Abstract
We have studied the lanthanum niobate system doped with strontium of nominal composition La_{0.95}Sr_{0.05}NbO_{4}. We observe two phases in the synthesized material. One of these phases is Sr-poor LaNbO_{4}. The other phase is to our knowledge an unreported oxide with a Sr:Nb ratio between 1:3 and 1:2. This phase has a large unit cell with lattice parameters a = 7.91 Å, b = 5.81 Å and c = 30.75 Å.
The well known Sr-poor phase, LaNbO4, transforms from a tetragonal to a monoclinic crystal structure upon cooling. This transformation is accompanied by domain formation. The observed orientation of the domain boundaries is in excellent agreement with theoretical considerations presented in this thesis. Furthermore, we have studied the atomic arrangement at the domain boundary by high resolution electron microscopy, and observe that the boundary is highly ordered.