Dedicated to Professor Dr. K. W. BOER on the occasion of his 70th birthday Measurements of dc and ac susceptibility, resistivity, Hall effect, and thermoelectric power (TEP) on pure phase Hg-1223 and mixed phase Hg-1212-1223 cuprates are reported. The mixed phase compounds show the highest critical temperature (magnetic susceptibility onset of 138 K). Both Hall effect arid TEP data support an underdoped state with hole-like conduction. Oxygen annealing reduces the Hall coefficient (increasing the concentration of holes) and lowers the TEI' (increasing the Fermi energy) features which can be explained by a simple Fermi-gas picture. The Hall coefficient which increases linearly with temperature and the negative temperature coefficient of the TEP, however, cannot be explained by single-hand Fermi-gas or -liquid theory, unless one introduces energy dependent carrier scattering. Estimates of the highest critical temperature based on the universality observed in the thermopower data for the cuprates suggest that 138 K is near the maximum one might expect for the Hg cuprates under optimal doping at atmospheric pressure.
Polycrystalline samples of Tl-based-1223 superconducting system were prepared with tin-addition to form (Tl 0.5 Pb 0.5 )SnxSr 1.6 Ca 2.4 Cu 3 Oy, for x = 0 -0.3. X-ray diffraction data showed the presence of a minor 1212 phase and impurities which increased with increasing x while the samples without addition of tin, x = 0, were nearly single phase of Tl,Pb-1223. The effects of tin addition to Tl,Pb-1223 on the magnetic properties were studied by dc susceptibility and hysteresis magnetization measurements. The magnetic results revealed a significant enlargement of hysteresis magnetization in tin-added samples with an onset transition temperature drop of only 1-4 K. According to recent measurements, the intragrain Jc reflects the transport critical current density in high field region. The critical current density in Tl,Pb-1223 was enhanced by as much as ten-times by tin-addition. An electron micro-probe technique was employed to locate the added tin in the samples, but Sn incorporated into the superconducting phase was below detectability. The variation in Tc with Sn addition and a reduction in pinning strength at Sn concentration above x = 0.05 show that the enhanced pinning is due to introduction of pinning centers into the superconducting phase even though micro-probe analysis indicates that most of the Sn is present in a nonsuperconducting phase. The field dependence and time dependence of the magnetization data showed an increased concentration of pinning centers without a change in pinning energy.
The addition of Sn in Tl,Pb(1223) (Tl 0.5 Pb 0.5 SnxSr 1.6 Ca 2.4 Cu 3 Oy) has been investigated using AES and Mössbauer Emission techniques to probe the valence and site geometry of Sn cations. It was found that Sn is present in tetravalent state in the compound at two sites with differing weights. The major part of the tin resides inside the grain and a minor part (weak doublet line in Mossbauer spectra) represents the contribution from the tin at the grain boundary.
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