The Transverse Effective Charges of the IV–VI Semiconductors

The quantum mechanical treatment of the crystal energy of the IV–VI semiconductors developed in our previous paper is extended so as to be applicable to the case in which there exists a relative ionic displacement corresponding to the TO phonon at point \(\). It is shown that the unusually large transverse effective charges in these crystals come partly from rather large electronic polarizabilities of their constituent ions and partly from covalent bonds contained partially in the interatomic bonds in these crystals. The theory contains only one adjustable parameter concerning local field correction which is related with penetration between nearest neighboring ions. Using its numerical value determined for PbS, we can explain the transverse effective charges of the IV–VI semiconductors quantitatively. The relation between Katayama–Kawamura's treatment and ours is discussed briefly.