Crystal relation with coordination number

If we neglect the H atom in LiOH then its structure corresponds to the anti-PbO type. Since the axial ratio and the free positional parameter of this tetragonal structure can vary in a certain range, different isopuntal structure [9] types are possible. Thus for c/a = V2 and z(anion) = j, a cubic close-packing of the anions results with the cations in tetrahedral holes. An axial ratio c/a = 1/V2 and an anion parameter z =, on the other hand, correspond to the CsCl type with coordination number 8. As follows from Table 56, LiOH approximates a cubic close-packing with Li in deformed tetrahedral coordination. The position of the lone electron pair of PbO is here taken by H (corrected O—H distance 0.98 A [325] similar to the lone pair-cation distance). The electron density corresponds to Li 0 ° H and one electron smeared between the layers [1012]. In Table 55, LiOH is compared with chemically related compounds. Lithium amide has a closely related structure in which the layers of tetrahedral cation sites are alternately I and i occupied (5T1 + IT2 and ti, respectively) instead of the completely occupied and completely empty layers of LiOH. This is obviously a consequence of the weaker dipole character of NHJ. LiF, with no dipole moment, crystallizes in the rocksalt structure. The structure of LiSH is similar to chalcopyrite whereas that of the hydrosulfides and hydroselenides of Na, K and Rb is a rhombohedrally deformed rocksalt type. 

Instead, we believe the electronic structure changes are a collective effect of several distinct processes. For example, at surfaces the loss of the bulk symmetry will induce electronic states with different DOS compared to bulk. As the particle sizes are decreased, the contribution of these surface related states becomes more prominent. On the other hand, the decrease of the coordination number is expected to diminish the d-d and s-d hybridization and the crystal field splitting, therefore leading to narrowing of the valence d-band. At the same time, bond length contraction (i.e. a kind of reconstruction ), which was observed in small particles [89-92], should increase the overlap of the d-orbitals of the neighboring atoms, partially restoring the width of the d-band. 

J6 The ammonium km is about the same size (r+ = 151 pm) as the potassium ion ir. 152 pm) and this is a usef ul fact to remember when explaining the resemblance in properties between these two tuns. For example, (he solubilities of ammonium salts arc similar to those of potassium sails. Explain the relation between ionic radius and soloWiiy. On the other hand, all of the potassium halides crystallize in the NaClstrocture with C.N. = 6 (see Chapter 4). but none of the ammonium halides does so. The coordination numbers of the ammonium halides are either four or eight- Suggest an explanation. 

There seems to be even less structural similarity for many other metal halides as the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, e.g., crystallizes in a hexagonal layer structure. Upon melting, and then, upon evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures they dissociate into monomers (Figure 9-58) [107], The coordination number decreases from 6 to 4 and then to 3 in this process. However, at closer scrutiny, even the dimeric aluminum trichloride molecules can be derived from the crystal structure. Figure 9-59 shows another representation of crystalline aluminum trichloride which facilitates the identification of the dimeric units. A further example is chromium dichloride illustrated in Figure 9-60. The small oligomers in its vapor have structures [108] that are closely related to the solid structure [109], Correlation between the molecular composition of the vapor and their source crystal has been established for some metal halides [110],... 

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