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Abstract  

Theoretical models for the melting of solids are inadequate because relatively little is known about the structures of liquids formed and the factors that control this phase transformation. In the present analysis of fusion phenomenon, usually considered to be a physical change, it is pointed out that, for many solids (e.g., metals and some simple ionic salts) melting involves the redistribution of primary valence bonds. Accordingly, this review includes examination some more chemical aspects of the controls of melting. The available data show that enthalpy and density changes during liquefaction and solidification of the metallic elements and of the alkali halides are small. From quantitative consideration of these values, it is concluded that ordered packing arrangements of atoms, ions, or molecules, comparable with those of crystals, must be extensively retained into the melt. The energy and molar volume changes on melting are too small to allow significant departure, in the liquid, from the regular, efficient space-filling arrays that characterize crystalline solids. The set/liq model for melting (dynamic equilibria between alternative ordered structures) is proposed to account for the properties of the liquid. A detailed and critical comparison of melting with solid state decompositions considers the kinetics and the mechanisms of the changes that occur during the supply/removal of energy to/from the melt/crystal contact interface. Other relevant aspects of melting are discussed including the factors that determine the magnitudes of the melting points of individual solids.

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Abstract  

Selected kinetic and mechanistic studies of thermal reactions of initially solid substances are reviewed with emphasis on the evidence that some of these chemical changes proceed with the essential participation of melting. The reactions considered are classified on the extent and the role of such melting and the various types of behaviour observed are discussed with reference to solid state rate processes in crystals. It is stressed that melting is an important feature in theoretical considerations of crystal reactivity because chemical changes often proceed more rapidly in a melt than in the solid state. However, literature reports concerned with reactions of solids do not always explicitly mention the possibility of melting during discussions of reactions mechanisms. The present paper comments on methods capable of detecting liquefaction during reaction, a feature of behaviour that is not always easily identified experimentally. Also considered here is the recognition of reaction intermediates, which provide important evidence concerning the course of the chemical changes through which the reactant is transformed into product. This short review draws attention to the considerable value of chemical evidence in elucidating mechanisms of reactions of solids including the necessity for identifying intermediates and the role of any melt or liquid participating.

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Abstract  

Aspects of the theories that are conventionally and widely used for the kinetic analyses of thermal decompositions of solids, crystolysis reactions, are discussed critically. Particular emphasis is placed on shortcomings which arise because reaction models, originally developed for simple homogeneous reactions, have been extended, without adequate justification, to represent heterogeneous breakdowns of crystalline reactants. A further difficulty in the mechanistic interpretation of kinetic data obtained for solid-state reactions is that these rate measurements are often influenced by secondary controls. These include: (i) variations of reactant properties (particle sizes, reactant imperfections, nucleation and growth steps, etc.), (ii) the effects of reaction reversibility, of self-cooling, etc. and (iii) complex reaction mechanisms (concurrent and/or consecutive reactions, melting, etc.). A consequence of the contributions from these secondary rate controls is that the magnitudes of many reported kinetic parameters are empirical and results of chemical significance are not necessarily obtained by the most frequently used methods of rate data interpretation. Insights into the chemistry, controls and mechanisms of solid-state decompositions, in general, require more detailed and more extensive kinetic observations than are usually made. The value of complementary investigations, including microscopy, diffraction, etc., in interpreting measured rate data is also emphasized. Three different approaches to the formulation of theory generally applicable to crystolysis reactions are distinguished in the literature. These are: (i) acceptance that the concepts of homogeneous reaction kinetics are (approximately) applicable (assumed by many researchers), (ii) detailed examination of all experimentally accessible aspects of reaction chemistry, but with reduced emphasis on reaction kinetics (Boldyrev) and (iii) identification of rate control with a reactant vaporization step (L’vov). From the literature it appears that, while the foundations of the widely used model (i) remain unsatisfactory, the alternatives, (ii) and (iii), have not yet found favour. Currently, there appears to be no interest in, or discernible effort being directed towards, resolving this unsustainable situation in which three alternative theories remain available to account for the same phenomena. Surely, this is an unacceptable and unsustainable situation in a scientific discipline and requires urgent resolution?

