Lanthanide distribution in some doped alkaline earth aluminates and gallates

High-resolution neutron and synchrotron X-ray powder diffraction studies are reported for the six oxides AB2O4 (A=Ca2+, Sr2+ and Ba2+ and B=Al3+ and Ga3+). These oxides all adopt a stuffed tridymite type structure, the precise nature of which depends on both the A- and B-type cations. Bond valence calculations reveal a range of values for the various A-type cations, in all cases at least one site is significantly underbonded. Conversely the tetrahedral B-type sites invariably exhibit unexceptional bond valencies. Attempts to dope the oxides with various lanthanides to the 1% level invariably resulted in some segregation into alternate phases located at the grain boundaries. The identity of the impurity phases is presented and the importance of bond valencies in understanding this segregation is highlighted.

Graphical abstractStructural studies of stuffed tridymite AB2O4 (A=Ca, Sr and Ba and B=Al and Ga) compounds indicate underbonding of the A2+ sites. Attempts to dope the oxides with lanthanides invariably resulted in some segregation. These impurity phases are identified and their formation explained by bond valencies.Figure optionsDownload full-size imageDownload as PowerPoint slide