Figure 1: Left to right, schematic representation of a PBU (oxygen ions represented by the small spheres, the T ion represented by the larger sphere), examples of SBUs (taken from [35]), and examples of TBUs formed form the SBUs. Note that in the representation of both the SBUs and TBUs the “T” ions sit at the vertices while the O ions are located in the middle of the aristae. This is done for the sake of simplicity.