*2.1. Chemicals*

Ga2O3 (99.999%), GeO2 (99.999%), LiNO3 (99.9%), Mn(NO3)2·*<sup>x</sup>*H2O (99.9% metals basis), Zn(NO3)2·6H2O (99%), ZnO (99.99%), and MnO (99.99%) were obtained from Aladdin (Shanghai, China). Hydrochloric acid (AR, 36.0–38.0%), nitric acid (AR, 36.0–38.0%), sodium hydroxide, concentrated ammonium hydroxide, anhydrous ethanol and polyvinyl alcohol (PVA) with analytical grade are stocked from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).
