Synthesis and structure of Sr_1.5Ba_0.5CuO₂(CO₃): An oxycarbonate containing infinite CuO₂ layers

The synthesis and structure of the layered oxycarbonate Sr1.5Ba0.5CuO2(CO)3, is reported here. The structure determined by time-of-flight neutron powder diffraction both at room temperature and at 4 K is tetragonal, space group P4/mmm, with lattice parameters a=3.9281(1)Å and c=7.6045(2)Å (room temperature). The structure of the oxycarbonate is composed of alternating carbonate and (CuO2)x layers separated by layers of alkaline-earth cations; considerable orientational disorder of the carbonate groups was evident even at 4 K. Based on the detailed structural features of the new oxycarbonates, we can now discuss possible limitations on their subsitutional chemistry; these will have important implications in attempts to drive the system metallic—and perhaps superconducting.