Reaction solid-state

Solid State Reactions.—There are three reports of kinetic and mechanistic studies on substitution in the solid state which may have some interest or relevance to cobalt(m) complex substitution mechanisms in solution. [Co(NH3)6](N3)3 decomposes to give cobalt nitride. The initial step, as in several solution mechanisms cited above, seems to be azide to cobalt electron transfer there is no evidence for nitrene intermediates. Decomposition of [Co(NH3)e]Cl3, which gives cobalt(ii) amongst the products, does not proceed via formation of [CoCl(NH3)5]Cl2, but for some nitrites, e.g. cis- and /ra j-[Coen2(NH3)2](N02)3, nitrite does enter the first co-ordination sphere of the cobalt in the course of reaction. Lastly, the mechanism of thermal and of photochemical decomposition of [Co(NH3)s(OH2)]X3 is said to be similar to the mechanism of reaction in solution, despite the ultimate formation of tetrahedral cobalt(ii) complexes in the solid state reactions.  

The name recombination centres given to such states reflects the fact that they also accelerate the back reaction, that is electron-hole recombination. Their eflicacy depends on the fact that they make possible indirect transitions (i.e. transitions with change of the wave number vector, see Chapter 2) by taking up energy and momentum. In this way the establishing of the local equilibrium is accelerated [128, 129]. 

A special role is played by auto-catalysis, in which the reaction rate is accelerated by the product. The resulting upward-spiral is fundamental to structure formation. This will be treated in Section 6.10. 

The starting materials for solid-state reactions are the oxides, hydroxides, sulfates, carbonates, and so on, of the various cations of the final product The drawback of the method is the need for intimate mixing, which is commonly achieved by powder milling. Normally, a grain size smaller than 1 pm caimot be obtained. This always causes problems when soHd electrolytes are prepared which contain ions with extremely low diffusion coefficients, even at relatively high temperatures 

Since the constituents are usually charged, the field of inorganic state reactions exhibits a substantial intersection with electrochemistry. The incorporation of a neutral component (e.g., O into an oxide) has already been considered as a most simple reaction. Here we concentrate on typical reactions that involve the formation of a new phase. Owing to the manifold of individual problems, a detailed treatment cannot be given here and only a few points relevant in our context shall be highlighted. 

Let us consider the oxidation of a metal, e.g., Zn to ZnO.4,75 265 The reaction starts with nucleation and early growth phenomena (lateral mass transport, tunneling, space charge phenomena). Usually at larger 

Since for ZnO, 7eon oi0n, and thus as - crion, the experimentally observed steep temperature dependence of the oxidation rate is explained by the activation enthalpy of Tion, Furthermore, the well-established doping effect266 by Al3+ or Li+ in depressing or enhancing the corrosion rate is immediately derived from the defect chemistry  

Alzn defects decrease Oi0n by decreasing [vq] (or Zn ) while the 

Li zn defects increase Ojon. The detailed partial pressure dependence follows through integration. 

When glass-ceramics based on these metastable phases are heated to a high-enough temperature, solid-state reactions inevitably occur, and the result is the development of the stable crystalline phase or phase assemblage. It is interesting to examine some of the types of forms these reactions commonly take. 

Invariably, however, the hexagonal quartz crystals will transform to the stable tetragonal phase P-spodumene, or as it is sometimes referred to, stuffed keatite. This phase transformation is isochemical with compositions along 

An illustrative and now classical example of methods for elucidating material transport mechanisms in solid-state reactions is the use of couple arrangements. Fig. 3.13 (Wagner, 1936 Reijnen, 1991). Couple experiments are helpful for illustrating the complexity of reaction mechanisms in ionic compounds in general, and ferrites in particular. 

The diffusion couple consists of two reactant pieces placed in contact. In some cases, the differences in colour of reactants and the product phase can be used to observe the progress of the reaction. In other cases, thin wires of Pt can be used as markers. After some time at high temperature, the product phase nucleates and grows, and therefore separates the reactants. For relatively simple reaction mechanisms, it is possible to deduce the nature of the diffusing species from the relative growth of the new phase. 

In practice, these techniques can be difficult. When significant changes 

A simple example of the use of couple experiments is the formation of the spinel MgAl204 from magnesia, MgO, and alumina, AI2Q3. The overall reaction is  

The relative amounts of the spinel product phase formed at the original boundary are indicative of the diffusion mechanism of the reaction. Experimental results show that the amounts of spinel product are formed in a ratio 1 3 at the marker (Reijnen, 1991), Fig. 3.13. 

Many chemical reactions take place in crystals. Detailed analyses of the stereo- and electronic effects are useful for an understanding of chemical reactions in general. Such solid-state reactions can be studied by X-ray diffraction techniques when the participating molecules are close together 

1 Solid-state photoreactions between double bonds 

The development of the principles of solid-state reactions of substituted cinnamic acids was pioneered by Gerhard M. J. Schmidt. The trans-SLcid was found to be polymorphic, and three different crystal forms (designated a, and 7) were identified by him. The finding that the nature of the cyclobutane derivatives formed by the solid-state photochemical reaction on crystals of frans-cinnamic acid depend on which polymorph is irradiated was of great interest. The products of the photo- 

FIGURE 18.1. Solid-state reactions that occur on irradiation of cinnamic acids. The reaction products are truxillic or truxinic acids, depending on the relative orientations of the molecules in the crystal. The distance between C=C bonds in the solid state must lie in the range 3.6-4.1 A. 

