IGHLY dispersed mixed oxides or oxide solid solutions H are useful as starting materials for special ceramics (magnetic, ferroelectric, and optical), monocrystals, and catalysts.Very few methods produce such oxides. Reactions between simple oxides at high temperatures yield poorly dispersed oxides. Methods using gas phase decomposition are mainly suitable for the preparation of single oxides (e.g. TiOz and A1203), although some mixed oxides have been obtained by special hydrolysis-pyrolysis techniques,' involving alkoxides or chelates, for example. Pyrolysis of precipitates as hydroxides or mixed salts (oxalato-complexes) is a widely used technique that often offers better prospects. However, the composition of the complex salts with two metallic ions is narrowly restricted by stoichiometric requirements; precipitates, as well as mixed salts with more than two metallic ions, are seldom homogeneous, due to composition changes in the liquid medium during the deposition of the solid phase. (For a critical bibliographic study of the various preparation methods, see Ref. 2.) Thus, it appeared advantageous to start, not with precipitates or complex salts, but with amorphous mixtures retaining all the necessary metallic ions in perfect homogeneity. Such a mixture, referred to as a precursor of the mixed oxide, can be obtained from solutions containing all the required ions and an organic polyfunctional acid possessing at least one hydroxy and one carboxylic function, such as citric, malic, tartaric, glycollic, or lactic acid. The solution usuaIly contains about 1 g-moI of acid function for each gram-ion of metal ions. The solution may be prepared from many metallic salts (e.g. nitrates, acetates, oxalates, and ammonium salts of metallic ions).The important step in the preparation of the amorphous precursor is the rapid dehydration of the solution at low temperature. Usually, the preparation starts with a concentrated aqueous solution of the necessary components ( a total of 3 to 10 g-ions of the metal ions). Dehydration is conveniently carried out in revolving evaporators under a pressure of a few mm Hg at 7OOC; it is stopped before the viscosity becomes high enough to interfere with discharging the vessel (a few thousand centipoises at 70°C). Dehydration is completed by drying for 3 to 24 h in a vacuum oven below 100°C. This procedure yields transparent glasses or solid foams constituted of cells, the walls of which are of a vitreous material. This material is transparent, uniformly colored (when the included ions are colored), and is completely amorphous to X-ray diffraction. It presents no evidence of inhomogeneity: i.e. no segregation or color fluctuations on a microscopic scale and no difference in behavior in different parts of the sample in thermogravimetric or spectroscopic infrared measurements. The crystallized mixed oxides produced are perfectly homogeneous. Chemical and infrared analyses indicate that this precursor is essentially an anhydrous mixed salt retaining all the ions introduced. The polyfunctiona...