Pyrochlores. VIII. Studies of some 2–5 Pyrochlores and Related Compounds and Minerals

Abstract

The cubic pyrochlores Cd,Nb,O,, Cd2Ta,0,. S n 2 N b 2 0 j . S n z T a 2 0 7 . Cd,Sb20,, Ca2Sb,0,, M n 2 S b 2 0 i , PbzSb,O,. and related compounds were prepared and investigated by a number of methods. On heating above 700'. Ca,SbzO,(pyrochlorc) transformed to Ca,Sb,O,(weberitej, while Pb,Sb201(pyrochlore) changed to a rhombohedrally distorted P b 2 S b 2 0 j pyrochlore. Refinement of the crystal structures of Cd,Nb,O, and C d z T a 2 0 , from powderdiffractometer intensities yielded 0.434(2) and 0.434(3) respectively as the best estimates of .\. (O,). Specimens of natural bindheimite and stibiconite were also examined. Stibiconite from San Luis Potosi (Mexico) was shown. on the evidence of its Mossbauer "'Sb spectrum, to contain Sb(V) and Sb(II1) in the approximate ratio of 0.2.BaCd,CI,.5HI0 and BaCd2CI,.2H20 both give powder diffraction patterns of the pyrochlore type. The chlorine could be partially replaced by Br to give mixed crystals BaCd,CI,-,Brl,5H,0 almost up to z = 2.The crystal chemistry of 2-5 oxide pyrochlores and the relationship of the weberite to the pyrochlore phases are discussed. On discute d e la cristallochiniie des pyrochlores oxygknes de type 2-5 ainsi que de la relation entrc les phases pyrochlore et wdberite. Canadian Journal of Chemistry. 50, 3648 (1972) As a sequel to our previous investigations of oxide pyrochlores of compositions A;+B;+O, ( 3 -4 pyrochlores) and Ai+B:+O, ( 1 -5 pyrochlores) (IPS), some corresponding 2-5 pyrochlores have been studied. Minerals of this type (pyrochlore. niicrolite etc.) have been known for a long time. and the discovery of ferroelectricity in Cd2Nb20, (9) initiated a search for similar compounds with unusual dielectric properties. This search has proved disappointing in that it has uncovered but a minuscule number of new ferroelectrics of this structure type. It showed, however, that for some combinations of divalent and pentavalent cations the composition of the pyrochlore (P) phase is not limited to the ideal A,B,O, stoichiometry, while for other combinations oiz!v hypostoichiometric P phases can be prepared by conventional methods. The 2-5 pyrochlores are then best described by the general formula A , + = B 2 0 6 +,. The range of z is determined by the particular combination of A and B. and may include unity. The stability of the P structure can be correlated, for 3-4 pyrochlores, with the ionic size and radius ratio of the cations (10). A similar correlation is less satisfactorv for the 2--5 pyrochlores ( 1 1). Pentavalent cations of suitable size and chemistry are relatively few, and the absence, among divalent elements. of an extensive sequence of closely related ions, such as that formed by the trivalent lanthanons, makes each of the 2-5 P phases more unique. The Can. J. Chem. Downloaded from www.nrcresearchpress.com by 34.208.6.16 on 05/10/18For personal use only.

Pyrochlores. X. Madelung energies of pyrochlores and defect fluorites

“…5). The stoichiometry A21'B22,5+F7 corresponds to that of the orthorhombics mineral weberite, Na2MgAlF7, whose structure type (W) is an alternative to P for compositions A2B2X7 (38). While the P and SThe true symmetry of natural weberite may in fact be lower than orthorhombic (personal communication from Dr. J.…”

“…However, the W phases appear to occur much more rarel! than the P phases (38), so that the decisive factors involved may be of a specialized n a t~r e .~ In mixed oxide-fluoride pyrochlores (Figs. 6 and 7) the problem of ordering in 8(a) is of interest.…”

Pyrochlores. VII. The Oxides of Antimony: an X-Ray and Mössbauer Study

“…0.425 to 0.4375 (33).1° The high values of the isotropic temperature factors B(16c) and B(8a) very likely have their origin in the incomplete occupancies of the two equipoints. In a stoichiometric P structure each oxygen atom 0, in 8(a) is surrounded by four A cations in 16(c) at the corners of a regular tetrahedron.…”

2–5 pyrochlore relaxor ferroelectric Cd2Nb2O7 and its Fe2+/Fe3+ modifications

“…15 ) and second phase than a pyrochlore. 16,17 On the other hand, an intriguing feature of Fe ions is also the facility to form a columbite especially during formation of the pyrochlore niobates. 18 The predisposition of Cd 2 Nb 2 O 7 to form a columbite for critical amounts of the impurity ions partially substituted for Cd 2ϩ seems to be interesting itself.…”