Rare-earth sesquioxides

Compensation parameters calculated from data reported by Winter for the decomposition of nitrous oxide (263) and nitric oxide (264) on various oxides are given in Table V, B and C, respectively. The variation of data in the latter reaction is relatively small (aL = 0.795, Table V, C) and the values of B and e can be reliably determined. Interpretation of the results for nitrous oxide decomposition is, however, less straightforward since the compensation trend for reactions on the rare earth sesquioxides (B = 19.831 and e = 0.0571) was significantly different from that for all other oxides studied (B = 23.226 and e = 0.0321) and the combined data give the intermediate values of Table V, B. Thus, we are unable to discriminate between the possibilities that either there are two distinct lines or the number of points available is insufficient to characterize fully the compensation effect at the observed level of standard deviation. 

C. Boulesteix, Defects and phase transformation near room temperature in rare earth sesquioxides 321... 

Rare Earth Sesquioxides. Oxides of the larger trivalent metal ions react (3, 17) with Pt02 at high pressure to form a AnTOnbtfH6 CffCTftfcal... 

A2Pt207, similar to those reported for tin, ruthenium, titanium, and several other tetravalent ions. Trivalent ions which form cubic platinum pyrochlores range from Sc(III) at 0.87 A to Pr(III) at X.14 A. Distorted pyrochlore structures are formed by lanthanum (1.18 A) and by bismuth (1.11 A). Platinum dioxide oxidizes Sb203 to Sb2(>4 at high pressure. The infrared spectra and thermal stability of the rare earth platinates have been reported previously and will not be repeated here, except to point out the rather remarkable thermal stability of these compounds decomposition to the rare earth sesquioxide and platinum requires temperatures in excess of 1200 °C. 

Fig. 9.15. Plot of the spin-orbit splittings, A 3,1, in the case of rare earth sesquioxides against the atomic number, Z. The broken and full curves are according to a (Z — Zo)4 and Dirac equation with screened Coulombic...

All three polymorphs of the rare earth sesquioxides are shown by either Ce203 (A-type), Pr203 (A and C-type) or Tb203 (A, B, and C-type). Much work is now underway on the existence and relationship... 

FIG. 12.7. The4-M203 structure of La203 and rare-earth sesquioxides. At the left is shown in projection the 7-coordination group around a ion, consisting of an octahedral group of ions with an additional " ion (heaviest circle) above one of the octahedron faces. 

Kouhik Biswas. Liquid phase sintering of SiC ceramics with rare earth sesquioxides [D]. Stuttgart University of Stuttgart, 2002. 

Acceptor-doped rare-earth sesquioxides were one of the first classes of non-per-ovskite stmctured oxides that were observed to exhibit proton conductivity [80,... 

Most rare earth oxides are thermally stable, as well as chemically active. As is seen below, the C-type sesquioxides R2O3, are related to the fluorite structure Cap2 or RO2, from which the C-type sesquioxides are derived by removing one-quarter of the oxide anions. In other words, the C-type sesquioxides have a lot of defects which may act as effective diffusion paths for reactants at chemical reactions. Rare earth sesquioxides of other type structures too are in similar situation. Relatively, low reactivity of rare earth dioxide, Ce02, are understandable form this point of view. 

The sesquioxides R2O3 crystallize in three forms, A-type(hexagonal), B-type(monoclinic) and C-type(cubic) structures, according to the ionic radius of the rare earth ion. Lighter rare earth ions, from La to Nd give A-form. These ions have happened to be seen to form the C-type stmcture, but this observation seems to be due to impurity stabilization or a metastable phase. An example of the B-type oxide is given by Sm203. Other rare earth sesquioxides yield the C-type oxides [3-6]. 

C. Boulesteix, Defects and Phase Transformation Near Room Temperature in Rare Earth Sesquioxides, in K.A. Gschneidner, Jr. and L. Eyring (eds.). Handbook of Physics and Chemistry of Rare Earths, Vol.5, Elsevier, Amsterdam, p.321, 1982. 

In accordance with the well known phase diagram for the rare earth sesquioxides [6], as much as five different structural varieties have been identified for them. They are referred to as A, B, C, H, and X types. A theoretical analysis of the equilibrium crystal lattice dimensions for A, B, and C structures in Ln203 has also been recently reported [29]. Three of the polymorphs above, the hexagonal, A-type, monoclinic, B-type, and cubic, C-type, are known to occur at room temperature, and atmospheric pressure, whereas H and X forms have only been observed at temperatures above 2273 K [6]. For the lighter members of the series. La through Nd, though not exclusively [6,30], the hexagonal, A-type, form is the most usually found, Fig. 2-1. By contrast, the heaviest lanthanoid sesquioxides, from... 

The rate of hydration is also sensitive to the preparation procedure and/or the pre-treatment applied to the oxides [111]. This suggests that the differences noted between results reported in [39] and [107,111,112] probably have a kinetic origin, and therefore, that there are no thermodynamic limitations to the bulk hydration of the whole series of rare earth sesquioxides. 

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