shannon farrell msc thesis abstract

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 a_pdecreases 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 (na_2/3th_1/3tio₃), 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 k₂la₂ti_3+no_10-2n, 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 [k_1.54(cr_1.43ti_6.52)_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