Abstract
Abstract A new high-pressure, high-temperature modification of Mn 4 Nb 2 O 9 , has been prepared at T =1700 °C, P =9 GPa. The XRD study based on Rietveld technique suggests the structure to be a rhombohedral (R3c) modification of Mn 4 Nb 2 O 9 , (LiNbO 3 type structure) with the chemical formula Mn(Mn 1/3 Nb 2/3 )O 3 and unit cell parameters a =5.34139(2), c =14.15807(7) A. However, electron diffraction (ED) and high resolution electron microscopy (HREM) images of crystallites found in an XRD single phase sample revealed frequent disorder, but also the existence of a new monoclinic modification of Mn 4 Nb 2 O 9 often intergrown with the corundum related P 3 c1-Mn 4 Nb 2 O 9 modification stable at ambient pressure and temperature. This new monoclinic modification has the unit cell parameters: a =9.93, b =5.34, c =13.20 A, β =92.9° and space group C2/c or Cc. It is closely related to the corundum lattice since its c ∗ axis is parallel to 〈2 0 2〉 ∗ R 3 c in the rhombohedral sublattice.
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