Hydrothermal phase equilibria in Ln2O3-H2O-CO2 systems

Hydroxide carbonates and oxide carbonate hydrate of rare earths grown on glass via a hydrothermal route

“…Instead, it is probable that the occurrence of Eu 2 O(CO 3 ) 2 Á H 2 O instead of EuOHCO 3 is controlled by the decrease in cation size of europium compared with lanthanum (approximately a 9% decrease in ionic radius [17]); i.e., the different phase is caused by the lanthanide contraction. It has been found in several related lanthanide hydroxide anion systems (anion= CO 3 2À , NO 3 À and halide) that the stable crystal phases of lanthanum and neighboring elements do not continue systematically across the entire lanthanide series [5,6,18]. Thus, it is quite reasonable for the stable phase of europium, which is approximately half way through the lanthanide series, to differ from that in the lanthanum and cerium systems.…”

Trends in Structure and Thermodynamic Properties of Normal Rare Earth Carbonates and Rare Earth Hydroxycarbonates

“…We have classified the various synthesis methods of the rare earth carbonates as conversion, precipitation, and decomposition. Based upon the findings by Kutty [43][44][45][46][47] and Caro [48][49][50], each of these synthesis types can be used to create the desired rare earth carbonate phase (normal vs. hydroxy). It should be noted that the most convenient means of creating the hydroxycarbonates are the decomposition methods.…”

“…It was demonstrated that working with very low solids loading of the oxides, at lower temperatures, increasing pCO 2 increased conversion yields while keeping conversion times relatively short (> 95% conversion in 1 hour) with increasing reaction times not giving measurable increases in yield. RE 2 O 3 -CO 2 -H 2 O systems have been studied using varied system pressure and temperature to determine hydrothermal phase equilibria of the system [43][44][45][46][47]. With sufficient pCO 2 and mole fraction of CO 2 , the hydrated normal carbonates are preferred over the hydroxycarbonates at lower temperatures.…”

“…The rare earth hydroxycarbonates have the general chemical formula of RE(OH)CO 3 ·xH 2 O that can assume the orthorhombic, hexagonal, or tetragonal structural variants depending upon reaction conditions. It should be noted that in the study of naturally occurring RE carbonates, particularly single RE carbonates, the hydroxycarbonates are of greater importance as they are the hydrolysis products of the normal carbonates at ambient conditions (pCO 2 = 3 × 10 −4 atm, 25 • C, total pressure = 1 atm) and are also the preferred carbonate phase at elevated temperatures [43][44][45][46][47]49].…”

“…At ambient conditions, Caro demonstrated the tendency of the lighter normal rare earth carbonates (La-Eu) to readily hydrolyze while the heavier normal carbonates are resistant to hydrolysis [49]. All normal rare earth carbonates will hydrolyze into their respective hydroxycarbonates in water when temperature is increased close to 100 • C. Studies on the hydrothermal behavior of the RE 2 O 3 -CO 2 -H 2 O ternary system by Kutty and coworkers [43][44][45][46][47] show that at isobaric conditions (fixed mole fraction CO 2 ) the hydroxycarbonates are the preferred phase at elevated temperatures.…”