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Abstract  

This critical survey argues that the theory, conventionally used to interpret kinetic data measured for thermal reactions of initially solid reactants, is not always suitable for elucidating reaction chemistry and mechanisms or for identifying reactivity controls. Studies of solid-state decompositions published before the 1960s usually portrayed the reaction rate as determined by Arrhenius type models closely related to those formulated for homogeneous rate processes, though scientific justifications for these parallels remained incompletely established. Since the 1960s, when thermal analysis techniques were developed, studies of solid-state decompositions contributed to establishment of the new experimental techniques, but research interest became redirected towards increasing the capabilities of automated equipment to collect, to store and later to analyze rate changes for selected reactions. Subsequently, much less attention has been directed towards chemical features of the rate processes studied, which have included a range of reactants that is much more diverse than the simple solid-state reactions with which early thermokinetic studies were principally concerned. Moreover, the theory applied to these various reactants does not recognize the possible complexities of behaviour that may include mechanisms involving melting and/or concurrent/consecutive reactions, etc. The situation that has arisen following, and attributable to, the eclipse of solid-state decomposition studies by thermal analysis, is presented here and the consequences critically discussed in a historical context. It is concluded that methods currently used for kinetic and mechanistic investigations of all types of thermal reactions indiscriminately considered by the same, but inadequate theory, are unsatisfactory. Urgent and fundamental reappraisal of the theoretical foundations of thermokinetic chemical studies is now necessary and overdue. While there are important, but hitherto unrecognized, delusions in thermokinetic methods and theories, an alternative theoretical explanation that accounts for many physical and chemical features of crystolysis reactions has been proposed. However, this novel but general model for the thermal behaviour and properties of solids has similarly remained ignored by the thermoanalytical community. The objective of this article is to emphasize the now pressing necessity for an open debate between these unreconciled opinions of different groups of researchers. The ethos of science is that disagreement between rival theories can be resolved by experiment and/or discussion, which may also strengthen the foundations of the subject in the process. As pointed out below, during recent years there has been no movement towards attempting to resolve some fundamental differences of opinion in a field that lacks an adequate theory. This should be unacceptable to all concerned. Here some criticisms are made of specific features of the alternative reaction models available with the stated intention of provoking a debate that might lead to identification of the significant differences between the currently irreconciled views. This could, of course, attract the displeasure of both sides, who will probably criticise me severely. Because I intend to retire completely from this field soon, it does not matter to me if I am considered to be ‘wrong’, if it contributes to us all eventually agreeing to get the science ‘right’.

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A view and a review of the melting of alkali metal halide crystals

Part 2. Pattern of eutectics and solid solutions in binary common ion mixtures

Journal of Thermal Analysis and Calorimetry
Author: A. K. Galwey

A representational model, proposed to account for the physical changes that accompany the melting of alkali halides, was described in Part 1 [1]. The liquid is portrayed as undergoing continual dynamic structural reorganization of its constituent ions between individual small domains, zones of various regular, crystal-type arrays. These alternative arrangements are stabilized by the enthalpy of melting, which, in liquids, relaxes the restriction for solids that only the single, most stable, crystal structure can be present. The dynamic character of the melt accounts for its fluid character and the loss of long-range order [1, 2]. This model is extended here to consider the phase diagrams of binary, common ion, alkali halide mixtures comprehensively reviewed in [3]. Factors determining whether each of these yields a eutectic, or a solid solution, on cooling are discussed and several trends in the 70-phase diagrams are identified. Eutectic formation, involving maintenance of the liquid state below the melting points of the pure components, is ascribed to the participation, in an extended dynamic equilibrium, of additional domains having the regular structures characteristic of double salts. The known crystalline double binary halides [3], Li/Cs or Rb/F, Cl, Br or I, melt at temperatures well below those of the simpler pure component salts. It is concluded that the set/liq model for melting, proposed in [1, 2], accounts for some important properties of the phase diagrams presented in [3].

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Summary  

Although melting is a most familiar physical phenomenon, the nature of the structural changes that occur when crystals melt are not known in detail. The present article considers the structural implications of the changes in physical properties that occur at the melting points, T m, of the alkali halides. This group of solids was selected for comparative examination because the simple crystal lattices are similar and reliable data are available for this physical change. For most of these salts, the theoretical lattice energies for alternative, regular ionic packing in 4:4, 6:6 and 8:8 coordination arrangements are comparable. Density differences between each solid and liquid at T mare small. To explain the pattern of quantitative results, it is suggested that the melt is composed of numerous small domains, within each of which the ions form regular (crystal-type) structures (regliq). The liquid is portrayed as an assemblage of such domains representing more than a single coordination structure and between which dynamic equilibria maintain continual and rapid transfers of ions. T mis identified as the temperature at which more than a single (regular) structure can coexist. The interdomain (imperfect and constantly rearranging) material (irregliq) cannot withstand shear, giving the melt its fluid, flow properties. From the physical evidence, it is demonstrated that the structural changes on melting are small: these can accommodate only minor modifications of the dispositions of all, or most, ions or larger changes for only a small fraction. This proposed representation, the set/liq melt model, may have wider applicability.

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Abstract  

Professor Vladimir V. Boldyrev has made numerous important contributions to a wide range of chemical topics, not only limited to studies of the decompositions of solids. Of particular value has been his emphasis on exploring, in detail, the chemical steps participating in the thermal reactions of solids by carefully designed experiments that rely on more observational evidence than the run-of-the-mill collection of overall kinetic data. Some of these major contributions to both the theory and the uses of solid-state reactions are identified here and discussed in relation to his illuminating and fundamental mechanistic studies of the thermal decompositions of silver oxalate, ammonium perchlorate, potassium permanganate and the dehydration of copper sulfate pentahydrate.

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The kinetics of isothermal dehydration of LiKC4H4O6.H2O single-crystals was investigated in the [001] crystallographic direction under a dynamic vacuum of 6.7×10−5 Pa with a quartz crystal microbalance. The removal of H2O molecules may be described by a diffusion equation for a semi-infinite medium. The diffusion coefficients vary from 2.13×10−11 cm2 s−1 at 391.7 K to 9.9×10−9 cm2 s−1 at 453.2 K. The scanning electron microscope data provide some evidence that the dehydration is not a topochemical reaction. From the experimental data it is concluded that the anhydrous product is in the state of “premelting”. This explains the anomalous diffusion energyE D=37±1 kcal mol−1 and preexponential factorD 0=5×109 cm2 s−1.

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