Phonon-Raman Scattering in the Two-Dimensional Diluted System K₂Cu_<i>x</i>Zn_1-<i>x</i>F₄

Abstract

Phonon-Raman spectra of K 2 Cu x Zn 1- x F 4 (1≧ x ≧0) measured at room temperature are reported. With the change of Cu 2+ -concentration ( x ), there appear two-types of Stokes lines; one is observable for 1≧ x ≧0, the other only for \(1{\geqq}x{\gtrsim}0.4\). The characteristic behavior of the latter type is closely related with the disappearance of the cooperative Jahn-Teller distortion well confirmed in K 2 CuF 4 . The results are discussed in connection with the percolation phenomena of a square lattice. Judging from the theoretical suggestion that the percolation thresholds for the first and second nearest neighbor couplings are 0.59 and 0.41 respectively, we point out that the coupling between the second nearest neighbors in the c-plane plays an important role on forming an infinite linkage of the cooperative distortion especially over the range of \(0.59{\gtrsim}x{\gtrsim}0.4\).