Polycationic disorder in [Bi6O4(OH)4](NO3)6: Structure determination using synchrotron radiation and microcrystal X-ray diffraction

The bismuth basic nitrate Bi₆O₄(OH)₄](NO₃)₆ crystallizes in a rhombohedral hexagonal unit cell with parameters , , , Z=6, space group R-3. The synthesis, formula determination, thermogravimetric analysis and nitrate assay, and finally, its crystal structure refinement determined at 150(2) K by synchrotron X-ray microcrystal diffraction are reported. Its structure is built from Bi₆O₄(OH)₄]⁶⁺ polycations, six per unit cell, disordered over two positions. Two oxygen atoms are common to the two antagonist polycations (full occupancy) while the remaining six are partially occupied. The Bi₆O₄(OH)₄]⁶⁺ hexanuclear clusters form columns along the c-axis. The cohesion between polycationic entities is effected by nitrate anions through either OH-ONO₂ hydrogen bonds or Bi-ONO₂ bonds. One of the two independent NO₃]⁻ groups is also disordered over two positions. Only a local order in the columns is obtained by formation of pairs of ordered Bi₆O₄(OH)₄]⁶⁺ polycations.