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 degrees C in Ar atmosphere, and its structure has been solved from X-ray synchrotron 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 are octahedrally coordinated by oxygen atoms and the three crystallographically different Mn atoms by 7, 4 + 4 and 4 oxygen atoms. Electron diffraction patterns show the presence of weak superstructure reflections corresponding td a primitive unit cell with a' = a and c'= 6c. The melting point df Mn3Ta2O8 is 1470 degrees C in Ar atmosphere: It is a semiconductor with an activation energy of 1.2 eV and a conductivity sigma= 3.7 x 10(-5) S cm(-1) at 600 degrees C. The magnetic susceptibility shows a maximum at 23 K and a Curie-Weiss behaviour at higher temperatures, with mu(eff)=5.7 (1) mu(B) per Mn atom. When Mn3Ta2O8 is oxidised at 1100 degrees C in air an Mn-Ta:oxide forms which has a wolframite type structure with unit cell a =4.7574(5), b = 5.7296(6), c = 5.1133(4) Angstrom and beta =91.202(9)degrees.