Page "learned" Paragraph 45

Douglass found Af to be trigonal, Laue symmetry Af, with Af, Af.

X-ray and experimental density showed one formula unit in the unit cell, corresponding to a paramagnetic ion density of Af.

The x-ray data did not permit Douglass to determine uniquely the space group, but a negative test for piezoelectricity led him to assume a center of symmetry.

Under this assumption the space group must be Af and the following are the positions of the atoms in the unit cell.

Af.

This space group requires the hydrogen bond to be symmetric.

Douglass found powder intensity calculations and measurements to agree best for Af.

These data lead to a structure in which sheets of Cr atoms lie between two sheets of O atoms.

The O atoms in each sheet are close packed and each Cr atom is surrounded by a distorted octahedron of O atoms.

The Af layers are stacked normal to the ( 111 ) axis with the lower oxygens of one layer directly above the upper oxygens of the neighboring lower layer, in such a manner that the repeat is every three layers.

The separate layers are joined together by hydrogen bonds.

A drawing of the structure is to be found in reference 6.