Principles determining the structure of high-pressure forms of metals: The structures of cesium(IV) and cesium(V).

Consideration of the relation between bond length and bond number and the average atomic volume for different ways of packing atoms leads to the conclusion that the average ligancy of atoms in a metal should increase when a phase change occurs on increasing the pressure. Minimum volume for each value of the ligancy results from triangular coordination polyhedra (with triangular faces), such as the icosahedron and the Friauf polyhedron. Electron transfer may permit atoms of an element to assume different ligancies. Application of these principles to Cs(IV) and Cs(V), which were previously assigned structures with ligancy 8 and 6, respectively, has led to the assignment to Cs(IV) of a primitive cubic unit cell with a = 16.11 A and with about 122 atoms in the cube and to Cs(V) of a primitive cubic unit cell resembling that of Mg(32)(Al,Zn)(49), with a = 16.97 A and with 162 atoms in the cube.