Decoupling of CuO2 Planes and Superconductivity in (Cu0.5Tl0.5)Ba2Ca3Cu4O12-δ

(Cu0.5Tl0.5)Ba-2(Ca3-xMgx)(Cu4-yZny)O12-delta (x = 0, 2; y = 0, 3) and (Cu0.5Tl0.5)Ba-2(Ca3-xSrx)Cu4O12-delta (x = 0, 1, 2, 3) samples have been synthesized at normal pressure by two-steps solid state reaction method and characterized by X-ray diffraction (XRD), resistivity measurements, and Fourier transform infrared (FTIR) absorption measurements techniques. These studies have shown that higher order CuTl-1234 phases cannot be stabilized in Mg doped samples. However, higher order CuO2 plane phases can be stabilized with the doping of Sr at the Ca sites in (Cu0.5Tl0.5)Ba-2(Ca3-xSrx)Cu4O12-delta (x = 0, 1, 2, 3) superconductors. Doping of Sr at the Ca sites promoted decoupling of CuO2 planes and resulted into new exciting high T-c phases. The XRD analysis of diffraction scans of the samples have shown increase in c-axis lengths with increased Sr doping concentration which evidenced the enhanced inter-planar distance and hence enhanced inter-planar decoupling. The onset temperature of superconductivity and zero resistivity critical temperature suppressed with the increased Sr doping in (Cu0.5Tl0.5)Ba-2(Ca3-xSrx)Cu4O12-delta (x = 0, 1, 2, 3) samples. Intrinsic doping of Sr in (Cu0.5Tl0.5)Ba-2(Ca3-xSrx)Cu4O12-delta (x = 0, 1, 2, 3) unit cell is confirmed by softening of apical oxygen mode of type Tl-O-A-Cu(2) and hardening of apical oxygen mode of type Cu(1)-O-A-Cu(2) and CuO2 planar oxygen mode. The decoupling of the CuO2 planes is also confirmed by the decrease in value of real and imaginary parts of dielectric constant and ac-conductivity with increased Sr concentration in the final compound. In the excess conductivity analysis of conductivity data, the coherence length along c-axis xi(c), inter-layer-coupling J, and the Fermi-velocity v(F) of the carriers are enhanced whereas the London penetration depth (except of samples with x = 1), B-c(T), B-c1(T), B-c2(T), J(c) suppressed with increased Sr doping. Ginzburg-Landau parameter kappa, however increased with the doping of Sr in the final compound. The decrease in the values of these parameters expressed the suppression of flux pinning characteristics of Sr-doped samples. The energy required to break-apart the Cooper-pairs increased but the the phase relaxation time of the carriers suppressed with increased Sr incorporation in the final compound.