The crystal structure of the title compound has been established by single-crystal X-ray diffraction methods from photographic data. The structure was solved by conventional Patterson and Fourier heavy-atom methods and refined by block-diagonal least-squares techniques to R 0·11 for 286 independent reflections. Crystals are monoclinic, space group p2₁/a, with Z= 2 in a unit cell with dimensions : a= 6·18(2), b= 8·19(2), c= 5·64(2)Å, β= 116·2(2)°. The asymmetric unit is one half of the formula unit. The structure consists of copper atoms occupying special positions of symmetry , co-ordinated by four planar oxygen atoms [Cu–O(1) 1·95(1); Cu–O(2) 1·90(1)Å, O(1)–Cu–O(2) 88·7(5)°), the distortion from the exact square plane not being significant. The copper atoms are bridged by, but not coplanar with, carbonate groups in an infinite two-dimensional polymeric puckered sheet in the ab plane, successive sheets being interleaved by sodium ions. The carbonate ions, although planar, are significantly distorted, the non-co-ordinated bond being much shorter [C–O(3), 1·22(2)Å) than the other two (C–O(1), 1·36(2); C–O(2), 1·29(2)Å]. The i.r. spectrum displays splittings providing supporting evidence for distorted stereochemistries of the CuO₄ and CO₃ groups, the metal–oxygen and carbon–oxygen asymmetric stretching modes being appreciably split, and possibly characteristic of the bridging carbonate group. The magnetic moment of 1·92 ± 0·04 BM over the temperature range 100—400 K is consistent with the requirements of square planar co-ordinated copper(II) and is indicative of little or no copper–copper interaction (Cu⋯Cu, 4·89 Å).