Sequence of phase transformations and inhomogeneous magnetic state in nanosized Sr<Subscript>2</Subscript>FeMoO<Subscript>6 – δ</Subscript>

We have studied the effect of synthesis conditions on the phase composition, grain size and morphology, degree of superstructural ordering, and magnetic properties of the Sr₂FeMoO_6–δ double perovskite. The results demonstrate that its magnetic state, dependent on the nonuniformity of grain morphology and the degree of superstructural ordering of the iron and molybdenum cations, correlates with the initial solution pH. Analysis of the sequence of phase transformations during strontium ferromolybdate crystallization in the citrate gel process from a pH 4 starting solution allowed us to propose combined conditions that ensure the preparation of single-phase Sr₂FeMoO_6–δ powder with an average grain size in the range 50–120 nm and the highest degree of superstructural ordering of the iron and molybdenum cations: P = 88%.     

Phase Transformations in the Systems Y2BaCuO5–“Ba3Cu5O8” and Y2BaCuO5–BaCuO2

Data are presented on the sequence of phase transformations leading to the formation of YBa₂Cu₃O_{7 –} textured ceramics and single crystals in the systems Y₂BaCuO₅–Ba₃Cu₅O₈ and Y₂BaCuO₅–BaCuO₂. During cooling in the Y₂BaCuO₅–BaCuO₂ system, YBa₂Cu₃O_{7 –} crystallization in the range 1260–1210 K occurs through the intermediate phase YBa₄Cu₃O_{9 –} , without an additional oxygen source. In the Y₂BaCuO₅–Ba₃Cu₅O₈ system between 1250 and 1210 K, YBa₂Cu₃O_{7 –} crystallization is accompanied by oxygen absorption.     

Phase Transformations in the Y2Cu2O5–BaCuO2 System

The sequence of phase transformations in the Y₂Cu₂O₅–BaCuO₂ pseudobinary system was studied during heating and cooling in the range 1170–1310 K. The results demonstrate that the reaction zone in BaCuO₂/Y₂Cu₂O₅ diffusion couples consists of BaCuO₂/YBa₂Cu₃O_{7 –} /Y₂BaCuO₅/Y₂O₃/Y₂Cu₂O₅ layers, corresponding to the sequence of chemical changes during YBa₂Cu₃O_{7 –} crystallization between 1170 and 1220 K. In the range 1260–1310 K, BaCu₂O₂ is formed. During cooling of a Y₂Cu₂O₅ + 4.3BaCuO₂ mixture, YBa₂Cu₃O_{7 –} crystallizes in a wide temperature range, between 1240 and 1190 K. The process depends on the presence of BaCu₂O₂ on the surface of Y₂BaCuO₅ grains in the high-temperature solution and the oxygen supply from the gas phase.     

Dielectric barrier formation and tunneling magnetoresistance effect in strontium iron molybdate

A comparative X-ray diffraction study of the initial single-phase metal-oxide compound-strontium iron molybdate Sr₂FeMoO_{6 ? δ} (SFMO)-and that subjected to additional isothermal annealing shows that this heat treatment leads to the appearance of a SrMoO₄ (SMO) phase. Small-angle neutron scattering measurements indicate that the SMO phase forms a dielectric shell surrounding SFMO grains, which has a characteristic thickness of 2–4 nm and extends above 120 nm. The character of the temperature dependence of the electric resistance corresponds to the metal-type conduction in single-phase SFMO and changes to a semiconductor type in the material with SMO dielectric shells, which is evidence of a tunneling mechanism of charge transfer. This conclusion is confirmed by an increase in the absolute value of the negative magnetoresistance of SFMO due to the appearance of a tunneling magnetoresistance component of the same sign.     

Interrelation among superstructural ordering,oxygen nonstoichiometry and lattice strain of double perovskite Sr2FeMoO6−δ materials

Journal of Materials Science - Double perovskite ceramics Sr2FeMoO6?δ having different amount of antisite disordering and oxygen content are prepared by the solid-phase reaction method...     

Oxygen Exchange in YBa2Cu3O7 – δ

The oxygen exchange in YBa₂Cu₃O_{7 –} materials of different densities was studied by thermogravimetry over wide ranges of temperatures and oxygen pressures. The results were correlated with x-ray diffraction (b – a), T _c (inductance method), and T _c data. According to polarized-light microscopy and coulometric titration results, the annealing conditions have a strong effect on the uniformity of oxygen distribution in the basal plane.     

Electrical Transport and Phenomenological Model of Oxygen Nonstoichiometry in YBa2Cu3O7 – δ

The electrical conductivity of YBa₂Cu₃O_{7 –} is found to change sharply near the T–OII and OII–OI structural phase transitions. This finding is interpreted using Kroger's quasi-chemical reactions and a phenomenological model of oxygen nonstoichiometry which considers the formation of oxygen divacancies at T < 750 K and < 0.5.     

Growth of YBa2Cu3O7 – δ Single Crystals in “Ba3Cu5O8”/Y2BaCuO5 and (“Ba3Cu5O8” + 0.2BaCuO2)/Y2BaCuO5 Diffusion Couples

YBa₂Cu₃O_{7 –} single crystals were grown in Ba₃Cu₅O₈/Y₂BaCuO₅ and (Ba₃Cu₅O₈ + 0.2BaCuO₂)/Y₂BaCuO₅ diffusion couples at temperatures between 1170 and 1270 K under optimized conditions. Copper nonstoichiometry was shown to have a significant effect on the superconducting properties of YBa₂Cu₃O_{7 –} crystals subjected to thermal cycling.