Study of phonons and normal mode analysis of perovskite-type superionic conductors

A de Launey angular force (DAF) model has been used to calculate the phonon frequency at zone centre (ZC) and normal modes in phase I of CsPbCl3 and t he gamma -phase of Ag3SI and Ag3SBr by assuming them to be ideal perovskite (P)-type structures. Three central force constants, alpha (1), alpha (2), and alpha (3), and three angular force constants, alpha'(1), alpha'(2) and alpha'(3), for Pb-Cl (S-Ag), Cs-CI (I ol Br-Ag) and Cs-Pb (S-I or Br) atoms are employed and determined from the experimental values of transverse inf rared (IR) active phonon frequencies at ZC. The rr, value between Cs and Ph is the strongest among all of these compounds. This is reasonable since bo th Binds of ions compose the cage lattice in the superionic conduction phas e. The normal coordinate of the highest frequency (I;) mode is assigned to be different from that for other perovskites. It reasonably interprets the higher values of nu (2) and nu (3) for CsPbCl3 than those of Ag3SX (X = Br, I) and also the lower value of nu (1) for CsPbCl3. Based on these results, the complete phonon dispersion curve is calculated. (C) 2000 Elsevier Scien ce B.V. All rights reserved.