg exhibits a high conductivity. The former undergoes a so-called a-transition at a temperature close to T^ (~120°C) for which the material is transformed into a paraelectric conductor with a sharp increase of conductivity in the c direction. With an increase of x in Fe,.^S, as for T^, the temperature for this atransition declines almost linearly [8,9].
To download the dissertation click on the link below:
No comments