High temperature-high pressure synthesis and crystal structure of the incommensurately modulated, α-PbO_2 related, compound MnTa_2O_6

摘要

A new high pressure-high temperature modification of MnTa_2O_6 has been prepared at T = 1400 ℃, P = 8 GPa. Electron and X-ray powder diffraction (XRPD) studies showed it to have a modulated α-PbO_2 structure. It crystallises in the four-dimensional space group P:P 112_1(αβ0). 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) A, b = 5.7453(3) A, c = 5.1496(3) A, γ=90.023(9)°, and the modulation vector is q = -0.1038(4)·a~* + 0.69107(7)·b~*.