Alkali halides electronic structure

Miller T M, Leopold D G, Murray K K and Lineberger W C 1986 Electron affinities of the alkali halides and the structure of their negative ions J. Chem. Phys. 85 2368-75... 

Sometimes the atomic arrangement of a crystal is such as not to permit the formulation of a covalent structure. This is the case for the sodium chloride arrangement, as the alkali halides do not contain enough electrons to form bonds between each atom and its six equivalent nearest neighbors. This criterion must be applied with caution, however, for in some cases electron pairs may jump around in the crystal, giving more bonds than there are electron pairs, each bond being of an intermediate type. It must also be mentioned that determinations of the atomic arrangement are sometimes not sufficiently accurate to provide evidence on this point an atom reported equidistant from six others may be somewhat closer to three, say, than to the other three. 

The alkali halides cire noted for their propensity to form color-centers. It has been found that the peak of the band changes as the size of the cation in the alkali halides increases. There appears to be an inverse relation between the size of the cation (actually, the polarizability of the cation) and the peak energy of the absorption band. These are the two types of electronic defects that are found in ciystcds, namely positive "holes" and negative "electrons", and their presence in the structure is related to the fact that the lattice tends to become charge-compensated, depending upon the type of defect present. 

Based on the ionic radii, nine of the alkali halides should not have the sodium chloride structure. However, only three, CsCl, CsBr, and Csl, do not have the sodium chloride structure. This means that the hard sphere approach to ionic arrangement is inadequate. It should be mentioned that it does predict the correct arrangement of ions in the majority of cases. It is a guide, not an infallible rule. One of the factors that is not included is related to the fact that the electron clouds of ions have some ability to be deformed. This electronic polarizability leads to additional forces of the types that were discussed in the previous chapter. Distorting the electron cloud of an anion leads to part of its electron density being drawn toward the cations surrounding it. In essence, there is some sharing of electron density as a result. Thus the bond has become partially covalent. 

The two-step process of epitaxial polymerization has been applied to symmetrically substituted diacetylenes First, the monomers have been crystallized epitaxially on alkali halides substrates from solution and the vapor phase. The oriented monomer crystals are then polymerized under the substrate s influence by gamma-irradiation. The diacetylenes in this study are 2,4-hexadiyn-l,6-diol (HD) and the bis-phenylurethane of 5,7-dodecadiyn-l,12-diol (TCDU). The polydiacetylene crystal structures and morphologies have been examined with the electron microscope. Reactivity and polymorphism are found to be controlled by the substrate. 

Pascal s work (11—13)) electron cloud radii also seem to be transferable. We begin therefore by comparing directly the structural distances D and the diamagnetic susceptibihties xm (the direct data of experiment) for the inert gases, their isoelectronic alkali halides, and their (almost) iso-electronic halogen molecules — three systems of very different bond-types. 

Br- (g). The electron affinity of Br (g) is calculable by the method of lattice energies. Selecting the crystal RbBr, because Rb+ and Br have exactly the same nuclear structure, and taking the exponent of the repulsive term to be 10, we have computed, for the reaction, RbBr (c) = Rb+ (g)+Br g), Dz= —151.2 whence the electron affinity of Br (g) becomes 87.9. Using the lattice energies of the alkali bromides as calculated by Sherman,1 we have computed the values 89.6, 85.6, 84.6, 83.6, and 89.6, respectively. Butkow,1 from the spectra of gaseous TIBr, deduced the value 86.5. From data on the absorption spectra of the alkali halides, Lederle1 obtained the value 82. See also Lennard-Jones.2... 

Thus increased covalent bonding resulting from Fajans-type phenomena can lower the transition temperatures. For example, the alkali halides (except CsCI, CsBr. and Csl) and the silver halides (except Agl) crystallize in the NaCI structure. The sizes of the cations are comparable Na = M6 pm. Ag = 129 pm, K = 152 pm, yet the melting points of the halides are considerably different (Table 8.6). The greater covalent character of the silver halide bond (resulting from the electron confi J ra-... 

It was Ziman [77] who has noted that there is little hope, at least at present, to develop an experimental technique permitting the direct measurement of these correlation functions. The only exception are the joint densities x / (r> ) information about which could be learned from the diffraction structural factors of inhomogeneous systems. On the other hand, optical spectroscopy allows estimation of concentrations of such aggregate defects in alkali halide crystals as Fn (n = 1,2,3,4) centres, i.e., n nearest anion vacancies trapped n electrons [80]. That is, we can find x mK m = 1 to 4, but at small r only. Along with the difficulties known in interpretating structure factors of binary equilibrium systems (gases or liquids), obvious specific complications arise for a system of recombining particles in condensed media which, in its turn, are characterized by their own structure factors. 

As it is known, I centres are the most mobile radiation-induced radiation defects in alkali halides and therefore they play an essential role in low-temperature defect annealing. It is known, in particular, from thermally-stimulated conductivity and thermally-stimulated luminescence measurements, that these centres recombine with the F and F electron centres which results in an electron release from anion vacancy. This electron participates in a number of secondary reactions, e.g., in recombination with hole (H, Vk) centres. Results of the calculations of the correlated annealing of the close pairs of I, F centres are presented in Fig. 3.11. The conclusion could be drawn that even simultaneous annealing of three kinds of pairs (Inn, 2nn and 3nn in equal concentrations) results in the step-structure of concentration decay in complete agreement with the experimental data [82]. 

Numerous data about the processes of the tunneling recombination of radiation defects have been obtained in studies on tunneling recombination luminescence. The recombination luminescence of y-irradiated alkali halide crystals was discovered in the mid-1960s [58, 59] in studying the transfer of electrons from Ag and T1 atoms (electron donors) to Cl2 particles (electron acceptor). The Ag and T1 atoms are formed as a result of the action of irradiation on alkali halide crystals which contain Ag+ or Tl+ additives in amounts of about 10 3M. The electrons generated by the irradiation reduce the Ag4 or TU ions to Ag° or Tl° while the hole centres are stabilized in the form of the Cl2 ion occupying two anion positions in the lattice. The hole centres of this kind, whose structure is depicted schematically in Fig. 16, are referred to as Vk-centres. 

The Madelung constant is unique ftk ejj h crystal structure and is defined only for those whose interatomic vectors are fixed by symmetry. The Born exponent, n, can be cslimatcd Hfor alkali halides by the noblc-gas-likc electron configuration of the Vigny It can also be estimated from the compressibility of the crystal system. For NaCl, n equals 9.1. 

These spectra exist, characteristically, of bands which may be very broad or very narrow, and which may show vibrational structure. Examples of very broad bands without any structure at all, not even at very low temperatures, are found in the spectra of the F centre (an electron trapped at a halide vacancy in the alkali halides), and the tungstate (WO2-) group in CaW04. The spectral width may approach a value of 1 eV, and the Stokes shift of the emission band may be 2 eV. 

In this review, the relationships between structure, morphology, and surface reactivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiO-MgO, CoO-MgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported. 2001... 

The new radii are much the same as those derived in an analogous way by Gourary and Adrian (18) and they reproduce r0 distances of the alkali halides of B1 structure to within about one per cent, with the exception of ro = 2.01 A in lithium fluoride. The electron distribution in the latter compound has been elucidated by Krug, Witte and Wolfel (19) and a map for the (100) plane is illustrated in Fig. 2. It will be noted that the ions in... 

These processes give rise to the electronic absorption bands of lowest energy observed in the pure undamaged single crystals which occur at 7.68 eV for MgO and 6.8 eV for CaO (142). Defects within the crystal structure are associated with optical absorption bands at reduced energies [for example, the anion vacancy band in the alkali halides (143)] because of the lower Madelung potential. The energy is still absorbed by the processes described in Eqs. (27) and (28), but the exciton is now bound to a defect and is equivalent to an excited electronic state of the defect. 

The optical absorption spectra were obtained earlier and interpreted for a very large number of ionic crystals. We have made use of such data in obtaining the numbers given in Tables 14-1 through 14-3. Reference was made in particular to the recent ultraviolet photoelectron-emission studies of the alkali halides by Poole et al. (1975), who reviewed other optical studies as well. We have noted the trends that are present in the electronic structure and that are reflected in these studies. 

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