Abstract
The Mn 2+ -containing oxide Mn 3 Ta 2 O 8 has been synthesised at 1200°C in Ar atmosphere, and its structure has been solved from X-ray synchrotron powder data (λ0.65 A) by direct methods. The structure was refined by the Rietveld method to R F =6.3%, with space group I4 1 /a, a=11.2728(2), c=9.8030(3) A, V=1243.47 A 3 , from 1190 reflections with d≥0.65 A. It is related to the fluorite structure with a ≊√5a f and c≊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 to a primitive unit cell with a′=a and c′=6c. The melting point of Mn 3 Ta 2 O 8 is 1470°C in Ar atmosphere. It is a semiconductor with an activation energy of 1.2 eV and a conductivity σ=3.7×10 –5 S cm –1 at 600°C. The magnetic susceptibility shows a maximum at 23 K and a Curie-Weiss behaviour at higher temperatures, with µ eff =5.7(1) µ B per Mn atom. When Mn 3 Ta 2 O 8 is oxidised at 1100°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) A and β=91.202(9)°.
Published Version
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