Bases lattice energy

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

More recently considered candidates are large molecular anions with delocalized anionic charge, which offer low lattice energies, relatively small ion-ion interaction, and hence sufficient solubility and relatively large conductivity. Delocalization of the charge is achieved by electron-with drawing substituents such as -F or - CF3. Furthermore, these anions show a good electrochemical stability to oxidation. In contrast to Lewis acid-based salts they are chemically more stable with various solvents and often also show excellent thermal stability. 

The table shows the lattice energy for some ionic compounds. Based on these data, which of these compounds would require the most energy to separate the bonded ions ... 

Conventional electrolytes applied in electrochemical devices are based on molecular liquids as solvents and salts as sources of ions. There are a large number of molecular systems, both pure and mixed, characterized by various chemical and physical properties, which are the liquids at room temperatures. This is the reason why they dominate both in laboratory and industrial scale. In such a case, solid salt is reacted with a molecular solvent and if the energy liberated during the reaction exceeds the lattice energy of the salt, the solid is liquified chemically below its melting point, and forms the solution. Water may serve as an example of the cheapest and most widely used molecular solvent. 

There is another use of the Kapustinskii equation that is perhaps even more important. For many crystals, it is possible to determine a value for the lattice energy from other thermodynamic data or the Bom-Lande equation. When that is done, it is possible to solve the Kapustinskii equation for the sum of the ionic radii, ra + rc. When the radius of one ion is known, carrying out the calculations for a series of compounds that contain that ion enables the radii of the counterions to be determined. In other words, if we know the radius of Na+ from other measurements or calculations, it is possible to determine the radii of F, Cl, and Br if the lattice energies of NaF, NaCl, and NaBr are known. In fact, a radius could be determined for the N( )3 ion if the lattice energy of NaNOa were known. Using this approach, which is based on thermochemical data, to determine ionic radii yields values that are known as thermochemical radii. For a planar ion such as N03 or C032, it is a sort of average or effective radius, but it is still a very useful quantity. For many of the ions shown in Table 7.4, the radii were obtained by precisely this approach. 

Fig. 6.5 The conductivity of PEO based networks containing different salts plotted at a reduced temperature (7 + T) as a function of the lattice energy of the incorporated salt.

An alternative (and probably more precise) method for evaluating defect energies is based on the calculation of lattice energy potentials. 

Table 4.2 Defect energies in forsterite and fayalite based on lattice energy calcnlations. 7 = ionization potential E = electron affinity = dissociation energy for O2 A77 = enthalpy of defect process (adapted from Ottonello et af, 1990).

Several issues remain to be addressed. The effect of the mutual penetration of the electron distributions should be analyzed, while the use of theoretical densities on isolated molecules does not take into account the induced polarization of the molecular charge distribution in a crystal. In the calculations by Coombes et al. (1996), the effect of electron correlation on the isolated molecule density is approximately accounted for by a scaling of the electrostatic contributions by a factor of 0.9. Some of these effects are in opposite directions and may roughly cancel. As pointed out by Price and coworkers, lattice energy calculations based on the average static structure ignore the dynamical aspects of the molecular crystal. However, the necessity to include electrostatic interactions in lattice energy calculations of molecular crystals is evident and has been established unequivocally. 

As was indicated in Sec. 1.2, the conclusion that the deformation phenomena play the smallest role in NaF was based first on the statement that its heat of sublimation (S) constitutes among the alkali halides the largest fraction of the lattice energy (17). The corresponding data in Table 5 show that the gradation of this fraction, SjU, is in fact closely parallel to that of the degree of polarity p both properties show a... 

Chalcogenides, 5 94-96 berkelium, 28 49, 53-54 lattice energies of, 1 192, 193 ligands, 45 16 Chalcogen(II) compounds binary halides, 35 274—280 complexes with Lewis bases, 35 293-295 halo-chalcogenates(ll), 35 280-288 mixed-valence compounds, 35 288-293 cationic species, 35 291-293... 

If local stresses exceed the forces of cohesion between atoms or lattice molecules, the crystal cracks. Micro- and macrocracks have a pronounced influence on the course of chemical reactions. We mention three different examples of technical importance for illustration. 1) The spallation of metal oxide layers during the high temperature corrosion of metals, 2) hydrogen embrittlement of steel, and 3) transformation hardening of ceramic materials based on energy consuming phase transformations in the dilated zone of an advancing crack tip. 

Comparison of measured heats of soln with estimated heats of soln based on chemical analyses of the samples indicated that the derived values of apparent lattice energy could be accounted for on the basts of changes in chemical compn alone, particularly the accumulation of free perchloric acid in the irradiated samples... 

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