Crystal structure, phase transitions and magnetic properties of pyrrhotite-type compounds Fe7−xTixS8

A series of Fe7−xTixS8 compounds with Ti concentrations up to x=4 has been synthesized by solid state reactions. The substitution of Ti for Fe in pyrrhotite (Fe7S8) modifies its crystal structure from the initial monoclinic 4C superstructure of the NiAs-type to the hexagonal 3C superstructure at x=1 and then to the 2C monoclinic superstructure at x≥2. The structural phase transition associated with the disordering of vacancies in metallic layers is found to persist in all the compounds. The growth of the Ti content in Fe7−xTixS8 is accompanied by a gradual decrease of the magnetic ordering temperature from ~590 K at x=0 down to 145 K at x=4, by a substantial increase of the low-temperature coercive field (up to 24 kOe) and reduction of the effective magnetic moment from ~5.8μB to 4.3μB. Unlike ferrimagnetic order in Fe7S8, the compound Fe6Ti1S8 shows an antiferromagnetic behavior, while further growth of the Ti concentration above x=2 increases the resultant magnetization up to a value almost in twice higher than the value for non-substituted Fe7S8. Such non-monotonous changes in magnetization behavior with increasing Ti concentration are ascribed to the preferential substitution of Ti for Fe in alternating metallic layers.