Lanthanum Oxide Selenides

The structure projected onto the x-y plane is shown in Fig. 224. There are three La atoms that are not equivalent La(1) is 6-coordinated by four 0 atoms and two Se atoms, La(2) is 8-coordinated by four O atoms and two Se-Se pairs, La(3) is 8-coordinated by four 0 and four Se atoms La(2) and La(3) form antiprisms. Layers of 0-La4 tetrahedra, centered on O atoms and parallel to the [010] axis, constitute a nearly tetragonal arrangement. Between these layers, planes of alternately arranged Se atoms and Se pairs are inserted. The Se atoms are bonded only to M with atomic distances from 3.21 to 3.33 A (mean value 3.24 A). The distance between Se-Se pairs Is 2.47 A showing covalent bonding. The La-0 distances are 2.34 to 2.47 A (mean value 2.41 A), Dugue et al. [1], also see the earlier publication of Khodadad et al. [2]. 

The beige colored La202Se was prepared by passing Se vapor in an inert gas carrier over La203 at 1200°C, Obolonchik, Mikhlina [3]. For the (possible) formation of La202Se during 

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La2Se2Ta3, Lanthanum tantalum oxide selenide (La2Ta308Se2), (134853-92-2, 30 146... 

Se2Ko7 Fa, Potassium tantalum selenide (Ko,7TaSe2). [158188-85-3). 30 167 Se2La20gTa3, Lanthanum tantalum oxide selenide (La2Ta30gSe2). [134853-92-2], 30 146... 

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

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