Certain crystal structures, however, that appear to fit the criteria just listed for a or /3 cinnamic acids do not, in fact, produce photodimers. For example, 3,4-dihydroxy-traras-cinnamic acid is photostable.It is suggested that this is because symmetry-related molecules are held together by strong hydrogen bonding which does not permit the molecular flexibility that requires the 4.0 A interaction to be reduced to approximately 1.5 A. It appears that there needs to be sufficient space in the crystal reaction cavity for the reaction to take place, and sufficient flexibility in the overall crystal packing for the required atomic spatial reorganization to occur. 

A third possibility, in which counter-diffusion of M + cations and anions occurs, is rare in oxide film formation and will be discussed in detail in Section 8.6.1. 

The oxidation of copper metal in a low partial pressure of oxygen produces cuprous oxide, CU2O, by a mechanism involving diffusion of Cu cations and electrons. The reaction is described by the chemical equation  

The initial reaction results in the formation of a continuous film of oxide that is firmly attached to the metal surface. The rate of growth of the film is controlled by the slow diffusion of the Cu ions. However, no corrosion could occur without the transport of electrons, as the mechanism depends on electron transport. The electronic conductivity of the film is therefore of major importance. The reason why both aluminium and chromium appear to be corrosion-resistant lies in the fact that, although oxide films form very rapidly in air, the films are insulators and prevent reaction from continuing. As the thin films are also transparent, the metals do not lose their shiny appearance. 

Reactions between two solids are analogous to the oxidation of a metal, because the product of the reaction separates the two reactants. Further reaction is dependent on the transport of material across this barrier. As with oxidation, cracking, porosity and volume mismatch can all help in this. In this section, the case when a coherent layer forms between the two reactants will be considered. The mechanism of the reaction may depend on whether electron transport is possible in the intermediate phase, and the rate of reaction will be controlled by the rate of diffusion of the slowest species. To illustrate the problems encountered a typical solid-state reaction, the formation of oxide spinels, is described. 

I thought of another moral, more down to earth and concrete, and I believe that every militant chemist can confirm it that one must distrust the almost-the same,. ..the practically identical, the approximate, the or even, all surrogates, and all patchwork. The differences can be small, but they can lead to radically different consequences, like a railroad s switch points the chemists trade consists in good part in being aware of these differences, knowing them close up, and foreseeing their effects. And not only the 

The subject of this chapter is the chemistry that can take place in solids, i.e., in a lattice of atoms. In solids, as in all chemistry, a necessary condition for reactions to be possible is sufficient atomic mobility. Atoms in the interior of crystallites or in grain boundaries are fixed. This means that reactions in solids need a comparatively high temperature. Moreover, there is no mobility in lattices without point defects such as vacancies or interstitials. In solids that have perfect lattices the atoms are not mobile and there is no chemistry.  

An understanding of solid state reactions is necessary for designing microstructures, properties, and processes for making novel solids. Processes that depend on solid state reactions and atomic transport are conversion reactions, ceramics processing, high-temperature corrosion, and ionic device operation. 

Most reactions of a solid are heterogeneous, occurring on the interface between the phases where the reaction and the reaction product are located. The microstructures of solids affect reaction rates and conversely, heterogeneous growth reactions can form many kinds of microstructures. Chapter 6 deals with surface chemistry and the chemical consequences of morphology are discussed in Chapter 7. Another typical feature of solid compounds apart from the presence of a surface is that nonstoichiometries often occur in solids and the stoichiometry strongly affects the properties and the reactions. 

Diffusion of reactant atoms determines the overall reaction rate and solid state reactions are diffusion-limited and have low rates. What is called the reaction mechanism in solid state reactions is not like a reaction mechanism in molecular chemistry. In the latter a mechanism for a reaction describes the path of the atoms in the reactant molecules during the conversion to the product molecules. The mechanisms in solid state chemistry are really diffusion mechanisms combined with atomic balances. 

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Chromates III). Mixed oxides, e.g. FeCr204, having spinel structures and prepared by solid state reactions. 

Copper(II) oxide, CuO. Black solid formed by heating Cu(OH)2, Cu(N03)2, etc. Dissolves in acid to Cu(II) salts, decomposes to CU2O at 800 C. Forms cuprates in solid state reactions. A cuprate(III), KCUO2, is also known. 

Plumbaies IV), e.g. ICjPbOj, are formed by solid state reactions and plunibates containing [Pb(OH)6] " ions are formed from aqueous solution. 

Manganaies IV), manganites. Mixed-metal oxides containing Mn(IV). Prepared by solid state reactions. 

