The A 1 (TO) phonon peaks are shifted by about 2.4 cm -1 towards lower wavenumbers for the sample doped with As (ZAs02_3) and Sb (ZSb01_3) as compared to the undoped one (inset in Fig. ). It is known that in A 1 (TO) Zn and O atoms move in opposite directions along the C3 axis 41 .