TOWARDS A UNIFIED DESCRIPTION OF THE AMOB2O5 (A = K, RB, CS, TL; M = NB, TA) FAMILY OF COMPOUNDS
Siegbert Schmid1, Ray L. Withers1, Deborah Corker2 and Pierre Baules3
1 Research School of
Chemistry, Australian National University, GPO Box 414, Canberra
ACT 2601, Australia
2 Nanotechnology, Cranfield
University, Bedford, MK42 8DF, UK
3 CEMES/CNRS, BP 4347, Toulouse
Cedex 4, France
Keywords: Pyroborates,
Modulated Structures, Crystal Chemistry
Non-centrosymmetric oxide pyroborates, AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta) [1 - 5], have recently attracted considerable interest, owing to their potential use as nonlinear optic materials. All members of that family possess a common underlying average structure as well as a superstructure of varying multiplicities (1, 2, 3, 5 and 8) along the b axis (corresponding to the Pmn21 setting of CsNbOB2O5 [4]). Electron diffraction evidence suggests that these phases are best described as modulated structures, thereby allowing for a unified view of their structures, underlying crystal chemistry and optical properties.
Despite the fact that all of these structures seem to be integer multiples of an underlying average structure, they appear to all be effectively incommensurately modulated owing to the lack of high order satellite reflections. This is further supported by the recent discovery of complete solid solutions in the Rb/K and Rb/Cs niobium borates [5]. Re-refinement of RbNbOB2O5 [1] as a modulated structure leads to an improved crystal chemistry and lower R-values, at the same time reducing significantly the number of refinable parameters.