Tantalate pyrochlores

Ikeda S, Fuboki M, Takahara YK, Matsumura M (2006) Photocatalytic activity of hydrothermally synthesized tantalates pyrochlores for overall water splitting. Appl Catal A 300 186-190... 

Partially oxidized tin(n) niobate and tantalate pyrochlores have been shown by Mossbauer Sn spectroscopy to be homogenous cubic phases containing tin(ii) and tin(iv) atoms. MTaO (M = Ti, V, Cr, or Fe) have been prepared from the corresponding oxides by ceramic techniques and X-ray and neutron diffraction, magnetic and Mdssbauer measurements have indicated that the metal atoms are distributed statistically over the metal sites of the rutile structure. The compounds M 5Mo.5Te30g (M = Sc, Cr, Fe, Rh, Ga, In, or Bi ... 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

Known oxyfluoroniobates (-tantalates) with compositions that correspond to X Me = 3 crystallize in typical structures of ReC>3, pyrochlore, and hexagonal and tetragonal tungsten bronze. Table 35 presents structural parameters of such compounds. 

The second group is the group of oxyfluorides that are derived from ferroelectric oxides by means of fluorine-oxygen substitution. The basic oxides are usually perovskite, tetragonal tungsten bronze, pyrochlore, lithium tantalate etc. [400]. 

Calcined samples indicated formation of carbonate groups at above 500"C at 700°C, crystalline phases started forming (apparently traces of both a pyrochlore phase and the desired tantalate). Heating at 850 C without soaking led to the crystallization of phase pure SrBi2Ta209. The particles approached sphericity with a porous and agglomerated form, and had an average size of about 60 nm. 

The MRVjOy (M = alkali metal) compounds have two structure types pyro-chlore and weberite (Chincholar, 1972). The pyrochlore structure is formed with lithium and for smaller rare earths also with sodium. The potassium compounds and sodium compounds with larger rare earth ions seem to have weberite structure, which is an orthorhombic distortion of cubic pyrochlore (Gabe and Chincolkar, 1979). The corresponding pentavalent niobates and tantalates have similar structural behaviour to the vanadates. 

Recent work by Allpress and Rossell (1978) and Rossell (1978) has shown that all rare earths other than scandium form niobates and tantalates with an O/M ratio of 1.75. These compounds are superstructures of fluorite they are orthorhombic wRh probable space groups Cmcm for the La compound and C222 for those containing Nd, Gd, Ho and Y. Both structures are very similar to each other and to both pyrochlore and weberite they are related to fluorite by a = [200]f, b = [011]f, c = [0T1]f. While the fluorite cation array is again intact, the anion arrangement results in the formation of slabs within which the cations all have cubic or... 

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