Effects of carbon incorporation on doping state of YBa2Cu3Oy

Abstract

Effects of carbon incorporation on the doping state of YBa 2 Cu 3 O y (Y-123) were investigated.Quantitative carbon analysis revealed that carbon could be introduced into Y-123 from both the precursor and the sintering gas. Nearly carbon-free (< 200 ppm) samples were prepared from a vacuum-treated precursor by sintered at 900 ˚C and cooling with 20 ˚C /min in flowing oxygen gas. The lower T c (= 88 K) and higher oxygen content (y = 6.98) strongly suggested the overdoping state, which was supported by the temperature dependence of resisitivity and thermoelectric power. The nuclear quadrapole resonance spectra and the Raman scattering spectra indicated that there was almost no oxygen defect in the Cu-O chain in these samples.On the other hand, in the same cooling condition, the samples sintered in air stayed at optimal doping level with T c = 93 K, and the intentionally carbon-doped sample was in the underdoping state. It is revealed that about 60% of incorporated carbon was substituted for Cu at the chain site in the form of CO 3 2+ , and the rest remains at the grain boundary as carbonate impurities. Such incorporation affected the oxygen absorption process in Y-123. It turned out that the oxygen content in Y-123 cannot be controlled only by the annealing temperature and the oxygen partial pressure but also by the incorporated carbon concentration.

Single crystal growth and physical properties of carbonsubstituted YBa 2 Cu 3 O 7– δ

“…From these results, we can easily conclude that carbon can be introduced into a crystal from atmosphere. In Table 1, we can find that the c-axis length decreases with increasing the carbon substitution, which is consistent with the previous reports [1][2][3][4]. It should be noted that even in "Air" sample, some carbon remains.…”

“…With increasing the carbon content, the crystal structure changes to tetragonal from the original (unsubstituted) orthorhombic one [1,2], and the c-axis parameter decreases [1][2][3][4]. In the early study, it was claimed that C substitution only degrades its superconductivity [1,2], while a further investigation shows that a slight C-substitution makes the crystal difficult to enter the carrier-overdoped region [4]. However all published reports were studies for polycrystals, where we need to consider a possibility that some carbon compounds are localized at the grain boundaries [5], which may act as barriers of oxidation [4].…”

“…In the early study, it was claimed that C substitution only degrades its superconductivity [1,2], while a further investigation shows that a slight C-substitution makes the crystal difficult to enter the carrier-overdoped region [4]. However all published reports were studies for polycrystals, where we need to consider a possibility that some carbon compounds are localized at the grain boundaries [5], which may act as barriers of oxidation [4]. Moreover, it is hard to see the Csubstitution effects on critical current density for poly-crystalline samples, because the degraded grain boundary can interrupt the current.…”