D.E. Morris

Stability of 124, 123, and 247 superconductors

Abstract The 124 phase YBa2Cu4O8 is synthesized by solid state reaction at 930°C in P(O2)>62;30 bar, while the 247 phase Y2Ba4Cu7O15−x is favored in the narrow range of P(O2) between 10 and 30 bar. The 123 phase YBa2Cu3O7−δ forms at P(O2) 62;6.6) may be thermodynamically unstable to decomposition into 124 (plus non-superconducting compounds) at all temperatures, with synthesis of 124 at low temperatures and oxygen pressure limited only by kinetics. Since very high oxygen pressures are not required, synthesis of the 124 and 247 phases is relatively easy, this may have considerable practical importance.

HgSr2CuO4+δ: A new 78 K superconductor by Mo substitution

Abstract A new series of strontium-based (Hg, Mo)-1201 superconducting compounds has been synthesized. The highest Tc of the series (78 K) is found for the nominal composition (Hg0.85Mo0.15)Sr2CuO4+δ, with lattice constants: a = 3.787 A and c = 8.844 A . X-ray diffraction and SEM with EDX analysis confirm that Hg and Mo are indeed incorporated in the structure. High-pressure oxygenation at 250 bar and 325°C, and also at 1 bar, reduces the Tc indicating that the samples are overdoped. The a-axis increases and c-axis decreases with increasing Mo concentration, suggesting that Mo substitutes in the Hg-O layer rather than in the CuO2 layer.

(Mg,Pb)Sr2(Y,Ca)Cu2O7, a new 60 K superconductor in the Pb-1:2:1:2 family

Abstract (Mg,Pb)Sr2(Y,Ca)Cu2O7, a new superconductor in the Pb-1:2:1:2 family, has been synthesized. The Y:Ca ratio and annealing conditions (oxygen pressure/temperature) have been optimized. Addition of silver oxide aids in increasing the superconducting volume fraction and raising Tc to 60 K. Since this Tc is comparable to that of other Pb-1:2:1:2 compounds, it is unlikely that Mg2+ replaces Cu in the CuO2 planes, in spite of its small radius ( r=0.72 A ); in that case a substantial depression of Tc would be expected. Instead, Mg2+ appears to go into the PbO layer in a rock-salt surrounding, in analogy with other divalent elements.

Dependence of the oxygen isotope effect upon hole density in Bi2Sr2Ca1−xYxCu2O8+δ and Y1−xCaxBa2Cu4O8

Abstract We have investigated the dependence of the oxygen isotope effect on the hole density in the high- T c cuprates Bi 2 Sr 2 CaCu 2 O 8 and YBa 2 Cu 4 O 8 . The hole density n was varied by partial replacement of Ca by Y (hole acceptor) in Bi 2 Sr 2 CaCu 2 O 8 , and by partial replacement of Y by Ca (hole donor) in YBa 2 Cu 4 O 8 . In both cases the dopant was introduced into the eightfold coordinated site between the two CuO 2 sheets. The hole concentration could be varied to reach maximum T c and upon further doping T c decreased. The oxygen isotope effect α 0 was found to be strongly correlated to n and T c . When n is optimized to give maximum T c ,α 0 is very small, close to zero and α 0 increases rapidly when n is either less than or greater than the optimum.

Destabilisation of 124 and 247 phases of Y-Ba-Cu-O by Fe substitution at elevated oxygen pressures

Abstract Single phase iron-substituted 124, (YBa2Cu4−xFexOy) has been obtained in the range of 0≤x≤0.05 at oxygen pressure of 200 bar and 100 bar. Greater Fe substitution at elevated P(O2) destabilised the 124 and 247 phases in favor of a tetragonal 123 phase (FeT-123). The solubility of Fe in FeT-123 phase increases with oxygen pressure. The lattice parameters do not change significantly with Fe content and P(O2). The formal average cooper valency (FACV) of single phase FeT-123 samples is high (e.g. FACV=2.58 for x=0.65). The reduction of Tc caused by Fe substitution in YBa2Cu3−xFexOy is about 6 K/%x over the range (0≤x≤0.2) for samples prepared at P(O2)=200 bar and cooled slowly under pressure.

Negative oxygen isotope shift in Bi-2223 (Bi1.6Pb0.4Sr2Ca2Cu3O10) with Tc ≈ 108 K

Abstract Oxygen isotope shift experiments were performed on several series of high temperature superconductors doped with various ions (isovalent and non-isovalent) at different sites. Among these was Bi-2223 which contains three CuO 2 sheets and has T c ≈ 108 K. Bi-2223 samples were substituted with 18 O to the extent of ≈90%. The oxygen isotope T c s were measured with high precision (resolution 2.5 mK, reproducibility 10 mK). We find Δ T c = -0.14±0.02K. Extrapolating to 100% exchange this yields α o =-0.013. In other high temperature superconducting compounds which we have also studied, substantial oxygen isotope shifts even above 70 K are found, but α o does not rise with decreasing T c in all cases. Changes in T c and α o can be correlated with the total hole concentration in several systems. Comparing different cuprate superconductors there appears to be an overall monotonic trend to lower values of α o with increasing T c and number of copper layers. This trend continues even above 100 K in Bi-2223 where α o becomes negative. Our results strongly suggest that phonon mediated electron pairing plays a vital role in high temperature superconductivity with near cancellation of α o by opposing effects when T c is high.

