Determination of the fluorite related structure of Mn3Ta2O8, using synchrotron X-ray powder and electron diffraction data

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 Å) 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) Å, V=1243.47 Å ³ , from 1190 reflections with d≥0.65 Å. 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) Å and β=91.202(9)°.