Structural analysis, magnetic and transport properties of the (Ru_1−xCo_x)Sr₂GdCu₂O₈system

Abstract

The effects of Co substitution on the structural and superconducting properties of the RuSr 2 GdCu 2 O 8 compound have been studied. Rietveld refinements of the x-ray diffraction patterns indicate that the cobalt ion progressively replaces ruthenium sites. This replacement induces significant changes on the crystal structure and on the magnetic and superconducting properties. The effects of Co substitution on the superconducting behaviour, and more particularly on the changes induced by the hole doping mechanism, were investigated in (Ru 1−x Co x )Sr 2 GdCu 2 O 8 by a 'bond valence sum' analysis with Co content from x = 0.0 to 0.2. The weak ferromagnetic transition at T M = 138.2 K is shifted to lower temperature, and suppressed at higher Co content. From the crystallographic point of view the Ru-O(1)-Cu bond angle, associated to the rotation of the RuO 6 octahedra around the c-axis, remains essentially constant when Ru is substituted by Co. Furthermore, increasing Co content has the effect of increasing the weak ferromagnetic moment, which may be interpreted as the main feature responsible for breaking the delicate balance between magnetic and superconducting ordering.

Evidence for Local Structural Distortion and Mixed States of Fe in <i>β</i>-NaFeO₂: A Bond Valence Sum Analysis

“…Consequently, a longer ionic bond is the result of the weak electrostatic field generated by less number of charges around its central ion. Hence, the appreciable smaller value of the bond valence for Fe ion indicates the disparity of formal charge distribution from its ideal oxidation state and hence was taken to be primary evidence of the lower charge distribution around Fe sites [4][5][6]. As suggested in our previous study, we roughly estimated that the oxidation state of iron ion in this compound was 2+ [16] and for that reason, we propose that the Fe sites are preferentially occupied by Fe 2+ or Fe 3+ .…”

“…In this paper, the correlation between the interatomic distances and the valence state of Fe ions due to the deficiency of sodium atom in β-NaFeO2 will be reported. We find that the sodium deficiency may lead to modify the angle of corner-sharing oxygen between the tetrahedra of FeO4 and NaO4, yield to mixed valence state of ions [4][5][6].…”