Solid State Chemistry and its Applications. Anthony R. West

to unit cell, nano and microscales. Many diffraction, spectroscopic and microscopic techniques are needed for full characterisation. Properties and applications Controlled by molecular formula and configuration; cannot be modified by doping. Some properties (e.g. pharmaceutical activity) may depend on molecular packing arrangements in crystals. Properties/applications depend on crystal structure, defects, dopants, surface structure, particle size and whether the material is a powder, single crystal, film, etc. Consider the diverse applications of Al₂O₃: films and ceramics used as insulators; powders used as abrasive; with Cr³⁺ dopants, ruby is used for lasers; porous solids used as catalyst supports.

      Thus, for toluene, once its formula and molecular structure had been determined there were few remaining issues to be resolved other than, perhaps, the detailed packing arrangement of molecules in crystalline toluene at low temperatures or the possible discovery and evaluation, even today, of as‐yet unknown chemical, biological or pharmaceutical properties of pure toluene.

      Alumina, by contrast, is a highly complex material; its properties, and therefore potential applications, depend on different aspects of its structure (bulk, defect, surface, nano), the methods needed to fabricate it in different forms and shapes, the possibility of doping to modify its properties and the characterisation or determination of its structure (and its composition, whether homogeneous or heterogeneous, if doped) across all length scales. This is solid state chemistry!

      The biggest contrast between molecular and non‐molecular materials is that the latter can be doped, allowing modification and control of properties such as magnetism, superconductivity and colour/optical band gap. By contrast, attempts to dope molecules are inevitably frustrated since replacing one atom in the molecule by another, or creating defects such as missing atoms, lead to entirely different molecules.

      In recent decades, materials chemistry has emerged as a distinct branch of chemistry which covers both non‐molecular, solid state materials (oxides, halides, etc.) and many molecular materials (especially, functional polymers and organic solids with potentially useful physical properties). Materials chemistry cuts across the traditional disciplines of chemistry but also includes something extra which is an interest in the physical properties of compounds and materials. In the past, solid state physics and materials science have been the usual ‘home’ for physical properties; but now, they are an intrinsic part of solid state and materials chemistry.

      The distinction between materials chemistry and materials science is often unclear but can be summarised broadly as follows:

      Materials chemistry

       Synthesis – structure determination – physical properties – new materials.

      Materials science

       Processing and fabrication – characterisation – optimisation of properties and testing – improved/new materials for engineering applications in products or devices.

      Materials science focuses on materials that are already known to be useful or have the potential to be developed for applications, either by compositional control to optimise properties or by fabrication into desired forms, shapes or products. Materials science therefore includes whatever aspects of chemistry, physics and engineering that are necessary to achieve the desired aims.

Materials chemistry is much more than just a subset of materials science, however, since it is freed from the constraint of a focus on specific applications; materials chemists love to synthesise new materials and measure their properties, some of which may turn out to be useful and contribute to the development of new industries, but they do this within an overarching interest in new chemistry, new structures and improved understanding of structure – composition – property relationships.

      A curious fact is that, in the early days of chemistry, inorganic chemistry had as its main focus, the elements of the periodic table and their naturally occurring or easy‐to‐make compounds such as oxides and halides. Inorganic chemistry subsequently diversified to include organometallic chemistry and coordination chemistry but interestingly, many traditional inorganic materials have returned to centre‐stage and are now at the heart of solid state materials science. Examples include: Cr‐doped Al₂O₃ for lasers; doped Si semiconductors for microelectronics; doped ZrO₂ as the solid electrolyte in solid oxide fuel cells; BaTiO₃ as the basis of the capacitor industry with a total annual production worldwide exceeding 10¹² units; copper oxide‐based materials for superconductor applications; and many, many more. The scope for developing new solid state materials/applications is infinite, judging by the ‘simple’ example of Al₂O₃ described above. Most such materials tend not to suffer from problems such as volatilisation, degradation and atmospheric attack, which are often a drawback of molecular materials, and can be used safely in the environment.

      It is important to recognise also that physical properties of inorganic solids often depend on structure at different length scales, as shown by the following examples:

      Thus, in the case of ruby, which is a natural gemstone and was the first material in which LASER action – light amplification by stimulated emission of radiation – was demonstrated, two structural aspects are important. One is the host crystal structure of corundum, α‐Al₂O₃ and the other is the Cr³⁺ dopant which substitutes at random for about 1% of the Al³⁺ ions in the corundum lattice: the Cr–O bond lengths and the octahedral site symmetry are controlled by the host structure; the two together combine to give the red ruby colour by means of d–d transitions within the Cr chromophore and the possibility of accessing the long‐lived excited states that are necessary for LASER action.

A remarkable example of the effect of crystal structure details at the unit cell scale on properties is shown by dicalcium silicate, Ca₂SiO₄ which is readily prepared in two polymorphic forms at room temperature. One, the β‐polymorph, reacts with water to give a semicrystalline calcium silicate hydrate which sets rock‐solid and is a main constituent of concrete; the other polymorph, γ‐Ca₂SiO₄, does not react with water. Just think, the entire construction industry rests on the detailed polymorphism of dicalcium silicate! It is not sufficient that one of the key components of cement has the right composition, Ca₂SiO₄; in addition, the precise manner in which ions are packed together in the solid state is critical to its hydration properties and whether or not it turns into concrete.

      At the nanoscale, crystalline particles may contain many hundreds of unit cells but often their properties are different from powders, ceramics or single crystals of the same material with larger‐sized grains simply because of the influence of surface energies. In small nanoparticles, surface free energies and structures increasingly dominate the total free energy of a material, as shown by the colour, and associated band gap, of CdS nanoparticles (or colloids in older terminology) which can be fine‐tuned by controlling the particle size.

      Some properties are determined by structure at the micron (1 μm = 10³ nm = 10⁴ Å = 10⁻³ mm) scale and this is the reason why ‘microstructure’ features strongly in the characterisation of metals and ceramics, primarily using optical and electron microscopy techniques. Frequently, impurities/dopants may precipitate at grain boundaries and surfaces and these can have a dramatic influence on for instance, the mechanical properties.

      These examples illustrate the awesome challenges that must be met before an inorganic solid can be regarded as fully characterised across the length scales. This, coupled with the enormous number of inorganic crystal structures that are known, and the possibility to introduce dopants which modify properties, underlines why solid state chemistry is a central subject to many areas of physical