Solid-state chemistry, also sometimes referred asmaterials chemistry, is the study of thesynthesis, structure, and properties of solid phase materials. It therefore has a strong overlap withsolid-state physics,mineralogy,crystallography,ceramics,metallurgy,thermodynamics,materials science andelectronics with a focus on the synthesis of novel materials and their characterization. A diverse range of synthetic techniques, such as the ceramic method andchemical vapour depostion, make solid-state materials. Solids can be classified as crystalline or amorphous on basis of the nature of order present in the arrangement of their constituent particles.[1] Their elemental compositions, microstructures, and physical properties can be characterized through a variety of analytical methods.
Because of its direct relevance to products of commerce, solid state inorganic chemistry has been strongly driven by technology. Progress in the field has often been fueled by the demands of industry, sometimes in collaboration with academia.[2] Applications discovered in the 20th century includezeolite andplatinum-based catalysts for petroleum processing in the 1950s, high-purity silicon as a core component of microelectronic devices in the 1960s, and “high temperature” superconductivity in the 1980s. The invention ofX-ray crystallography in the early 1900s byWilliam Lawrence Bragg was an enabling innovation. Our understanding of how reactions proceed at the atomic level in the solid state was advanced considerably byCarl Wagner's work on oxidation rate theory, counter diffusion of ions, and defect chemistry. Because of his contributions, he has sometimes been referred to as thefather of solid state chemistry.[3]
Given the diversity of solid-state compounds, an equally diverse array of methods are used for their preparation.[1][4] Synthesis can range from high-temperature methods, like the ceramic method, to gas methods, likechemical vapour deposition. Often, the methods prevent defect formation[5] or produce high-purity products.[6]
The ceramic method is one of the most common synthesis techniques.[7] The synthesis occurs entirely in the solid state.[7] The reactants are ground together, formed into a pellet using a pellet press and hydraulic press, and heated at high temperatures.[7] When the temperature of the reactants are sufficient, the ions at the grain boundaries react to form desired phases. Generally ceramic methods give polycrystalline powders, but not single crystals.
Using amortar and pestle, ResonantAcoustic mixer, orball mill, the reactants are ground together, which decreases size and increasessurface area of the reactants.[8] If the mixing is not sufficient, we can use techniques such asco-precipitation andsol-gel.[7] A chemist forms pellets from the ground reactants and places the pellets into containers for heating.[7] The choice of container depends on the precursors, the reaction temperature and the expected product.[7] For example,metal oxides are typically synthesized in silica or alumina containers.[7] Atube furnace heats the pellet.[7] Tube furnaces are available up to maximum temperatures of 2800oC.[9]
Molten flux synthesis can be an efficient method for obtaining single crystals. In this method, the starting reagents are combined with flux, an inert material with a melting point lower than that of the starting materials. The flux serves as a solvent. After the reaction, the excess flux can be washed away using an appropriate solvent or it can be heat again to remove the flux by sublimation if it is a volatile compound.
Crucible materials have a great role to play in molten flux synthesis. The crucible should not react with the flux or the starting reagent. If any of the material is volatile, it is recommended to conduct the reaction in a sealed ampule. If the target phase is sensitive to oxygen, a carbon- coated fused silica tube or a carbon crucible inside a fused silica tube is often used which prevents the direct contact between the tube wall and reagents.
Chemical vapour transport results in very pure materials. The reaction typically occurs in a sealed ampoule.[10] A transporting agent, added to the sealed ampoule, produces a volatile intermediate species from the solid reactant.[10] For metal oxides, the transporting agent is usually Cl2 or HCl.[10] The ampoule has a temperature gradient, and, as the gaseous reactant travels along the gradient, it eventually deposits as a crystal.[10] An example of an industrially-used chemical vapor transport reaction is theMond process. The Mond process involves heating impurenickel in a stream ofcarbon monoxide to produce pure nickel.[6]
Intercalation synthesis is the insertion of molecules or ions between layers of a solid.[11] The layered solid has weakintermolecular bonds holding its layers together.[11] The process occurs viadiffusion.[11] Intercalation is further driven byion exchange,acid-base reactions orelectrochemical reactions.[11] The intercalation method was first used in China with the discovery ofporcelain. Also,graphene is produced by the intercalation method, and this method is the principle behindlithium-ion batteries.[12]
It is possible to usesolvents to prepare solids byprecipitation or byevaporation.[5] At times, the solvent is ahydrothermal that is under pressure at temperatures higher than the normalboiling point.[5] A variation on this theme is the use offlux methods, which use a salt with a relatively low melting point as the solvent.[5]
Many solids react vigorously with gas species likechlorine,iodine, andoxygen.[13][14] Other solids formadducts, such asCO orethylene. Such reactions are conducted in open-ended tubes, which the gasses are passed through. Also, these reactions can take place inside a measuring device such as aTGA. In that case,stoichiometric information can be obtained during the reaction, which helps identify the products.
