Studies of spinels. VII. Order-disorder transition in the inverse germanate spinels Zn2−x(Co, Ni)xGeO4(x ⋍ 1)

A series of germanate spinels Zn2−x(Ni, Co)xGeO4 has been synthesized and investigated by X-ray diffractometry and infrared spectroscopy. Synthesis at 1200°C leads to cubic phases characterized by an inverse, disordered distribution of Ge and bivalent cations (essentially Ni or Co) over the octahedral sites; however, the presence of some short-range order is suggested by the infrared spectrum. Tempering at an appropriate temperature (between 800 and 900°C, depending on the Co/Ni ratio) leads to tetragonal spinel phases, corresponding to the 1:1 order between Ge and bivalent cation on octahedral sites. The transformation is sluggish and proceeds at a significant rate only in a narrow temperature range, just below the transition temperature. A comparison with the behavior of known inverse, ordered titanate spinels shows that, within the family of inverse II–IV spinels M2IIMIVO4, the ratio of the octahedral cationic radii rMII/rMIV is one of the factors determining the order-disorder transition temperature, and the importance of the tetragonal distortion. © 1980, All rights reserved.