Crystal structure of Na4[Cu4O2(SO4)4]•MeCl (Me: Na, Cu, □) -: the synthetic Na-analogue of piypite (caratiite)

The crystal structure of Na-4[Cu4O2(SO4)(4)]. MeCl (Me: Na,Cu,square) has been solved by direct methods and refined to R1 = 0.028 using 2466 independent reflections. The compound is tetragonal with space group P4/n, a = 18.451(1) Angstrom, c = 4.9520(2) Angstrom, Z = 4. The two main building units running parallel to the c-axis comprise: (1) slabs containing SO4 and OCu4 tetrahedra; and (2) chains of corner sharing Me(Cl,O)(6) octahedra. The displacement ellipsoids of the atoms occupying the Me sites and of the Cl ions indicate a short-range static disorder. Copper in the slabs exhibits a (4+1) coordination. Both elements form a tunnel structure in which additional Na atoms are incorporated for charge compensation. The structure is closely related to the mineral piypite (K4Cu4O2(SO4)(4). MeCl (Me: Na,Cu,square)): a approximate to root2.a(piy.); c approximate to c(piy.). Furthermore, the structure has a pronounced pseudo-translational symmetry: apart from some of the oxygen sites, all of the nineteen crystallographically distinct positions are related by a pseudo F centring.