The charge density distribution and antiferromagnetic properties of azurite Cu-3[CO3](2)(OH)(2)

The structure and bonding in azurite are investigated on the basis of accur ate single-crystal X-ray diffraction data. Both spherical IAM and pseudoato m models have been used in the refinements. The deformation electron densit y: dynamic (IAM) and static (pseudoatom) are mapped for the CO3 group and f or Cu(1) and Cu(2) squares in different sections. The carbonate group in az urite, not constrained to have trigonal symmetry, exhibits peaks in both st atic and dynamic maps which result from sigma -bonds between C-sp 2 hybrid orbitals and O-p orbitals with some delocalisation of density in the dynami c map because of the thermal motion of oxygens. For the analysis of crystal fields and for the multipole calculations, coordinate systems on the Cu-at oms have been chosen as for a Jahn-Teller octahedron, but with the normal t o the square as the z-axis instead of the absent apical oxygens. In both Cu squares there are peaks which result from single Cu-O sigma -bonds. Most r emarkable is the preferential occupation of the non-bonding 3d orbitals of Cu-atoms being above and below the Cu-squares. The centre of these peaks fo r the Cu(1)-atom makes an angle with the c-axis similar to 53 degrees in th e ac plane. This direction corresponds to the maximum magnetic susceptibili ty at ambient temperature. The real atomic charges of Cu-atoms in azurite d etermined from multipoles are close to Cu+1. The occupancies of the 3d atom ic orbitals show that non-bonding orbitals in both Cu-atoms are most popula ted, in contrast to bonding orbitals, as is typical for the Jahn-Teller oct ahedron. The absence of apical oxygens makes this effect even more pronounc ed. It is suggested that the antiferromagnetic structure below 1.4 K will b e collinear and commensurate with b ' = 2b.