Elastic constants covalent crystals

The perfect-crystal model of Karasawa et al. considers a basic force-field approach for the analysis of crystal properties. The model contains covalent-bonded interactions along the polymer chain as well as non-bonded van der Waals interactions between molecules and Coulombic interactions when relevant, all with appropriate temperature dependences. Table 4.3 lists some of the temperature-dependent elastic moduli and some elastic constants cy of ideal polyethylene, determined by Karasawa et al. (1991), which will be of interest to us in later chapters. These are shown also in Fig. 4.1. Of these Ec (= l/ssj) gives directly the main-chain Young s modulus of polyethylene. Also listed is the transverse shear elastic constant c e, which can be considered to be a good measure of... 

Lattice relaxation Shm = K (K is the elastic constant) accompanies electronic transition in the covalence and ionic crystals. The physical situation is presented by the configurational coordinate diagram in Fig. 4.25. The FT is indicated in Fig. 4.25 by a blue arrow. The IT creates the system in the + e state (solid blue... 

Plausible elastic constants of cBN are listed in Table 5 by summing up the theoretically expected (10,94-96,124,140,141) and experimentally observed data (74,143-145). Griineisen parameters (143,144) and linear thermal expansion coefficients near room temperature (146,147) are also listed. The elastic constants of cBN are about 20-40% lower than those of diamond and several times larger than those of other covalent crystals such as Si and GaAs. Although cBN is stiff, it is elastic and can be distorted the volume of cBN decreases elastically by about 20% at 100 Gpa (Fig. 21) (74). 

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