PTOT notation

We have seen examples of the 1 1 stoichiometry corresponding to NaCl (3-2PO) and ZnS (3-2PT or 2-2PT). Rutile (Ti02) has P layers filled by O2 with Ti4+ in O sites. The stoichiometry is 1 2 and this is clear from the notation 2-2POi/2. Only 1/2 of the sites in the O layer are filled and the P layers are hep. Later we will examine a more systematic treatment of applications of the PTOT notation. It applies to thousands of inorganic crystal structures including more complex structures. It is not limited to metals and MX, MX2 and M2X com-... 

In this book we are particularly interested in simple descriptions of structures that are easily visualized and providing information of the chemical environment of the ions and atoms involved. For metals, there is an obvious pattern of structures in the periodic table. The number of valence electrons and orbitals are important. These factors determine electron densities and compressibilities, and are essential for theoretical band calculations, etc. The first part of this book covers classical descriptions and notation for crystals, close packing, the PTOT system, and the structures of the elements. The latter and larger part of the book treats the structures of many crystals organized by the patterns of occupancies of close-packed layers in the PTOT system. 

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 structure of p-quartz is unique and much different from those of tridymite and cristobalite. It does not follow the rules for the PTOT system, but the notation can be applied with some qualification. In general, for close-packed structures between packing layers there are two T layers, and half way between the P layers there is an O layer. The... 

K2PtCl6 illustrates the versatility of the PTOT system. It can be handled as a 2 1 salt (Li20 structure) treating PtCljj as a unit (3 3PTT, Section 6.3.3, Figure 6.14). If we consider it as a PO structure, the P layers are three-quarters filled by Cl and one-quarter by K. The Pt atoms are in alternate layers, occupying one-quarter of the O sites, only those sites surrounded by Cl atoms. This gives the notation 3 3Pi/4 3/4P1/4 3/4O1/4. 

Approximately 300 structures are described using the PTOT notation. These structures are encountered for many thousands of compounds. The system and the notation reveal similarities and differences among structures. Figures have been selected and prepared carefully. For many of these, labels for layers or atoms have been added to identify important features and aid in visualizing the structure. 

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