Layered cuprates with the T* structure: Structural and conducting properties

The structural and conducting properties of the oxide La_1.2Tb_0.8CuO_4+δ and its Sr²⁺ and Ce⁴⁺ lightly doped derivatives (T* phases) are examined as functions of temperature. The properties of the isostructural heavily doped La_1.0Dy_0.8Sr_0.2CuO₄ are also investigated. High-resolution powder neutron structural results confirm that the correct space group is tetragonal P4/nmm, with no low symmetry distortions apparent to low temperatures and an essentially perfect lanthanide ordering into two crystallographically inequivalent sites. Excess oxygen (its presence also revealed by thermogravimetric analysis measurements) is accommodated as an O^2- ion in the interstitial space defined by the lanthanum ion bilayers in the rock-salt part of the unit cell. No excess oxygen is needed to stabilize the T* structural type for large Sr²⁺ doping levels, as in La_1.0Dy_0.8Sr_0.2CuO₄. Analysis of the resistivity data shows the occurrence of two-dimensional variable-range hopping, implying that the electronic states at the Fermi level are localized. A crossover to a three-dimensional hopping mechanism occurs with increased carrier density in La_1.0Dy_0.8Sr_0.2CuO₄ at low temperatures, with high temperature transport dominated by small-polaron behavior. The charge carriers are essentially confined within the boundaries of the unit cell for all compositions.