Abstract
Abstract
A previous electron diffraction study during a student project indicated that Er1.96Ca0.04Ti2O6.98 exhibit a primitive Bravais lattice. This result has not been repeated during the current investigations. The same TEM specimen as used during the student project was reinvestigated as well as new samples prepared by both a dry- and a wet method of synthesis. Er1.96Ca0.04Ti2O6.98 has been determined to exhibit a face centered cubic Bravais lattice and to crystallize in the Fd3m (no. 227) space group by SAD and CBED. This is in accordance with the reported space group for Er2Ti2O7. None of the following factors have been found to affect the Bravais lattice of the structure: doping with divalent calcium ions, holmium impurities, phase transitions, order- disorder transitions of defects, water content and the method of synthesis. No explanation has been found for the inconsistency in exhibited Bravais lattice.
Lanthanum tungstate with a nominal La/W ratio of 5.6 was studies by electron diffraction. Samples were prepared by the freeze-drying method. CBED patterns indicated that the compound crystallize in a quasi- Fd3m space group. However SAD patterns indicated a primitive cubic lattice where the d- glide plain symmetry breaks down. Magrasó et al. have reported that this compound crystallizes in the F-43m space group by XRD and NPD. CBED, SAD and XRD/NPD results are in disagreement, which might be explained by small range structural disorder, but this warrants further investigation.