AbstractsChemistry

The present thesis consists of six publications and a summary of the experimental results reviewed together with relevant literature data. Two transition metal oxide (TMO) systems were investigated: Sr2Qn−1B2O3n+1−δ (Q = Y, Ca, Sr; B = Co, Fe; n = 1, 2, 3 and ∞) compounds with a Ruddlesden-Popper (RP) crystal structure and the InFeMO4 (M = Mg, Co, Ni, Cu and Zn) compounds with a spinel structure (for Mg, Co and Ni). For both systems, polycrystalline samples were synthesized and characterized for magnetic and (magneto)transport properties which were then interpreted in terms of the site-specific cation composition, oxygen content and the oxidation state of the transition metal species. The interplay between oxygen stoichiometry and structural properties was studied for the RP oxides, Sr2(Ca,Y)Co2O7−δ. It was shown that the maximum value of oxygen content significantly depends on the average valence of the (Y,Ca)-site cations, increasing with increasing YIII-for-CaII substitution level. The strong impact of oxygen content on transport properties was confirmed, as the highly oxygenated samples exhibited significantly lower resistivity (by several decades) compared to the oxygen-depleted samples. In addition, concentration of oxygen vacancies in the (Ca,Y)O layer was observed to have a large effect on the magnetic properties, as filling the vacancies facilitated the emergence of ferromagnetic order and increased the strength of the magnetoresistance (MR) effect. The tendency of the RP oxides to absorb additional layers of water was studied for the Srn+1FenO3n+1−δ series (n = 1, 2, 3 and ∞) by subjecting the samples to humidity under carefully controlled conditions. In addition, the intercalation reaction was also studied under extreme exposure by directly immersing some samples into distilled water. As result, novel layered water-derivative phases were obtained and identified by means of powder XRD measurements. The importance of the (SrO)2 double layer of rock-salt structure was demonstrated, as it was shown that the n = ∞ member lacking the layer did not absorb water. Moreover, it was revealed that the rate of the derivative-phase formation depends on n and also on the concentration of oxygen vacancies. The narrow overall cation composition ranges and the distribution of each cation species between the two cation sites in the InFeMO4 (M = Mg, Co, Ni, Cu and Zn) system were studied by means of ICP measurements and Rietveld refinement of powder XRD data. The impacts of cation stoichiometry and magnetic dilution on the magnetic properties were investigated as well. In addition, a novel fitting method for the Mössbauer spectra of the cation-disordered spinel phases was developed in order to more accurately determine the cation distribution, iron oxidation state and magnetic order in these phases.