High temperature–high pressure synthesis and crystal structure of the incommensurately modulated, α-PbO 2 related, compound MnTa 2O 6

Abstract

A new high pressure–high temperature modification of MnTa 2O 6 has been prepared at T=1400 °C, 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: P112 1/ n( 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) Å, b=5.7453(3) Å, c=5.1496(3) Å, γ=90.023(9)°, and the modulation vector is q =−0.1038(4)· a ∗+0.6910(7)· b ∗ .