Antiferromagnetic order in Ca₂CuO₂Cl₂, Sr₂CuO₂Cl₂ and Sr₂CuO_3+x

The magnetic order in the tetragonal M₂CuO₂Cl₂, M=Ca,Sr and in Sr₂CuO_3+x compounds was studied by using Mossbauer spectroscopy of 0.5% ⁵⁷Fe-doped samples. These compounds crystallize in a K₂NiF₄ - type tetragonal structure, isostructural to La_2-xSr_xCuO₄ superconductors. In this structure, one crystallographic site exists for copper atoms, which form CuO₂ planes. Sr₂CuO₃ prepared under ambient pressure has an orthorhombic structure, while the tetragonal Sr₂CuO_3+x is obtained when prepared under high oxygen pressure. Both copperoxychlorides are antiferromagnetically ordered with T_N≈260K, very similar to T_N in La₂CuO₄ or Nd₂CuO₄. The strontium-cuprates are magnetically ordered at 4.2K with T_N below 10K.