Chalcogenides of transition metals (M) with the formula M7Se8 have a layered structure of the NiAs-type. Selenium ions form a hexagonal close-packed structure, and the metal cations occupy octahedral voids. The position of the cations and the anions in this structure is unequal. It is known that the compound Fe7Se8 depending on the preparation conditions and the position of different types of ordering may be implemented or superstructure type 3C or 4C with tripled or quadruple periods in a direction perpendicular to the layer plane, respectively. Under heating to the critical temperature of the compound Fe7Se8 has been observed a structure phase transition of the "order-disorder" associated with the disordering of vacancies in the cation layers. The compound Fe7Se8 is a ferrimagnet with the Neel temperature of 450 K. The
magnetic moments of iron atoms are ferromagnetically arranged inside each layer, but coupled antiferromagnetically between adjacent layers. The presence of vacancies in every second iron layer leads to incomplete cancellation of magnetic moments and
hence to the appearance of a resultant magnetization and ferrimagnetism in Fe7Se8. Therefore, the resultant magnetization is a good tool for study of the distribution of vacancies in cation layers. The presence of vacancies in the cation layers and their
ordering, as well as substitution on sub lattice, has a strong influence on the physical properties of these compounds. The substitution of iron atoms with cobalt in Fe7-yMySe8 to the range of concentrations y = 7, leads to a strong decrease (to ~ 10% for y
= 7) in the lattice parameter c0, characterizing the mean interlayer distance. Substitution of titanium atoms in the limited concentration of y = 3, distance between layers is observed increasing with titanium concentration. Magnetic measurements of Fe7-yMySe8 samples (M = Ti, Co) are shown that the magnetic ordering temperature decreases monotonically with substitution, in spite of fundamental differences in the behavior of the interlayer distance in these systems. From the data obtained it was found that both titanium and cobalt substituting Fe atoms in compounds Fe7-yMyX8 act as a diluent iron magnetic subsystem, as they have zero or very small magnetic moment. Under slow cooling compound V7Se8 crystallizes in the monoclinic system with a superstructure 4C. According to the magnetic properties of the compound V7Se8 is either anti ferromagnet, or Pauli paramagnetic. In the
literature, there is no studying on the effect of substitution of Fe atoms of V atoms in compounds Fe7-yVySe8. |
