Sesquioxides cubic forms

Antimony Trioxide. Antimony(III) oxide (antimony sesquioxide) [1309-64-4] Sb203, is dimorphic, existing in an orthorhombic modification valentinite [1317-98-2] is colorless (sp gr 5.67) and exists in a cubic form and senarmontite [12412-52-17, Sb O, is also colorless (sp gr 5.2). The cubic modification is stable at temperatures below 570°C and consists of discrete Sb O molecules. The molecule is similar to that of P40 and As O and consists of a bowed tetrahedron having antimony atoms at each corner united by oxygen atoms lying in front of the edges. This solid crystallizes in a diamond lattice with an Sb O molecule at each carbon position. 

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. 

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... 

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. 

The dioxide is known for all the actinides from thorium through californium. Attempts to prepare einsteinium dioxide have not been successful. All the dioxides crystallize with the fluorite face-centered cubic structure. Actinides that form both a dioxide and a sesquioxide may form complex intermediate oxides, which have O/M ratios between 1.5 and 2.0. 

The structure of platinum dioxide and its reactions with some di, tri, and tetravalent metal oxides have been investigated. Ternary platinum oxides were synthesized at high pressure (40 kUobars) and temperature (to 1600°C). Properties of the systems were studied by x-ray, thermal analysis, and infrared methods. Complete miscibility is observed in most PtO2-rutile-type oxide systems, but no miscibility or compound formation is found with fluorite dioxides. Lead dioxide reacts with Pt02 to form cubic Pb2Pt207. Several corundum-type sesquioxides exhibit measurable solubility in PtOz. Two series of compounds are formed with metal monoxides M2PtOh (where M is Mg, Zn, Cd) and MPt306 (where M is Mg, Co, Ni, Cu, Zn, Cd, and Hg). 

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. 

Arsenic trioxide [1327-53-3] (arsenic(III) oxide, arsenic sesquioxide, arsenous oxide, white arsenic, arsenic), As203, is the most important arsenic compound of commerce. The octahedral or cubic modification, arsenolite [1303-24-8], AH 2Q8, —1313.9 kJ/mol (—314 kcal/mol) L°C29g, 214 J/(mol-K) (51 cal/(mol-K)), is the most common form and has been known from early times. The monoclinic form, daudetite [13473-03-5], AH°(NC. ... 

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... 

Due to its unique spectroscopic and emission dynamic properties, Nd " ion can be used for laser emission over a wide range of temporal regimes, from CW to very short pulses of below picosecond. The transitions from the two Stark levels of the manifold F3/2 to the Stark levels of the manifolds " ll 1/2, and " Ii3/2, corresponding to the ranges of 0.9, 1.0, and 1.3 pm, respectively, showed pretty large emission cross sections. Various cubic crystalline materials, including garnets and sesquioxides, have been used as hosts for Nd laser emission, which formed the major part of ceramic laser materials. 

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