Physical properties of A₂Cu₃O₄X₂ (A = alkali-earth elements; X = halogen) with the Cu₃O₄ plane

First, we have studied the stability of Sr_2-xBa_xCu₃O₄Cl_2-yBr_y of the 2342 phase with the Cu₃O₄ plane, which is composed of the Cu(1)O₂ and extra Cu(2) sublattices, in the temperature range between 600 and 950 °C. It has been found that a mixture of (Sr,Ba)₂CuO₂(Cl,Br)₂ with the CuO₂ plane and CuO becomes stable instead of the 2342 phase with the Cu₃O₄ plane for smaller values of x and y at low temperatures. Next, the electrical and magnetic properties of Sr_2-xBa_xCu₃O₄Cl_2-yBr_y with the Cu₃O₄ plane have been investigated. The electrical resistivity exhibits semiconductive behavior. The magnetic susceptibility shows three-dimensional antiferromagnetic order below T_N(1) = 300-400 K and T_N(2) to approximately 30 K for the Cu(1) and Cu(2) sublattices, respectively. A broad peak due to the two-dimensional antiferromagnetic order in the Cu(2) sublattice is also observed around T_N^2D(2) = 100 K. With increasing x and y, the lattice parameters increase, the electrical resistivity increases and values of both T_N(1) and T_N^2D(2) decrease. These results are explained as being due to the decrease of the overlaps between Cu3d and O2p, Cl3p, Br4p orbits.