DETERMINATION OF THE FLUORITE RELATED STRUCTURE OF MN3TA2O8, USING SYNCHROTRON X-RAY-POWDER AND ELECTRON-DIFFRACTION DATA

The Mn2+ -containing oxide Mn3Ta2O8 has been synthesised at 1200 degre es C in Ar atmosphere, and its structure has been solved from X-ray sy nchrotron powder data (lambda approximate to 0.65 Angstrom) by direct methods. The structure was refined by the Rietveld method to R-F = 6.3 %, With space group I4(1)/a, a = 1.1.2728(2), c = 9.8030(3) Angstrom, V = 1243.47 Angstrom(3), from 1190 reflections with d greater than or equal to 0.65 Angstrom. It is related to the fluorite structure with a approximate to root 5a(f) and c approximate to 2a(f). The Ta atoms ar e octahedrally coordinated by oxygen atoms and the three crystallograp hically different Mn atoms by 7, 4 + 4 and 4 oxygen atoms. Electron di ffraction patterns show the presence of weak superstructure reflection s corresponding td a primitive unit cell with a' = a and c'= 6c. The m elting point df Mn3Ta2O8 is 1470 degrees C in Ar atmosphere: It is a s emiconductor with an activation energy of 1.2 eV and a conductivity si gma= 3.7 x 10(-5) S cm(-1) at 600 degrees C. The magnetic susceptibili ty shows a maximum at 23 K and a Curie-Weiss behaviour at higher tempe ratures, with mu(eff)=5.7 (1) mu(B) per Mn atom. When Mn3Ta2O8 is oxid ised at 1100 degrees C in air an Mn-Ta:oxide forms which has a wolfram ite type structure with unit cell a =4.7574(5), b = 5.7296(6), c = 5.1 133(4) Angstrom and beta =91.202(9)degrees.