High-temperature phase changes in RuSr 2GdCu 2O 8 and physical properties

Abstract

A sol–gel method was successfully applied to synthesize RuSr 2GdCu 2O 8 (Ru-1212) as a single phase. The crystallization of Ru-1212 arises at 950 °C under O 2 flow with a small amount of secondary phase. The pure Ru-1212 phase is achieved after heating at 1020 °C in O 2 flow. The X-ray diffraction (XRD) pattern is refined (Rietveld) in the tetragonal space group P4/mmm with lattice parameters a=3.83904(9) Å and c=11.5678(4) Å. In situ high-temperature XRD and differential thermal analysis coupled with thermal-weight measurement show a structural decomposition of the phase at T d=1050 °C followed by a partial melting at T m=1118 °C. The decomposition produces crystallized Sr 2GdRuO 6, SrRuO 3 phases and a mixture rich in copper containing “Sr, Gd, Cu, O” that does not diffract X-ray. This phase reduces to Cu + at T m with an important weight loss and a significant amount of liquid. The grain size and/or inappropriate grain boundaries of the pure phase treated below T m does not permit to detect in sol–gel samples superconductivity otherwise observed in compounds prepared by solid-state reaction.