Shannon Farrell MSc thesis abstract

Thesis Title: 
Crystallographic studies of selected perovskite-group compounds

This study investigates the tausonite-loparite solid solution series.  Members of this series are important in some alkaline complexes, thus making studies of their crystallography essential.  Rietveld refinement of the crystal structures of the tausonite-loparite solution series, using X-ray diffraction powder patterns, indicates that there is a reduction in symmetry from cubic (Pm3m) to orthorhombic (Pnma), by way of an intermediate tetragonal (P4/mbm) modification.  The symmetry changes appear to occur at about ~66.6 and ~33.3 wt% tausonite, which are consistent with formulae of approximately Sr2(NaLa)Ti3O9 and Sr(NaLa)2Ti3O9, respectively.  The pseudo-cubic cell parameter apdecreases with increasing loparite content, while the [111] tilt angle F (F=0 in Pm3m) on inception at 50 wt% loparite achieves a maximum and decreases thereafter with increasing loparite content.  Rietveld refinements indicate that no ordering at the A-site exists throughout the solid solution series. 

This study also investigates a titanium perovskite (Na2/3Th1/3TiO3), which is unusual in that it contains a tetravalent cation at the A-site.  This thorium titanium perovskite was synthesized in an attempt to determine its structure.  Although power diffractometry suggests an Fm3m space-group, attempts at Rietveld refinement of the structure show the actual space-group must be of reduced symmetry.

This study also provides data on the pseudo-binary system between hollandite (K2Cr2Ti6O16) and the n=3 member of the homologous series K2La2Ti3+nO10-2n, i.e., K2La2Ti6O16.  This series is important in understanding the location and environment of the rare-earth cations in natural hollandite specimens and the capability of hollandite (i.e., SynRock) to immobilize large elements of varying charge and size.  This pseudo-binary system is characterized by the presence of the following phases:  hollandite [K1.54(Cr1.43Ti6.52)7.95O16];  perovskite-2 (LaCrO3); and perovskite-3 (La2Ti2O7).  Complete solid-solution between the end-members of this system does not occur.  The hollandites (space-group 14/m) have an A-site occupancy of approximately 75-82%, and exhibit no significant substitution of La3+ at any of the cation sites.  Perovskite-1 is considered to be a non-stoichiometric A-site deficient perovskite.  Potassium hexatitanate is the only main phase that is stoichiometric and contains no substitution of Cr3+ in any of the cation sites.  All the Cr3+ excluded from the potassium hexatitanate structure is incorporated into perovskite-2.

A copy of the thesis can be downloaded here