Sesquioxides hexagonal forms

The hexagonal (A-type) of the sesquioxide (Baybarz 1973, Baybarz and Haire 1976) is very difficult to prepare, as it only exists in a narrow phase region between the monoclinic form and the liquid (melting point reported to be 1750°C). This crystal form of CfjOj has also been obtained from old hexagonal forms of BkjOj after about five half-lives (97% transformation) following the beta decay of Bk-249 oxide. The transformation of the cubic form to the monoclinic form of sesquioxide occurs between 1100- 1400°C and the transformation of the monoclinic form to the hexagonal form occurs at about 1700°C (Baybarz and Haire 1976). 

From fig. 19, it would be expected that plutonium sesquioxide would form both a cubic and a hexagonal phase, and that curium and berkelium sesquioxides would both form cubic, monoclinic, and hexagonal phases. This is what is found experimentally. For americium sesquioxide, a cubic and a hexagonal phase would be expected, with a slight possibility that a monoclinic phase would also form. There has been some disagreement whether or not a pure Am monoclinic sesquioxide phase exists there is a good probability that it does not. For californium and einsteinium sesquioxides, fig. 19 would predict the existence of a cubic and monoclinic phase for each experimentally. 

Another observation that can be made from fig. 22 is that the hexagonal structure appears to persist to a smaller molecular volume in the actinide series than in the lanthanide series (e.g., curium sesquioxide should be the highest member to form a hexagonal phase based on the lanthanide radius relationship). The hexagonal form of californium sesquioxide has a very narrow phase field and it is very difficult experimentally to retain this structural form of the sesquioxide at room temperature (e.g., to quench-in this form). This latter factor may have some bearing on this volume discrepancy (e.g., the volumes may be affected by the preparation). Finally, the volume data for the cubic forms of both series, the hexagonal forms of the actinides and the monoclinic forms of the lanthanides, all show a cusp at elements with half-filled shells. 

In the sesquioxides the hexagonal A form has a seven coordination about the metal atoms. The coordination group MOT can be c escribed as an octahedron... 

Solid rhodium sesquioxide, Rh203, was first identified in 1927,1148 and has the corundum structure1149 (hexagonally close packed O2- anions distorted by the presence of Rh3+ cations in of the octahedral interstices). Each Rh is surrounded by six oxygens, three at 2.03 A and three at 2.07 A. An orthorhombic phase forms above 750 °C.1150... 

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]. 

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... 

Two crystal forms of Am203 (pink colored) have been reported (see table 25) a hexagonal (A-type) and a bcc (C-type) form. Which crystal form is obtained depends on the temperature of the preparation the C-type is the low-temperature (less than 600°C) form. The A-type sesquioxide has been reported to be stable from 900 to 1500°C (Berndt et al. 1974). It appears that a B-form of this oxide does not exist if it does, it has a very limited temperature range. Incorporation of small amounts of lanthanide oxides (e.g., Sm203) with Am203 can stabilize the monoclinic form of AmjOj (Schultz 1976). 

The Cm203 (white) displays three crystal modifications, namely the A-, B- and C-type lanthanide structures as shown in table 23 (Eller and Pennemann 1986). A small uptake of oxygen can cause the oxide to acquire a tan to light brown appearance. The C-type (bcc) structure is the low-temperature form, which converts to the B-type (monoclinic) structure above 800°C, which in turn changes to the A-type (hexagonal) structure above 1600°C (Baybarz and Haire 1976). It is the C-type structure that is readily oxidized to higher oxides the monoclinic form is very resistent to oxidation and the monoclinic to cubic transformation via temperature treatment is very diflicult ( irreversible transformation). The B to A and the A to B transformations occur more readily with temperature. Self-irradiation (especially noticeable with the more readily available, shorter-lived Cm-244 isotope) converts the C-form of the sesquioxide to the A-form (Wallmann 1964, Noe et al. 1970). 

A summary and a comparison of the different phases of the lanthanide and actinide sesquioxides is given in fig. 22 taken from Baybarz and Haire (1976), where the molecular volumes of the hexagonal, monoclinic and cubic forms are plotted. There is a considerable densification in going from the cubic (six coordinated) to the monoclinic (six and seven coordinated) and finally to the hexagonal (seven coordinated) forms of these oxides. It is evident that the monoclinic form has been observed at a larger molecular volume in the lanthanide sesquioxide series than in the actinide sesquioxide... 

Additional support for this latter structure being a form of metallic californium is that samples exhibiting this structure have been converted by thermal treatment to the fee structure having the parameter Oq = 0.494 nm, and vice versa [72], The second dhep structure [71] listed in Table 11.3 and the other hexagonal structure [70] may represent the same material. If they are metallic californium, they would represent a hexagonal structure for the divalent form of the metal. They are at present not well-established structures for the metal. The fee structure with the parameter Uq = 0.540 nm [68,69] has been observed in earlier work, where very small quantities of californium were prepared. In a later study [70], this structure (oq == 0.540 nm) was also observed in thin films of californium metal that had been heated to 200-300°C in air. It should be noted that poorly crystallized samples of californium sesquioxide (Cf203. c, body-centered cubic, Oo = 1.080-1.083 nm see Section 11.7.2) can be indexed as an fee strueture with Go = 0.540-0.542 nm. If the fee structure with Oo = 0.540 nm was indeed a metallic phase of californium, the lattice parameter would imply that the metal had an intermediate valence between 2 and 3. 

Fig. 20.4 Unit cells of the three forms of actinide sesquioxides A, hexagonal B, monoclinic and C, cubic. Only those atoms are included which are needed to show the coordination of the metal ions in each instance.

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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