Rare-Earth Polymorphism and Phase Equilibria in Rare-Earth Oxide-Water Systems

Abstract

A high-water-pressure high-temperature study has been conducted of the systems Y2O3-H2O, Smz03-Hz0, Ndr03-Hz0, and Laz03-H20. The trihydroxides La(OH)3, Nd(OH)r, and Sm(OH)3 are isomorphous, but Y(OH)a does not belong to this group. On the other hand, the oxyhydroxides YOOH, SmOOH, and NdOOH are isomorphous; LaOOH, if it exists, cannot be stable in the presence of water at room temperature. Pressuretemperature curves for the equilibrium between the trihydroxide, oxyhydroxide, and a new hydroxy compound have been determined for each of the three smaller ions. The reversibility of the transition between the A-, B-, and C-type rare earths has been established, and new X-ray data on the B structure are listed. The relations between the polymorphs as a function of ionic size are changed from those given by Goldschmidt. The very marked difference in stability in air between the A-and C-type structxires suggests the obvious desirability of converting rare-earth oxide ceramics into the C structure with proper compositional variation and thermal treatment.

Low Temperature Stability of Cubic Zirconia

“…For example, as the cubic and the tetragonal crystal structures are much alike, the dissociation of water molecules on the cubic surface and the formation of Zr±OH, Y±OH and YO(OH) in the cubic lattice are also possible. In the work of Lange et al [2], a thin foil of a composition of 6.6 mol% Y 2 O 3 ±ZrO 2 was annealed for 16 h at 250 C in an environment containing water vapor, a subsequent TEM observation of the foil found that clusters of Y(OH) 3 [14]. But in Lange's experiment, no evidence proved that the Y(OH) 3 crystallites, at least partly, were not formed from the cubic grains.…”

Phase relations between iridium and the sesquioxides in air

“…Th e hexagonal A type (a=3.831 A, c=5 .999 A) [8], is ge nerally regarded as the stable modification of Nd"203 although th ere has been so me co ntrov ersy regarding the existence and stability of the Band C types [6,7]. Th e melting point of Nd 2 0 3 is not well established.…”

“…N eo dymium ses quioxide (Nd 2 0;]) has bee n reported to occ ur in three polymorphi c form s; th e A, B, and C rare earth oxide s tru ct ure types [6,7] . Th e hexagonal A type (a=3.831 A, c=5 .999 A) [8], is ge nerally regarded as the stable modification of Nd"203 although th ere has been so me co ntrov ersy regarding the existence and stability of the Band C types [6,7].…”

Controlling the morphology of yttrium oxide through different precursors synthesized by hydrothermal method

“…According to the phase diagram of Y 2 O 3 -H 2 O system [18], Y 2 O 3 is not stable under hydrothermal conditions at temperature lower than 550 °C, so that the products of hydrothermal reaction are normally precursors of yttrium oxide rather than oxide itself. The precursors decompose into the oxide through heat treatments.…”