Changes in the Atomic Coordinates and State of Anion and Cation Order Induced by Alloying and Temperature Change in Y2[M(2)yM(1)1−y]2O7 Pyrochlore Solid Solutions
Pyrochlore oxides, A2B2O7 , are superstructures based upon a fluorite-type (Cl) arrangement of ions. Usually the B cation is tetravalent, and A is a larger trivalent ion. These species are ordered among the 8-coordinated interstices in the simple cubic anion array. To achieve charge balance, one-eight of the anions in the parent fluorite array are omitted in an ordered fashion such that only six anions coordinate B. The nearest-neighbor oxygen ion to the vacant anion site is displaced from its ideal location and relaxes toward the vacancy. The driving force for the ordering that creates the pyrochlore superstructure is believed to be the size difference between the A and B cations. Some, but not all, pyrochlores disorder when heated to very high temperature (usually above 2000° C). It has recently been observed that substitution of a larger tetravalent cation in solid solution in the B site, A2(B′yB1−y)2O7, will progressively drive the system toward disorder to a non-stoichiometric fluorite structure which lacks long-range order.
Details of the atomic arrangements that accompany chemically-driven disorder in a number of pyrochlore solid solutions were determined through Rietveld analyses of neutron and X-ray powder diffraction patterns. Results have been obtained for solid solutions with A = Y and pair-wise substitution of Zr, Sn or Ti in the B site. One would anticipate the rates of disorder in the cation and anion arrays to be coupled through Coulombic interactions. Remarkably, anion and cation arrays disorder at independent rates in all systems. The (Sn-Ti) solid solutions displayed little disorder despite appreciable overlap in the range of radius ratio RA/RB with that of other systems. Thermally-induced disorder was examined with in-situ diffraction measurements at temperatures up to 1500° C and revealed behavior similar to the effects induced by solid solution.
KeywordsDisorder Process Tetravalent Cation Anion Disorder Positional Coordinate Fractional Substitution
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