The essentially non-destmetive nature of Rutherford backscattering spectrometry, combmed with the its ability to provide botli compositional and depth mfomiation, makes it an ideal analysis tool to study thm-film, solid-state reactions. In particular, the non-destmetive nature allows one to perfomi in situ RBS, thereby characterizing both the composition and thickness of fomied layers, without damaging the sample. Since only about two minutes of irradiation is needed to acquire a Rutherford backscattering spectmm, this may be done continuously to provide a real-time analysis of the reaction [6]. 

The application of RBS is mostly limited to materials applications, where concentrations of elements are fairly high. RBS is specifically well suited to the study of thin film stmctures. The NMP is usefiil in studying lateral inliomogeneities in these layers [30] as, for example, in cases where the solid state reaction of elements in the surface layers occur at specific locations on the surfaces. Other aspects, such as lateral diffusion, can also be studied in tluee-dimensions. 

Theron C C 1997 In situ, real-time characterization of solid-state reaction in thin films PhD Thesis University of Stellenbosch... 

Karge FI G 1997 Post-synthesis modification of microporous materials by solid-state reactions Stud. Surf. Sol. Catal. 105 1901-48... 

It is noteworthy that it is the lower cross-over temperature T 2 that is usually measured. The above simple analysis shows that this temperature is determined by the intermolecular vibration frequencies rather than by the properties of the gas-phase reaction complex or by the static barrier. It is not surprising then, that in most solid state reactions the observed value of T 2 is of order of the Debye temperature of the crystal. Although the result (2.77a) has been obtained in the approximation < ojo, the leading exponential term turns out to be exact for arbitrary cu [Benderskii et al. 1990, 1991a]. It is instructive to compare (2.77a) with (2.27) and see that friction slows tunneling down, while the q mode promotes it. 

Since only the vibrational degrees of freedom take part in a solid-state reaction, the sole reason for this change may be the increase in their frequencies in the transition state... 

H. Schmalzried, Solid State Reactions, (Verlag Chemie, Basel 1981). 

Heating the crystalline salt 2-aminopyridinium propiolate (346) at 100 °C in the solid state led to a 10 9 mixture of 2/f-pyrido[l,2-n]pyrimidin-2-one and ( )-3-(2-imino-l,2-dihydro-l-pyridyl)acrylic acid (347). Analysis of differental scanning calorimetry data shows unambiguously that the reaction takes place in the solid state. An endothermic peak at 81.1 °C corresponds to a solid state reaction, and a peak at 122-123 °C is attributed to melting. The product ratio of 2//-pyrido[l, 2-n]pyrimidin-2-one and 347 is 1 2.5 at 60°C, and 1 1.4 at 80°C (94MI12). 

The corrosion rate is greatest close to the reversible Pb02/PbS04 potential as a result of a solid state reaction between PbOj and the underlying lead surface". This corresponds to the rest or open circuit condition. 

Fej04 . A similar correspondence between theory and practice has been found for growth of Fej04 by the solid state reactions from FeO and Fe, , between 600 and 1 200°C. The growth rate of FeO is within 10% of the theoretical rate expected from Fe lattice diffusion, calculated according to the Wagner theory . 

The Pb02/PbOx border slowly penetrates into the metal, but only at a very slow rate as a solid-state reaction. Cracks are formed when the oxide layer exceeds a given thickness, on account of the growth in volume when lead becomes converted into lead dioxide (Table 7). Underneath the cracks the corrosion process starts again and again. As a whole, the corrosion proceeds at a fairly constant rate. It never comes to a standstill, and a continually flowing anodic current, the corrosion current is required to re-establish the corrosion layer. 

The third aspect, the stability range of solid electrolytes, is of special concern for alkaline-ion conductors since only a few compounds show thermodynamic stability with, e.g., elemental lithium. Designing solid electrolytes by considering thermodynamic stability did lead to very interesting compounds and the discovery of promising new solid electrolytes such as the lithium nitride halides [27]. However, since solid-state reactions may proceed very slowly at low temperature, metasta-... 

Both of these mechanistic representations have been widely applied in interpretations of observations on solid state reactions and there is ample experimental evidence for their existence in most, but not necessarily all, systems. 

Classification of solid state reactions according to Boldyrev [100] Initial step Reaction mechanism Examples... 

Magnetic resonance techniques, EPR (ESR) and NMR, can be used [341,342] to obtain information about atomic, ionic, molecular and crystallographic states before, during and after solid state reactions. Only a very restricted use has been made of the NMR of solids [342—345]. 

Additional information concerning the mechanisms of solid—solid interactions has been obtained by many diverse experimental approaches, as the following examples testify adsorptive and catalytic properties of the reactant mixture [1,111], reflectance spectroscopy [420], NMR [421], EPR [347], electromotive force determinations [421], tracer experiments [422], and doping effects [423], This list cannot be comprehensive. Electron probe microanalysis has also been used as an analytical (rather than a kinetic) tool [422,424] for the determination of distributions of elements within the reactant mixture. Infrared analyses have been used [425] for the investigation of the solid state reactions between NH3 and S02 at low temperatures in the presence and in the absence of water. 

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