Sesquioxides polymorphism

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... 

The id-MaOs and 5-M2Q3 stmctures are associated particularly with the 4f and 5f sesquioxides. When there is polymorphism the A, B, and C structures are characteristically those of high-, medium-, and low-temperature forms respectively. The structures of some 4f and 5f oxides are set out in Table 12.6. A striking feature of the /I-M2O3 structure, which has been confirmed by neutron diffraction, is the unusual 7-coordination of the nietal atoms (Fig. 12.7). In La203 the nearest oxygen neighbours of La are 3 at 2-38, 1 at 2-45, and 3 at 2-72 A. 

Because of the inner nature of the 4f orbitals, the dififerenees of eleetron configuration between the lanthanoid elements are associated to eleetrons relatively well screened from the chemical surroundings by the outer (5s p ) shell. This implies weak crystal fields splitting effects [28], and a relatively small eovalent contribution to the bonding, particularly in the sesquioxides. Accordingly, the ionic model plays an important role in determining their chemistry [21]. Also related to these chemical characteristics, the lanthanoid compounds exhibit a rich variety of stmctures, often reflected in the occurrence of polymorphism phenomena. 

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... 

Figure 3-17. Polymorphic transformations of the lanthanoid sesquioxides. (Reproduced with permission from ref. Copyright 1966 Revue International des Hautes Temperatures...

The phase diagram for all the lanthanoid sesquioxides is given in Fig. 3-17 and shows the different transformation temperatures. From the diagram it is obvious that promethium, Pm, was the first metal for which the existence of all three polymorphs and the transition temperatures could be established ]39]. For praseodymium, Pr, under atmospheric pressure the C-type structm-e is formed above 500 °C and transforms to A-type above 700 °G. Depending on the ionic radii only for the rare earth metals from Pm to Gd all five polymorphs will be formed. With increasing temperature the order of transition is C B—>A. 

Roth, R. and Schneider, H. (I960) Stability relations of the polymorphic forms of tare earth sesquioxides. J. Res. Natl. Bur. Stand., 64A 309-314. 

Hoekstra HR, Gingerich KA (1964) High-pressure B-type polymorphs of some rare-earth sesquioxides. Science 146 1163-1164... 

Fig. 8 Polymorphic transformation temperatures for the lanthanide sesquioxides (after Foex and Traverse 1966b, Warshaw and Roy 1961).

The actinide oxides have received intensive scrutiny because their refractory nature makes them suitable for use as ceramic fuel elements in nuclear reactors. UO2 melts at 3150 K, and Th02 has the highest melting point of any oxide, about 3465 K. The actinide oxides are complicated by deviations from stoichiometry, polymorphism, and intermediate phases. The sesquioxides are basic, the dioxides are much less basic, and UO3 is an acid in solid state reactions. The reactivity of these oxides depends greatly on their thermal history. If ignited, they are much more inert. Table XI contains some representative data on actinide oxides. 

Ito and Johnson (1968), Warshaw and Roy (1964) and BocquiUon et al. (1973) used Greek letters to distinguish the polymorphic forms of rare earth disiUcates. Felsche (1970,1973) foUowed the nomenclature of the polymorphs of the rare earth sesquioxides, designating the various structure types by the capital letters A, B, C, D, E, F, G. This method of naming is followed in the present review. 

All the rare earth elements, under suitable conditions, form a sesquioxide. Polymorphism is common and no fewer than five distinct crystalline types have... 

Fig. 27.2. Polymorphic forms and transition temperatures of the rare earth sesquioxides (after Coutures et al., 1975).

Hoskins and Martin (1975) have shown the polymorphic relationships of the sesquioxides in terms of their coordinated defect model. Their discussion adds new insight into possible ways of viewing these structures. 

The individual lanthanide sesquioxides crystallize in one or more of three main polymorphic forms designated A, B, and C. Two further high-temperature forms, H and X, have also been reported by Foex and Traverse (1966). The C-and X-forms are both cubic, the A- and H-forms are hexagonal, while the B-form is monoclinic. Details of these structures, their interrelationships, and their stability ranges are discussed in ch. 27, but the dominant factor in determining which structure is assumed by a given sesquioxide under specified conditions is the cation radius. The trend is A->B- C with decreasing radius (i.e. La ->Sc ). 

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