Mg-doped TlBa2(Ca2-xMgx)(Cu2.2Ti0.8)O10-δ (x = 0, 0.05, 0.15, 0.25, 0.35) superconductors

Abstract

Ti-doped TlBa2(Ca2-xMgx)(Cu2.2Ti0.8)O10-δ (x = 0, 0.05, 0.15, 0.25, 0.35) samples are synthesized at 860 °C by two step solid state reaction method. All these samples have shown metallic variations of resistivity from room temperature down to onset of superconductivity. The room temperature resistivity systematically increases with increased Mg contents in the final compound. The onset temperature of superconductivity [Tc(onset)], as observed in the resistivity and susceptibility measurements, and the zero resistivity critical temperature are found to increase for Mg-doping of x = 0.05 and 0.15; however, the values of these parameters are suppressed for Mg-doping of x = 0.25 and 0.35. Magnitude of the superconductivity also increases for Mg-doping of x = 0.05 and 0.15, whereas its values decrease for Mg-doping of x = 0.25 and 0.35. All TlBa2(Ca2-xMgx)(Cu2.2Ti0.8)O10-δ (x = 0, 0.05, 0.15, 0.25, 0.35) samples have shown orthorhombic crystal structure and the c-axis lattice parameter is systematically suppressed with the increase of Mg-doping. In the phonon modes assignment as observed from Fourier transform infrared spectrometer absorption measurements, the planar oxygen mode is softened, whereas the apical oxygen mode of type Tl-OA-Cu(2)/Ti is hardened with increased Mg-doping. The Oδ mode of oxygen atoms increases in intensity with increased Mg incorporation in the final compound. It is most likely that the density of phonon modes increases till mg-doping of x = 0.15, but for x = 0.25 and 0.35 the c-axis length decreases further, which may enhance the population of an-harmonic modes of oscillations. Such competitive modes of oscillations are most likely produced due to the presence of two different masses existing in the conducting CuO2/TiO2 planes, i.e., Ti (47.90 amu) and Cu (63.54 amu). Since the distance between the planes decreases with increased Mg-doping, the equilibrium population of the phonon modes in CuO2/TiO2 planes may also change and hence the Tc and magnitude of superconductivity. These observations strongly suggest the existence of electron-phonon interactions essential for the mechanism of high Tc superconductivity.