Nano-sized powders of Mn⁺² doped ZnO and Cr⁺³ doped ZnO have been synthesized in a sonochmical reactor and in a microwave domestic oven. The sonochemical reaction was carried out At 20 KHz frequency, with Power density 100 W/cm², under a pressure of 0.15 MPa of argon for 3 hours. The average temperature of the process did not exceed 70^oC. The process was carried out in water and ethylene glycol. The reaction yield was 15 to 55% depending on synthesis conditions. Better results have been obtained for ethylene glycol. The microwave driven hydrothermal reaction has been carried out at 150^oC and 280^oC for 40 min. and he yield was over 50%. The reaction substrates were a mixture of zinc acetate, Mn²⁺ acetate or Cr³⁺ acetate in the corresponding solvent.

The density, grain size, and lattice constants of the samples have been investigated. The lattice constant increases with Mn content, but the rate of increase decreases above 8% of Mn content. This result speaks for a substitution of Zn ions by Mn ions, however for a limited solubility of Mn.

A phase analysis of the powders obtained shows that although only XRD peaks corresponding to ZnO have been observed for 10% Mn concentration, for the other elements at 10% initial concentration a peak corresponding to of their oxides appears, indicating a lower solubility limit in the ZnO lattice than in the Mn case. The grain size of the powders obtained is in the range of 20 - 40 nm.
Besides the structure investigations, also optical and magnetic properties of the samples were measured.

The results obtained indicate that both the ultrasound driven and microwave driven reactions are suitable for doping ZnO with transition metals ions. However, the microwave reaction was more efficient, most likely because of the higher average temperature comparing to the ultrasound driven one.