Chemical vapour deposition is a method widely used for the preparation of coatings andsemiconductors from molecular precursors.[15] A carrier gas transports the gaseous precursors to the material for coating.[16]
This is the process in which a material’s chemical composition, structure, and physical properties are determined using a variety of analytical techniques.
Synthetic methodology and characterization often go hand in hand in the sense that not one but a series of reaction mixtures are prepared and subjected to heat treatment.Stoichiometry, a numerical relationship between the quantities of reactant and product, is typically varied systematically. It is important to find which stoichiometries will lead to new solid compounds or solid solutions between known ones. A prime method to characterize the reaction products ispowder diffraction because many solid-state reactions will produce polycrystalline molds or powders. Powder diffraction aids in the identification of known phases in the mixture.[17] If a pattern is found that is not known in the diffraction data libraries, an attempt can be made to index the pattern. The characterization of a material's properties is typically easier for a product with crystalline structures.
Once the unit cell of a new phase is known, the next step is to establish the stoichiometry of the phase. This can be done in several ways. Sometimes the composition of the original mixture will give a clue, under the circumstances that only a product with a single powder pattern is found or a phase of a certain composition is made by analogy to known material, but this is rare.
Often, considerable effort in refining the synthetic procedures is required to obtain a pure sample of the new material. If it is possible to separate the product from the rest of the reaction mixture,elemental analysis methods such asscanning electron microscopy (SEM) andtransmission electron microscopy (TEM) can be used. The detection of scattered and transmitted electrons from the surface of the sample provides information about the surface topography and composition of the material.[18]Energy dispersive X-ray spectroscopy (EDX) is a technique that uses electron beam excitation. Exciting the inner shell of an atom with incident electrons emits characteristic X-rays with specific energy to each element.[19] The peak energy can identify the chemical composition of a sample, including the distribution and concentration.[19]
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Similar to EDX,X-ray diffraction analysis (XRD) involves the generation of characteristic X-rays upon interaction with the sample. The intensity of diffracted rays scattered at different angles is used to analyze the physical properties of a material such as phase composition and crystallographic structure.[20] These techniques can also be coupled to achieve a better effect. For example, SEM is a useful complement to EDX due to its focused electron beam, it produces a high-magnification image that provides information on the surface topography.[18] Once the area of interest has been identified, EDX can be used to determine the elements present in that specific spot.Selected area electron diffraction can be coupled with TEM or SEM to investigate the level of crystallinity and the lattice parameters of a sample.[21]
X-ray diffraction is also used due to its imaging capabilities and speed of data generation.[22] The latter often requiresrevisiting and refining the preparative procedures and that are linked to the question of which phases are stable at what composition and what stoichiometry. In other words, what thephase diagram looks like.[23] An important tool in establishing this arethermal analysis techniques likeDSC orDTA and increasingly also, due to the advent ofsynchrotrons, temperature-dependent powder diffraction. Increased knowledge of the phase relations often leads to further refinement in synthetic procedures in an iterative way. New phases are thus characterized by their melting points and their stoichiometric domains. The latter is important for the many solids that are non-stoichiometric compounds. The cell parameters obtained from XRD are particularly helpful to characterize the homogeneity ranges of the latter.
In contrast to the large structures of crystals, thelocal structure describes the interaction of the nearest neighbouring atoms. Methods ofnuclear spectroscopy use specificnuclei to probe the electric and magnetic fields around the nucleus. E.g.electric field gradients are very sensitive to small changes caused by lattice expansion/compression (thermal or pressure), phase changes, or local defects. Common methods areMössbauer spectroscopy andperturbed angular correlation.
For metallic materials, their optical properties arise from the collective excitation of conduction electrons. The coherent oscillations of electrons under electromagnetic radiation along with associated oscillations of the electromagnetic field are calledsurface plasmon resonances.[24] The excitation wavelength and frequency of the plasmon resonances provide information on the particle's size, shape, composition, and local optical environment.[24]
For non-metallic materials orsemiconductors, they can be characterized by their band structure. It contains aband gap that represents the minimum energy difference between the top of the valence band and the bottom of the conduction band. The band gap can be determined usingUltraviolet-visible spectroscopy to predict the photochemical properties of the semiconductors.[25]
In many cases, new solid compounds are further characterized[26] by a variety of techniques that straddle the fine line that separates solid-state chemistry from solid-state physics. SeeCharacterisation in material science for additional information.
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