Phase Transformation of Metastable Cu₂ZnGeO₄ with a Wurtz-Kesterite Structure at Elevated Temperatures

The thermal stability and high-temperature phase transformation of metastable Cu₂ZnGeO₄ were investigated in an Ar atmosphere by thermogravimetry, differential thermal analysis, and high-temperature X-ray diffraction. Three Cu-deficient Cu^I_2-ZnGeO_4-2/ phases with a wurtzite-related structure were observed, with varying amounts of copper deficiency. The metastable Cu₂ZnGeO₄ was stable at approximately 275 °C and transformed into intermediate phases. The intermediate phases had a wurtz-kesterite structure with a small number of copper and oxygen vacancies, which later transformed into a high-temperature phase at approximately 425 °C. The crystal structure of the high-temperature phase was assumed to be a deficient wurtzite-related structure with hexagonal closely packed oxygen and deficient copper sites on the order of tens of a percent. The high-temperature phase decomposed into stable Cu₂O, GeO₂, and Zn₂GeO₄ phases above 550 °C. The mechanism for the formation of the phase with a large amount of copper deficiency is discussed, leading to an understanding of the formation process for the copper-deficient phase of complex compounds containing monovalent copper.