NB5O12F:
A TERNARY NIOBIUM OXIDE FLUORIDE COMPOUND WITH AN INCOMMENSURATE SUPERSTRUCTURE
Franziska E. Rohrer, Ann-Kristin Larsson and Sven Lidin
Inorganic Chemistry, Arrhenius Laboratory,
Stockholm University, 106 91 Stockholm, Sweden
The crystal structures of several ternary compounds in the niobium oxide fluoride system are already known [1-5]. All these are related to a-Nb2O5, ie they contain blocks of ReO3 type and differ just in the arrangement of the blocks of octahedra.
Nb5O12F was prepared by heating a mixture of NbO2F and Nb2O5 in a sealed gold tube 2 and the unit cell found to be related to TT- Nb2O5. The aim of this work was to investigate this phase closer by means of X-Ray and Electron Diffraction and High Resolution Electron Microscopy techniques.
The Single Crystal X-ray examination of selected crystals shows an incommensurate modulation along the a-axis of the orthorhombic subcell (a=3.6674, b=3.970, c=6.147 A). The symmetry of the 4D super spacegroup is P:Bmm2:-11s. The modulation can be described by the vector q ~ 0.606 0 0* (ie similar to the commensurate one reported by Andersson & Astrom 1964). The reciprocal lattice of the modulated structure can hence be described as H = G + mq, where G is the set of Bragg reflections for the orthorombic sublattice, and m is an integer.
Electron diffraction pattern were taken from many different crystallites. In some of these the Bmm2 sublattice could be found. Additional features in term of diffuse scattering or modulations were always observed. In some cases the modulations wave vector, q could be found and in that case m = 1-4 could be observed (the same for the X-ray diffraction pattern).
On moving the beam about on the crystallites, domains with diffuse scattering were frequently found. Streaking along the q-vector was prominent and could dominate the scattering (in addition to G). HREM images recording the nature of the modulations could be recorded.