Ionic crystals stability

Table 3.1-5 Melting points and heats of fusion for isomeric [BMIM][PFg] and [PMIM][PFs] ionic liquids, showing melting point and crystal stability increasing with the degree of branching in the alkyl substituent.

Ionic bond, 287, 288 dipole of, 288 in alkali metal halides, 95 vs. covalent, 287 Ionic character, 287 Ionic crystal, 81, 311 Ionic radius, 355 Ionic solids, 79, 81, 311 electrical conductivity, 80 properties of, 312 solubility in water, 79 stability of, 311... 

Of the three principal classes of crystals, ionic crystals, crystals containing electron-pair bonds (covalent crystals), and metallic crystals, we feel that a good understanding of the first class has resulted from the work done in the last few years. Interionic distances can be reliably predicted with the aid of the tables of ionic radii obtained by Goldschmidt1) by the analysis of the empirical data and by Pauling2) by a treatment based on modem theories of atomic structure. The stability,... 

The Relative Stability of Alternative Structures of Ionic Crystals.—... 

Cryptands, 42 122-124, 46 175 nomenclature, 27 2-3 topological requirements, 27 3-4 Cryptate, see also Macrobicyclic cryptate 12.2.2], 27 7-10 applications of, 27 19-22 cylindrical dinuclear, 27 18-19 kinetics of formation in water, 27 14, 15 nomenclature, 27 2-3 spherical, 27 18 stability constants, 27 16, 17 Crystal faces, effect, ionic crystals, in water, 39 416... 

This simple rule restricts greatly the acceptable structures for a substance, and it has been found useful in the determination of the structures of complex ionic crystals, including especially the silicate minerals. The rule is satisfied nearly completely by most, of the structures that have been reported for the silicate minerals, deviations by as much as i being rare. Somewhat larger deviations from the rule are occasionally found for substances prepared in the laboratory, for which stability as great as for minerals is not expected. 

If all the packing atoms are no longer neutral (e.gn half are cations and half are anions), the closest packed structures are no longer the most stable, as can be seen from the similar two-dimensional case (see above). However, these structures may still be useful when considered as limiting cases for certain ionic crystals. Consider lithium iodide, in which the iodide anions are so much larger than the lithium cations that they may be assumed to touch or nearly touch. They can be considered to provide the framework for the crystal. The much smaller lithium ions can then fit irto the small interstices between the anions. If they expand the lattice slightly to remove the anion-anion contact, the anionic repulsion will be reduced and the crystal stabilized, but the simple model based on a closest packed system of anions may still be taken as the limiting case and a useful approximation. 

A gradient of electrical potential constitutes the classic (external) force field for ionic solids. Let us study the effect of this electric field on the interface morphology and stability. The thermodynamic driving force in ionic crystals is Vi/,(= +... 

However, doubly ionized oxygen, O2-, in Cu oxides, emits an electron in a vacuum, but is to be stabilized in an ionic crystal, and the author found that delocalization of electrons on the oxygen site causes the antiferromagnetic moment on the metal site. The analysis was performed by changing width and depth (including zero depth) of a well potential added to the potential for electrons of oxygen atom in deriving numerical trial basis functions (atomic orbitals). (The well potential was not added to copper atom.) The radial part of trial basis function was numerically calculated as described in the previous... 

A model has been given for the breaking up of an ionic crystal into free ions which stabilize themselves in solution with solvent sheaths. One central theme guided the account, the interaction of an ion with its neighboring water molecules. 

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