Microemulsion mediated synthesis of nanocrystalline (K-x,Na1-x)NbO3 powders

The present study reports on the synthesis of nanocrystalline (K-x,Na1-x)NbO3 powders prepared via microemulsion mediated hydrolytic decomposition of mixed alkoxide solutions. Compositions with different K/Na ratios ranging between the two end members KNbO3 and NaNbO3 have been synthesized and characterized with respect to stoichiometry, purity, crystalline structure, particle size and powder morphology using X-ray diffraction, Raman spectroscopy and inductively coupled plasma with optical emission spectroscopy. Both raw as well as calcined powders were investigated. For the technically relevant and piezoelectric most active composition (K-0.50,Na-0.50)NbO3 the results are presented and discussed in comparison to micron-sized and submicron-sized powders, that have been prepared by solid state reaction for reference. The study of the crystallographic structure of these reference powders by XRD and Raman spectroscopy confirms the size induced phase transition between the thermodynamically stable monoclinic modification for large particles towards a new triclinic polymorph, which has been reported for nano-powders of this composition by us previously. As possible origin for this phenomenon, internal OH- groups, variations in K-/Na-site occupancy and mechanical stresses arising from the large surface curvature of the nanocrystalline powders are addressed.