is the reduced effective mass of positive and negative ions contributing to phonon mode with masses M + and M _ , correspondingly; ε and ε ∞ are the static and high frequency dielectric permittivity. Note, that expression (2) gives reasonable estimate for A 1 (TO) peak position k 0 =358 cm -1 in ZnO. For this estimate we took Millikan charge calculated for ZnO clusters q=0.9e (e is the elementary charge) 43 , ε ∞ =n 0 2 =4 (n 0 is the refractive index) 44 , ε=8.6 45 . This