HfO 2-based refractory compounds and solid solutions : 2. Kinetics and mechanism of compound formation in the systems HfO 2[sbnd]M 2O 3 (MO)

Abstract

In a previous communication 1 we considered the papers published in the USSR on studies of phase diagrams for the systems hafnia-alkali-earth oxides and hafnia — rare-earth oxides. The purpose of many investigations was the preparation of new refractory compounds and solid solutions which could create the basis of high-refractory materials. In addition to the great analogy between binary systems with hafnia and zirconia, our interest in hafnium-containing compounds was promoted by their higher melting temperatures compared with those of zirconia-based compounds and by several peculiar physico-chemical properties characteristic of hafnia-based compounds and solid solutions. It has been shown 1 in the HfO 2 MO systems, strontium forms the greatest number of compounds: SrHfO 3, Sr 2HfO 4, Sr 3Hf 2O 7 and Sr 4Hf 3O 10; calcium forms two compounds: CaHfO 3 and CaHf 4O 9; barium and magnesium each produce one compound: BaHfO 3 and Mg 2Hf 5O 12, respectively. Rare earth oxides with large ionic radii usually form pyrochlore-type (P) compounds of composition Ln 2Hf 2O 7, whereas rare-earth oxides with small ionic radii combine with hafnia to form wide regions of fluorite-like solid solutions. In the region of compositions corresponding to the pyrochlore-type compounds rare-earths with the smallest ionic radii (Sc, Yb) form compounds with rhombohedral structures (R) of the composition M 4Hf 3O 12(M 7O 12). Together with the latter, compounds M 7O 11.5 and M 7O 11 were found for gadolinium and erbium and M 9O 17 and M 7O 13 for scandium. Most of these compounds are stable only over a certain temperature range and transform into disordered F-type cubic solid solutions at elevated temperatures. In addition, there exist limited regions of solid solutions based on the monoclinic (M) and tetragonal (T) modifications of HfO 2 as well as on various structural modifications (C, B, A, H, X) of rare-earth oxides.