High temperature-high pressure synthesis and crystal structure of the incommensurately modulated, α-PbO2 related, compound MnTa2O6

A new high pressure-high temperature modification of MnTa2O6 has been prepared at T = 1400degreesC, P = 8 GPa. Electron and X-ray powder diffraction (XRPD) studies showed it to have a modulated alpha-PbO2 structure. It crystallises in the four-dimensional space group P: P112(1)/n (alphabeta0). The structure model could be refined by the Rietveld technique, using XRPD data. An incommensurate ordering of the Mn and Ta atoms gives rise to a modulation of the oxygen and metal atoms' positions. The occupational modulation is described by a combination of a Crenel and a sinusoidal function, whereas the positional modulations can be described purely by sinusoidal functions. The unit cell parameters are a = 4.7472(2) Angstrom, b = 5.7453(3) Angstrom, c = 5.1496(3) Angstrom, gamma = 90.023(9)degrees, and the modulation vector is q = -0. 1038(4) (.) a* + 0.6910(7) (.) b*. (C) 2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved.