In this final chapter we widen our horizons a little by moving away from the traditional crystal structures that are of importance to the materials community. We will cover molecular solids, in which the individual building blocks are entire molecules instead of atoms, and a variety of important biological structures, including DNA, RNA, and several virus structures. We will see that many of the techniques and concepts developed for regular materials remain applicable to these classes of structures.

Introductory remarks

Molecular solids are those for which the building blocks are conveniently described in terms of molecular, rather than individual atomic constituents. We have already seen that it can be useful to represent some ceramic and silicate structures in terms of molecular units. This chapter emphasizes structures based on low atomic number constituents, such as C, H, O, N, … Organic chemistry is defined as the chemistry of carbon compounds; this encompasses all molecules that occur in living organisms and in materials important for life. An older definition of organic as “compounds derived from living organisms” is broadened here to include synthetic materials, which are important in man-made compounds such as polymers and fullerene-based solids.

Molecular crystals often have strong bonding within the molecular units, with weaker intermolecular interactions that give rise to a weak solid cohesion. In many instances, the solid is held together by van der Waals forces or hydrogen bonding.

In this chapter we take a closer look at minerals, both terrestrial and extraterrestrial. We begin with the classification of minerals into 12 classes (native elements, sulfides, sulfosalts, oxides, halides, carbonates, nitrates, borates, phosphates, sulfates, tungstates, and silicates), and we provide many examples of minerals based on the connectivity of SiO4 units.We conclude the chapter with some remarks about minerals that have been identified on the planet Mars.

Classification of minerals

Klein and Hurlbut (1985) define a mineral as a naturally occurring homogeneous solid with a definite (but generally not fixed) chemical composition and a highly ordered atomic arrangement. This is a very broad definition that includes a huge variety of compounds. Intuitively, we think of minerals as gemstones, but not every gem stone is a mineral, since coral, opal, and pearl, for instance, are formed by organic processes. There are more than 3000 recognized minerals, and in this chapter we will introduce mostly members of the silicate class as well as hydroxides and oxyhydroxides of iron. The first class, the silicates, is of interest because of their relative abundance on the planets. The second is of interest because of their association with life forms. Before we do so, we consider briefly the classification of minerals into classes.

In this chapter, we begin the process that will take up the remainder of this book, of describing about 100 important crystal structures. Since metallic structures form an important class of structures, with many practical applications, we will take this and the following chapter to introduce a few basic and a long list of derived structures. We begin with a brief description of the most important parent structures and introduce the Hume–Rothery rules and some basic phase diagrams. Then we cover a more systematic approach to the description of derivative and superlattice structures in fcc, bcc, diamond, and hcp-derived structures. We conclude the chapter with a discussion of structures with interstitial alloys, alternative stacking sequences, and natural and artificial (commensurate and incommensurate) long-period superlattices, including how they can be identified using X-ray diffraction methods.

Introductory comments

It is often useful to go beyond the description of a crystal in terms of the Bravais lattice and the unit cell decoration. In this chapter, we examine ways to disassemble and understand a crystal structure in terms of:

1. • Derivative structures: New structures can often be derived from simpler structures by substitutions of one atom for another.

2. • Interstitial structures: New structures can result from the ordered occupation of subsets of the interstitial sites in simple structures.

3. […]

We are grateful to the many readers, students and teachers alike, who have sent us comments and corrections, or who simply expressed their appreciation of the first edition of our book. As always, it is difficult, if not impossible, to please everyone and to accommodate all requests for changes or additional material. As we prepared this second edition of Structure of Materials, we attempted simultaneously to shorten the text and make it more complete by adding sections on magnetic symmetry (time-reversal symmetry, magnetic Bravais lattices, and magnetic point and space groups). The new text has 24 chapters, as before split into 1–13 (crystallography and symmetry) and 15–24 (examples of important structures), with Chapter 14 as a transition chapter, applying the material from the first half of the book. In addition to the new material on magnetic symmetry, we have added sections on the oxides of iron (Chapter 21) and magnetic minerals on Mars (Chapter 23), and we have made numerous small changes throughout the text. The resulting text is more succinct, and, we hope, a significant improvement over the first edition.

Each chapter now has an introductory and summary section, and a short set of four new problems. Additional new problems, as well as all the problems from the first edition, can be found on the book's website, http://som.web.cmu.edu/, for a total of nearly 600 problems. Solution sets to all problems are made available to instructors via the publisher. In addition, PowerPoint files with enlarged versions (some in color) of all the figures from the book are available from the website. We hope that these files will become a valuable teaching resource.

In this book, we will introduce many concepts, some of them rather abstract, that are used to describe solids. Since most materials are ultimately used in some kind of application, it seems logical to investigate the link between the atomic structure of a solid, and its resulting macroscopic properties. After all, that is what the materials scientist or engineer is really interested in: how can we make a material useful for a certain task? What type of material do we need for a given application? And why can some materials not be used for particular applications? All these questions must be answered when a material is considered as part of a design. The main focus of this book is on the fundamental description of the positions and types of the atoms, the ultimate building blocks of solids, and on some of the experimental techniques used to determine how these atoms are arranged.