Studies on (Hg, Bi)-1212 A 92 K superconductor

A series of (Hg, Bi)-1212 superconducting compounds has been synthesized. The highest Tc of the series (92 K) is found for the composition (Hg0.67Bi0.33)Sr2(Y0.5Ca0.5)Cu2O7-δ, with lattice constants: a=3.811 A and c=12.002 A. X-ray diffraction and SEM with EDX analysis confirm that Hg is indeed incorporated in the structure. High-pressure oxygenation at 250 bar and 400°C changes the demagnetization curve, adding a second transition at ∼42 K; the 92 K transition remains unchanged. This behavior is explained in terms of phase segregation into oxygen “rich” and “poor” phases.

Stability of Ba, Sr and Ca cuprate phases as a function of oxygen pressure

Abstract Alkaline earth-copper oxide binary mixtures were reacted in elevated oxygen pressure up to 200 bar in the temperature range 880–950°C. The phases which were formed have been identified by X-ray powder diffraction analysis. The compounds which become stabilized at higher P (O 2 ) are Ba 2 Cu 3 O 5+ δ , Sr 2 CuO 3+ δ , (Sr 14 Cu 24 ) 1- x O 41 and Ca 4 Cu 5 O 10 . These compounds have formal copper valence (FCV) in the range of 2.4 to 2.8 and are therefore favored at higher P (O 2 ) over the ABO 2 and A 2 BO 3 compounds obtained at 1 bar which have FCV=2. The transition pressures to the more oxidized cuprates are ∼10 bar for Ca 2 CuO 3 /Ca 4 Cu 5 O 10 , 17.5 bar for BaCuO 2 /Ba 2 Cu 3 O 5+ δ and 40 bar for SrCuO 2 /(Sr 14 Cu 24 ) 1- x O 41 at 880–950°C.

Isotope effects and possible pairing mechanism in optimally doped cuprate superconductors

We have studied the oxygen-isotope effects on Tc and in-plane penetration depth l ab(0) in an optimally doped three-layer cuprate Bi1.6Pb0.4Sr2Ca2Cu3O101y (Tc;107 K). We find a small oxygen-isotope effect on Tc (a O50.019), and a substantial effect on l ab(0) @Dlab(0)/l ab(0)52.560.5%#. The present results along with the previously observed isotope effects in single-layer and double-layer cuprates indicate that the isotope exponent a O in optimally doped cuprates is small while the isotope effect on the in-plane effective supercarrier mass is substantial and nearly independent of the number of the CuO2 layers. A plausible pairing mechanism is proposed to explain the isotope effects, high- Tc superconductivity, and tunneling spectra in a consistent way. The pairing mechanism responsible for high-T c superconductivity is still controversial. In conventional superconductors, a strong effect of changing ion mass M on the transition temperature Tc implies that lattice vibrations ~phonons! play an important role in the microscopic mechanism of superconductivity. An isotope exponent a(52d ln Tc /d ln M )o f about 0.5 is consistent with the phonon-mediated BCS theory. A nearly zero oxygen-isotope effect ( a O.0.03) was earlier observed in a double-layer cuprate superconductor

Phase diagram and new phases in the Y-Ba-Cu-O system at high oxygen pressure

Abstract The Y-Ba-Cu-O phase diagram has been determined at 980°C at high oxygen pressure 200 bar=20 mPa. It shows several differences from the phase diagram at 1 bar. Highly oxidized phases are stabilized: BaO2 instead of BaO and YBa2Cu4O8 instead of YBa2Cu3O7−x. Two new phases have been observed: YBa5Cu2O10−x has a tetragonal unit cell with a=5.88 A and c=8.04 A, consistent with a four layered structure such as CuO2-BaO-( 1 2 Y + 1 2 Ba)O-BaO, while YBa2Cu2O6−x appears to be isomorphous with the superconductor La1.6Sr0.4CaCu2O6. In addition, we find a high-pressure BaCuO∗x phase, its X-ray diffraction (XRD) pattern indicates a perovskite related layered structure which has a unit cell a=5.68 A, c=14.40 A. The formation of compounds on the YO1.5-BaO line: Y2Ba2O5, Y2BaO4 and Y2Ba4O7 is suppressed at high P(O2). Tie lines are observed to join CuO-2:1:1, 2:0:2-2:1:1, YO1.5-2:1:1, 2:1:1-BaO2, CuO-1:2:4 and 2:1:1-1:2:4.