Influence of the synthesis conditions and carbonate content on the properties of Y₂Ba₄Cu₇O_14+x

A study of the thermodynamic, magnetic, structural and physical properties of the Y2Ba4Cu7O14+x (247) phase is presented in this paper. The stability range of the 247 phase in the P-T phase diagram forms a narrow band between that of the 124 and 123 phases. The critical temperature Tc of fully oxygenated ceramic samples varies from 50 to 93 K as the content of the carbonate changes from 6000 to 500 ppm. Susceptibility measurements performed on pure sintered samples with low CO2 content show a Tc onset at 93 K with a sharp transition. At a low magnetic field a shoulder caused by weak links has been observed. NQR measurements of ceramic samples and single-crystal X-ray investigations proved a high structural perfection of the samples with Tc#62;90 K can clearly showed the existence of many defects in the materials with lower Tc. The increase of Tc from 4 to 90 K in the 247 crystals is coupled with a decrease of the apical bond length. © 1994.