Structures of the Elements and Some Molecular Crystals

Crystal Structures of the Elements and Some Molecular Crystals 

In the introductory chapters the examples using the PTOT notation have been simple inorganic compounds. Most of these structures have a close-packed arrangement of anions with cations occupying T and / or O sites. Most of the elements are monatomic, and close packing is expected. The structures of most elements with diatomic molecules and even those with larger molecules can be described also in the PTOT system. 

The 3P (ccp) structure is centrosymmetric, and there are three equivalent packing directions along the cubic diagonals. The six neighbors of an atom in any close-packed layer for 2P (hep) or 3P (ccp) are at the corners of a centrosymmetric hexagon. For hep, the six neighbors in 

The group IIA metals have the outer electron configuration ns2 with no singly occupied orbital for bonding. The structures are hep for Be and Mg and ccp for Ca and Sr. The structures are bcc for Ba and Ra, for which promotion to (n — l)d1ns1 is more favorable than for the other metals of the family. Metals with filled or nearly filled orbitals (Co, Ni, Pt metals, Cu family, Zn, and Cd) have ccp (3P) or hep (2P) structures. 

Among the lanthanides and actinides there are several metals with the unusual 4P (ABAC) structure, and Sm has the strange 9P (ABAB CBCAC) structure. There are uncertainties of the structures of some of the metals beyond U. For most of these metals only small samples are available, purity is a problem, and in some cases samples are deposited on a filament. Impurities and deposition on another metal can change the structure. 

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