Bonding in Solids Some Illustrative Cases

The extent and character of directed valence bonds depends on the completeness of s-p hybridization, and this in turn depends on the apparent s-p energy difference in the bonded state not in the free atom). Some ways in which this apparent energy difference can be defined are discussed in the next section. In any case, it is interesting to utilize here the completeness of s-p hybridization as a means of illustrating the differences in chemical bonding in molecules and crystals. For molecules it is clear, from the richness and variety of organic compounds, that directed bonds are most easily formed with atoms from the first period. In crystals, on the other hand, perfect hybridization occurs most often with atoms from the second period (especially Si, P, and S, which are most likely to form directed bonds in solids). This difference arises primarily because of the greater... 

Section 4 is entirely devoted to ferroelectric and H-bonded systems. It also provides a nice illustration of results that always maintained the utility of proton NMR in solid state, even wideline, or how the old question of the order disorder or displacive nature of some ferroelectric phase transitions were reopened by progresses in NMR resolution. A number of structural phase transition is discontinuous, but the examples of coexistence in solid-state and kinetic studies are rather scarce this is the object of Section 5. Section 6 is devoted to single-crystal studies that allow very precise comprehension of subtle phase transition mechanisms. Section 7 introduces the salient features of NQR that represent an interesting alternative to NMR in some cases. The section ends with a table of miscellaneous phase transitions that complete the references given in the text. Section 8 concludes and presents some perspectives in NMR phase transition studies. 

In addition, the 4f electrons are shielded by the outer 5s and 5p orbitals, and the M-X chemical bond is only minimally affected by a 4f electronic excitation. Consequently, equilibrium bond lengths in the ground and excited states are similar as illustrated in Fig. 8 (right). Excitation and relaxation of Ln ions can therefore be dominated by electronic transitions with only minimal involvement of vibrational modes E kEa, no Stokes shift), thereby ehminat-ing much of the internal heating that is inherent to Stokes-shifted parity-allowed transitions. This key feature makes Ln ions particularly attractive for solid-state laser cooUng because it allows anti-Stokes fluorescence to dominate in some special cases. 

Tautomerism may be of many kinds as illustrated in this book and in Ref. [1]. However, tautomerism may be less obvious depending on the rates of interconversion or the equilibrium constant may be very much in favor of one form. A typical example of the latter is acetone and other ketones. Nevertheless, tautomerism plays a major role in the chemistry of alkyl ketones. Tautomerism may occur both in the gas phase, namely the liquid and condensed phases, and in the solid state, namely in the ground state as well as in the excited state. As tautomerization often involves the movement of light atoms, proton transfer is the usual case and hydrogen bonding will often be involved. Tautomerization can of course be both intra- and intermolecular, and in the latter case a solvent molecule may also be involved. For some characteristic cases, see Ref. [1, Chapter 3]. 

---