Structure-composition-property Relations in B-site Deficient Hexagonal Perovskite Systems

This thesis describes the structural and preliminary electrical characterisation of
various “shifted” hexagonal B-site deficient perovskites, of generic formula, A4B3O12. These ceramics are reported to possess promising microwave dielectric properties, which are a requirement for use in the ever evolving mobile telecommunications industry.

The 12R crystal structures of the Ba3-xSrxLaNb3O12 series (x = 1 – 3) were refined at variable temperatures using NPD data in space group R-3 and some interesting behaviour was exhibited. All compositions were found to be antiphase tilted, the extent of which being dependent on the A-cation size. Upon heating, the tilt angle for all members of the series was found to decrease, although no tilt transitions from R-3 to R-3m were witnessed for all phases up to 900 °C, and a change in the magnitude of permittivity at 1 MHz was observed for all members except, BaSr2LaNb3O12. Additionally evidence from preliminary low temperature data suggest relaxor ferroelectric type behaviour. Ba2SrLaNb3O12 was found to possess the most
promising dielectric properties although room exists for further improvement of the
quality factor (Q value).

To facilitate further tuning of the dielectric properties, specifically Q, the Ba3-
xSrxLaNb3-yTayO12 series (x = 0 - 3, y = 1 - 2) was investigated, with the introduction of Ta5+ on the B-site. Some interesting ordering of A- and B-sites was found to occur, and furthermore, Nb5+ was found to display a preference to inhabit the distorted environment of B(1), adjacent to the “shifted” region, compared to the undistorted
B(2) site, in the perovskite block. Unusual microwave dielectric properties were displayed with all compositions exhibiting small and positive temperature coefficient of resonant frequency (TCF) values and significantly small Q values, the reverse of what is expected, thus implying that the B-site has some influence on TCF values in this case. The interesting dielectric properties may be explained by several factors, including, the increased strength associated with the covalency of the Ta-O bond
compared to that of Nb-O and the presence of some significant impurity phases, particularly in the intermediate compositions that are richer in Ta5+. The LCR data collected revealed similar results as found for the Ba3-xSrxLaNb3O12 series (x = 1 – 3), highlighting the limiting nature of the temperature range which did not, in fact
reflect the true response of the compositions’ behaviour. All of the compositions are antiphase tilted at room temperature therefore the collection of variable temperature diffraction data is warranted.

A range of novel Ta-based intergrowth compounds were synthesised and preliminarily characterised. No detailed structural or electrical data were collected for these compounds, however, the ability to form Ta-based intergrowths, in the first instance, has been established. It is postulated that by the formation of these
intergrowth phases, the dielectric properties will be effectively tuned in comparison to
those of the parent oxides that they are composed of, due to the regularisation of the octahedral layers that occurs upon the formation of such intergrowth structures. This often leads to improved TCF values, and by the introduction of Ta5+, can also give improved Q values. Ta5+ was substituted for Nb5+ in the untilted 5x6y (x = 1 – 3, y = 1) layered compounds, however, it was found that to facilitate this, the replacement
of Ba2+ for Sr2+ was also required on the A-site. This double substitution was a requirement to obtain compositions closer to single phase Ta-rich phases compared to Nb-rich compositions.