Meta-stable phases in the Mn-Ta-O system

摘要

Studies of compound formation in the system Mn-Ta-O showed the existence of six new meta-stable phases. The compound MnTa_2O_6 was found to exhibit, in addition to the stable tantalite structure, modifications related to the tri-rutile, wolframite, and wodginite structures. These were characterized by powder X-ray diffraction and, for the wodginite modification, also single crystal X-ray diffraction. Three new phases, RI-Mn_4Ta_2O_9, RII-Mn_4Ta_2O_9 and R-Mn_(11)Ta_4O_(21) were found with rhombohedral symmetries and corundum related structures. The structures of RI-Mn_4Ta_2O_9 and RII-Mn_4Ta_2O_9 were determined and refined, respectively, from X-ray powder and single-crystal diffraction data. Electron diffraction studies indicate the existence of a chemical inter-growth between corundum and rock-salt type structures.