Studies of spinels. VII. Order-disorder transition in the inverse germanate spinels |
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Authors: | J Preudhomme P Tarte |
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Institution: | University of Liège, Institute of Chemistry, B-4000 Sart-Tilman par Liege 1, Belgium |
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Abstract: | 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 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 is one of the factors determining the order-disorder transition temperature, and the importance of the tetragonal distortion. |
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