Die Kristallstruktur der orientierungsfehlgeordneten kubischen Hochtemperaturphase des Silberperchlorats AgClO4

Abstract

AbstractThe crystal structure of the orientationally disordered cubic high-temperature phase of AgClO4has been investigated using single crystal X-ray diffraction data. The lattice constant isa= 703(1) pm (170°C), the space group isFm3m,Z= 4. The ClO4−-anions are orientationally disordered and occupy statistically 6 equilibrium orientations, characterized by the fact that in each orientation only one of the three [unk]-axes of a ClO4-tetrahedron lies parallel to a cubic crystal axis. The structure can be described by choosing Cl in 4a (000), Ag in 4b (1/21/21/2) and oxygen in 96j (0yzetc., occupation factor61). The conventional least-squares refinement yieldsy= 0.120,z= 0.160. The Cl – O bond length amounts to 148(4) pm (corrected for thermal motion), the angles in the ClO42212-anion are 109(2)° and 110(2)° (corrected for thermal motion).A rigid-body-motion analysis using the isotropic temperature parameter of chlorine and the anisotropic temperature parameters of oxygen yields an angle of 17° for the libration of the C1O4−-anion around its [unk]-axis and of 10° around the axes perpendicular to the [unk]-axis.In order to account for the large librations of the ClO44−-anions an additional structure refinement has been carried out, using a method worked out by Hohlwein (1984) for structures with orientationally disordered groups. The refinement confirms the existence of 6 equilibrium orientations of the kind described above. The oxygen parameters (y= 0.122,z= 0.170) correspond to a Cl – O bond length of 147(3) pm. The thermal motions of the ClO4-group and of the Ag- and O-atoms agree well with those derived by the conventional refinement.