Physical properties of nanomaterials often diverge from those expected based on the known behaviour of macro‐sized and three‐dimensional materials. Thus, quantum confinement effects lead to band structure modification and variable electronic and optical properties in colloidal systems. The importance of surface structure and properties is increasingly dominant in nano‐sized materials because most of the material is close to, or at, a surface. Surface structures always differ from bulk structures because of modified coordination environments and bonding arrangements, but in the study of bulk materials, surface effects are frequently ignored in favour of the dominant bulk structure and properties. They cannot be ignored in nanomaterials however, because nano‐size local bonding/structure effects underpin industries that are often based on and include heterogeneous catalysis, sensors, smart windows and solar cell technology.
High quality chemical science thrives on well‐crystallised, homogeneous materials, whose structures, bonding and properties can be well‐understood, rationalised and integrated into a coherent overview of the subject. However, many materials properties and applications depend on heterogeneity, either through compositional gradients in doped materials or through the formation of composite structures. In such cases, emergent phenomena often arise in which the overall properties are not simply a sum of the component parts but have additional, value‐added characteristics. Examples include:
reversible semiconductor–insulator transitions on heating doped, inhomogeneous barium titanate ceramics, used as temperature controllers in for example, hair dryers.
voltage‐induced insulator–semiconductor transitions in lightning and voltage overload protection devices, based on doped zinc oxide. These two examples are the exact opposite of thermodynamically stable, well‐characterised homogeneous materials. Their study can lead to practical frustrations: if one attempts to prepare high quality, homogeneous materials in order to better understand their behaviour, then the key properties of interest may be lost!
it is becoming increasingly possible to design and fabricate entirely new materials which are kinetically stable but thermodynamically metastable or unstable, either by moving individual atoms in an electron microscope or depositing specific layers of atoms, one layer at a time, using techniques such as atomic layer or molecular beam epitaxy, ALD and MBE. A very eye‐catching example is the creation of so‐called electron gases at metallic interfaces in two‐dimensional structures consisting of alternating layers of the insulating oxides SrTiO3 and LaAlO3. Another consequence of reduction in size of nano‐structures is that they become increasingly disordered and lose their regular three‐dimensional periodicity. The local structure involving coordination numbers and bond lengths is usually retained but, depending on the length scale, they may be better regarded as amorphous. Glasses are traditionally made by the rapid cooling of liquids so as to avoid their crystallisation; amorphous nanomaterials may never pass through a high temperature, liquid state but nevertheless, show many of the characteristic properties of glasses.
In spite of the enormous diversity in the chemistry of non‐molecular, solid state materials, examples of which are given above, it is important not to lose sight of the fundamental principles associated with structure, bonding, properties, thermodynamics and kinetics of solids, since these provide a framework on which to understand and appreciate them in all their glory and complexity.
This book focuses on inorganic solids: their crystal structures, defect structures and bonding; the methods used to synthesise them and determine their structures; their physical properties and applications. Organic and other molecular materials are included in the coverage if their properties in the solid state complement, or relate to, those of inorganic solids. Physical properties are an intrinsic part of solid state chemistry since the whole area of structure–property relations requires the insights and input of chemistry to synthesise and characterise materials, as well as a good understanding of physical properties and the factors that control them.
1 Crystal Structures, Crystal Chemistry, Symmetry and Space Groups
Solid state chemistry is concerned mainly with crystalline inorganic materials, their synthesis, structures, properties and applications. A good place to begin is with crystal structures and crystal chemistry. All necessary crystal structure information is contained in data on unit cells, their dimensions and the positions or atomic coordinates of atoms inside the unit cell. Crystal chemistry combines this basic structural information with information about the elements, their principal oxidation states, ionic radii, coordination requirements and preferences for ionic/covalent/metallic bonding. A working knowledge of the Periodic Table and the properties of elements is, of course, invaluable to be able appreciate crystal chemistry, but conversely, knowledge of crystal structures and especially crystal chemistry provides a very useful way to gain increased understanding of the elements and their compounds.
Many of the properties and applications of crystalline inorganic materials revolve around a surprisingly small number of structure types. In this chapter, the main families of inorganic structures are reviewed, especially those which have interesting properties; more details of the vast array of structures may be found in the encyclopaedic text by Wells and also in the Wyckoff Crystal Structures book series. First, however, we must consider some basic concepts of crystallography.
1.1 Unit Cells and Crystal Systems
Crystals are built up of regular arrangements of atoms in three dimensions; these arrangements can be represented by a repeat unit or motif called the unit cell. The unit cell is defined as the smallest repeating unit which shows the full symmetry of the crystal structure. Let us see exactly what this means, first in two dimensions. A section through the NaCl structure is shown in Fig. 1.1(a); possible repeat units are given in (b) to (e). In each, the repeat unit is a square and adjacent squares share edges and corners. Adjacent squares are identical, as they must be by definition; thus, all the squares in (b) have Cl− ions at their corners and centres. The repeat units in (b), (c) and (d) are all of the same size and, in fact, differ only in their relative position. The choice of origin of the repeat unit is to some extent a matter of personal taste, even though its size, shape and orientation are fixed. The repeat unit of NaCl is usually chosen as (b) or (c) rather than (d) because it is easier to draw and visualise the structure as a whole if the repeat unit contains atoms or ions at special positions such as corners and edge centres. Another guideline is that usually the origin is chosen so that the symmetry of the structure is evident (Section 1.3).
In the hypothetical case that two‐dimensional (2D) crystals of NaCl could form, the repeat unit shown in (e), or its equivalent with Cl at the corners and Na in the middle, would be the correct unit. Comparing (e) and, for example, (c), both repeat units are square and show the 2D symmetry of the structure; as the units in (e) are half the size of those in (c), (e) would be preferred according to the above definition of the unit cell. In three dimensions, however, the unit cell of NaCl is based on (b) or (c), rather than (e) because only they show the cubic symmetry of the structure (see later).
Figure 1.1 (a) Section through the NaCl structure, showing (b–e) possible