Enhancement of Tc by Sr substitution for Ba in Hg-2212 superconductor

Abstract

The Ba substitution by Sr has been studied in two Hg-2212 The y = 0 compound of the first series is the non superconducting Hg 2 Ba 2 YCu 2 O 8−δ prototype. In the second series, this y = 0 compound is already superconducting at 21 K. Indeed the members of this series present a higher charge carrier density in their CuO 2 superconducting planes than their homologues of the first series due to the doping introduced by the substitution of 20 % of Y by Ca. The compounds of both series were synthesized in high pressure (3.5 GPa) -high temperature (950 -1050˚C) conditions. In both cases Sr substitution was successful up to the full replacement of Ba (y = 1.0). The Hg-2212 phases were characterized by XRD, SEM, EDX and a.c. susceptibility.

The effect of Sr substitution on superconductivity in Hg2(Ba1 ySry)2YCu2O8 : I. A neutron powder diffraction study

“…The T c increased from 0 K for the unsubstituted sample (y = 0) to 42 K for full substitution (y = 1.0). This enhancement was not only observed in the Y based Hg-2212 series but also confirmed in a second Hg-2212 series, doped with 20% of Ca on the Y site, where T c increases now from 21 K (y = 0) to 58-60 K (y = 0.7-1.0) [24,25]. Finally, we performed neutron powder diffraction (NPD) analysis on both series to better determine the origin of this T c enhancement.…”

“…As shown by ac susceptibility measurements, both samples, with y = 0.50 and 1.0, are superconducting (see figure 5 in [25]). This is surprising in view of the absence of superconductivity generally found in Hg 2 Ba 2 YCu 2 O 8−δ [3][4][5].…”

“…with a higher doping level. In this second series, T c increases from 21 K (y = 0) to 58-60 K (y = 0.7-1.0) [24,25]. Thus the phenomenon of T c increase seems to be intrinsically due to Sr.…”

“…Nevertheless, even with optimized time conditions, impurities phases were observed (figure 1): HgO, CuO and an unknown phase whose amount increases with the nominal Sr content. This phase containing Hg, Sr and oxygen has been reported to be Sr 0.76 Hg 1.24 O 2 according to x-ray diffraction (see the tick marks in the bottom part of figure 1) and EDX data [24,25]. We assume this phase to have a unit cell close to that of the Ca 0.76 Hg 1.24 O 2 phase identified in Ca rich (Hg, Pb)-2212 compositions [9] and occurring also in Hg-1223 HP-HT synthesis [29,30].…”

The effect of Sr substitution on superconductivity in Hg2(Ba1 ySry)2Y Cu2O8 : II. A bond valence sum approach to the hole distribution

“…This enhancement can also be obtained by mechanical pressure which increases T c for pure Ba based Hg-2212 by 50 K under 20 GPa [5]. Moreover, T c max can be pushed to 60 K by Ca doping in Hg 2 Sr 2 (Y 0.80 Ca 0.20 )Cu 2 O 8−δ [6,7].…”