REPOSITORY > RESULTS

Abstract

This PhD thesis presents experimental results from the synthesis, crystal structure and
microwave dielectric characterization of BaO-rich phases within the BaO-Nb2O5 and BaONb2O5-
WO3 system. I was primarily interested in two compounds with unknown crystal
structure, i.e., the binary Ba4Nb2O9 and the ternary Ba6Nb2WO14, which lie on a common
virtual tie-line Ba4Nb2O9–Ba2WO5. As such, the investigations presented in this PhD thesis were
focused on:
(a) A detailed study of the formation conditions of Ba4Nb2O9, the phase transitions in
Ba4Nb2O9 and the phase stability of the individual polymorph. These investigations were
followed by decomposition studies of Ba4Nb2O9, induced by various thermal and atmosphere
treatments.
(b) The phase relations and the phase stability of the BaO-rich hexagonal phases
within the BaO–Nb2O5–WO3 ternary system. The first part of the ternary system investigation is
dedicated to the phase formation and microstructure of the hexagonal-perovskite based
ceramics, while the second part is related to the characterisation of the microwave dielectric
properties of the ceramics and to the hexagonal-perovskite crystal structure of the hightemperature
Ba6Nb2WO14 polymorph.
(c) The crystal-structure characterization of the Ba4Nb2O9 and Ba6Nb2WO14
polymorphs. The structural characterizations are based on a special approach, i.e., the TEMtilting
method in combination with a Rietveld refinement. The outcome of the applied
methods is a space-group determination of the high-temperature γ-Ba4Nb2O9 and
Ba6Nb2WO14, and the crystal structure of the low-temperature α-Ba4Nb2O9. Therefore the
presented tilting method represents a challenging starting point for the crystal-structure
determination of inorganic ionic compounds.