Moessbauer and X-ray studies of the oxygen-deficient perovskite-related oxides with 8-8-20 structure

Full text: The cuprates La8-xSrxCu8O20-δ (1.28 <≤ x ≤ 1.92) have metal type of conductivity and very low specific resistance. High electro conductivity featured to these connections, it is caused by presence in their components of the mixed valent states of copper Cu2+-Cu3+. Due to it these structures show metal behaviour. Ability of trivalent iron to acceptance octahedral coordination and to a lesser degree pyramidal, assumes an opportunity to replace copper by iron in the ordered way to create one-dimensional conducting behavior along a c-axis. The selective doping method allows to receive not simply substituted cuprates of type 8-8-20, but also to stabilize complicated oxides of the given structure in systems with others rare-earth elements. On a measure of growth of the iron contents transition from metal to semimetal behavior is observed. The oxygen-deficient perovskite-related oxides with structure 8-8-20 Pr8-ySryCu8-xFexO20 (y = 2.8-3.6, x = 1.6-2.4) have been studied by the 57Fe Moessbauer spectroscopy (room-temper.). It is found decrease of iron concentration results in amplification of magnetic ordering in samples; it arises in spectra as occurrence and growth magnetic components. Moessbauer spectrum of the Pr4.4Sr3.6Cu5.6Fe2.4020 consists of three paramagnetic doublets that are in part kept in spectra of other researched cuprates with arising sextets both. In a spectrum of the Pr5.2Sr2.8Cu6.4Fe1.6O20 sextets compose the basic part. For finding-out of the reasons of such behavior of Moessbauer spectra X-ray researches have been carried out for the studied samples. X-ray patterns have shown one-phase nature of samples. All diffraction peaks are described within the framework of tetragonal symmetry with space group P4/mbm. Thus, the following explanation was offered. It is known, that in oxygen-deficient structures iron atoms prefer to acceptance high-coordination positions, including due to attraction in their placement of additional oxygen atoms from other sites. Hence, each iron atom that has accepted in its site additional oxygen atom simultaneously increases coordination number of the neighbor position so a)so probability of that this position also will be occupied with iron atom. In a result at small iron concentration its atoms form regular distributed clusters, that is in structure areas where substitution of copper by iron increased are formed. In these areas there is a magnetic ordering of the iron atoms causing presence magnetic a component in spectra of samples. At increase of magnetic atoms concentration in a sample there is more equivalent distribution of iron at the structure. We hope the final explanation will be received by additional researches. The present work has been performed with the support of the grants of Tatarstan Academy of Sciences No. 06-6.1-142, CRDF (REC-007), Scientific School-1708.2003.2