  In this paper we report structural and optical properties of Magnesium
substituted Zinc Oxide (Zn1-xMgxO) nanocrystals (~10-12nm) synthesized by low
temperature route. In the low temperature synthesis route it was possible to
reach x = 0.17 without segregation of Mg rich phase. The exact chemical
composition has been established by quantitative analysis. Rietveld analysis of
the XRD data confirms the Wurzite structure and a continuous compaction of the
lattice (in particular the c-axis parameter) as x increases. There is an
enhancement of the strain in the lattice as the Mg is substituted. The bandgap
also gets enhanced as x is increased and reaches a value of 4eV for x = 0.17.
From the TEM and the XRD data it has been concluded that when there is a phase
segregation for x > 0.17, there is a shell of Mg(OH)2 on the ZnO. The
absorption also shows persistence of the excitoinc absorption on Mg
substitution. The nanocrystals show near band edge photo luminescence (PL) at
room temperature which shows blue shift on Mg incorporation. In addition to the
near band edge emission the ZnO and Zn1-xMg xO alloy nanocrystals show
considerable emission in the blue-green region at wavelength of ~550 nm. We
find that the relative intensity of the green emission increases with the Mg
concentration for very low x (upto x = 0.05) and on further increase of the Mg
concentration there is a sharp decrease of relative intensity of the green
emission eventually leading to a complete quenching of blue emission. It is
concluded that due to phase segregation (for x \geq 0.20), the formation of the
shell of Mg(OH)2 on the ZnO leads to quenching of the green emission .However,
this shell formation does not have much effect on the near band edge PL.
