  In this paper we report phonon dynamics in chemically synthesized Zn1- xMgxO
(0\leqx\leq0.07) and Zn1-yCdyO (0\leqy\leq0.03) alloy nanostructures of sizes
~10 nm using non-resonant Raman and Fourier Transformed Infrared Spectroscopy
(FTIR). Substitution by Mg makes the unit cell compact while Cd substitution
leads to unit cell expansion. On alloying, both A1(LO) and E1(LO) mode of
wurtzite ZnO show blue shift for Zn1-xMgxO and red shift for Zn1-yCdyO alloy
nanostructures due to mass defect and volume change induced by the impurity
atoms. Significant shift has been observed in E1(LO) mode for Zn1-xMgxO (73
cm-1 for x = 0.07) and Zn1-yCdyO (17 cm-1 for y = 0.03) nanostructures. The
variation in Zn(Mg,Cd)-O bond length determined from the blue (red) shift of IR
bands on alloying with Mg (Cd) is consistent with their respective ionic sizes
and the structural changes predicted by X-ray diffraction study. However, on
progressive alloying one can detect phase segregation (due to presence of
interstitial Mgand Cd ions) in the alloy nanostructures for relatively higher
Mg and Cd concentrations. This is confirmed by the gradual absence of the
characteristic IR and Raman bands of wurtzite ZnO near 400-600 cm-1 as well as
by X-Ray and TEM studies.
