Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 14

Chemistry is an evolving subject! Traditionally, there have been three branches of chemistry: organic, physical and inorganic, with some arguments in favour of including analytical as a fourth branch. An alternative, fairly new classification (favoured by the author!) divides chemistry into two broad areas: molecular (which includes liquids and gases) and non‐molecular (or solid state). The ways in which we think about, make, analyse and use molecular and non‐molecular substances are completely different, as shown by a comparison of one ‘simple’ substance in each category, toluene and aluminium oxide:

Comparison of the chemistries of molecular and non‐molecular materials

Characteristic	Toluene	Aluminium oxide
Formula	Fixed, C6H5CH3	Usually fixed, Al2O3, but for other oxides may be variable, e.g. Fe1−x O
Are defects present?	Not allowed: missing or mis‐placed atoms give rise to different molecules	Unavoidable: small concentration of vacancies, interstitials and dislocations are always present
Doping possibilities	Not possible without producing a different molecule	Doping or solid solution formation allows control and optimisation of properties, e.g. ruby is Cr‐doped Al2O3
Structure and its determination	Molecular structure can be determined spectroscopically: NMR/Mass Spec/IR. Determine packing arrangement, bond lengths and angles, by single crystal X‐ray diffraction. Usually, structural information is then complete.	Full characterisation of a solid requires structural and compositional information across the length scales from local, 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: