Investigation into oxide-phosphates of selected lanthanoids and transition metals

The general goals of this work were the syntheses of selected lanthanoid and transition metal oxide-phosphates. Moreover, solving the crystal structures of Eu7O6(PO4)3 and Eu8O9(PO4)2 were targets. The experiments in this thesis confirm the existence of oxide-phosphates with compositions Ln3O3(PO4 ) and Ln7O6(PO4 )3 for Ln: Nd, Sm and Eu. Eu3O3(PO4) decomposes at 1600 °C to Eu7O6(PO4)3 and "Eu8O9(PO4)2". Similar experiments suggest for "Ln8O9(PO4)2" ( Ln : Sm, Eu and Gd) a slightly higher phosphate content and the approximate formula Ln14O15(PO4)4.

For Eu7O6(PO4)3 the synthesis of Serra et al. (Rev. int. hautes Tempér. Réfract. 1978,15, 287–313) was confirmed. The unit cell derived in this work differs from the one given in literature. However, TEM measurements confirm our unit cell. The preliminary structure model was constructed out of powder diffraction data and contains layers built from [OEu4 ] tetrahedra and [OEu6 ] octahedra separated by orthophosphate groups as structural motif. The model shows slight crystallochemical inconsistencies that have to be improved in future work. In a different approach DFT calculations for structure optimization might be applied. The metal-oxygen substructure [Eu7O6]9+ is related to a section from the Fluorite structure type.

Sm7O6(PO4)3 was synthesized and the corresponding lattice parameters and those of Nd7O6(PO4)3 on the basis of the powder pattern stated in literature [Serra, ibid.] were refined using the preliminary structure model of Eu7O6(PO4)3. Thus, the samarium and neodymium compounds were confirmed to be isotypic to Eu7O6(PO4)3.

The X-ray powder diffraction pattern reported in literature [Serra, ibid.] for "Eu8O9(PO4)2" is confirmed, yet indexing led to a different monoclinic unit cell. The structure model for "Eu8O9(PO4)2 " is in agreement with the anticipated slightly higher phosphate content. The structure model for "Eu8O9(PO4)2", as the one for Eu7O6(PO4)3 is related to the fluorite structure family with the occasional substitution of [EuO3 ]3- by [PO4]3- or Eu3+ by (P-O)3+. The 3D ordering for this substitution, which should be similar to the incorporation of [SiO4]4- tetrahedra into MnO2 in the Braunite structure type still has to be unveiled. "Sm8O9(PO4)2", isotypic to "Eu8O9(PO4)2" was synthesized under similar conditions.
The unit cells of "Sm8O9(PO4)2" and "Gd8O9(PO4)2 " obtained in this work fit well into the row of lanthanoid oxide-phosphates.

For summary, synchrotron diffraction data is needed to improve the structure models. Serra [ibid.] stated that the lanthanoids from terbium to lutetium are able to form an additional oxide-phosphate with the composition Ln12O15(PO4)2. Terbium is the most promising candidate for future experiments. Serra stated that terbium is capable of forming all mentioned oxide-phosphates making it the only lanthanoid forming four different oxide-phosphates.
The transition metal oxide-phosphates "Cr3O3(PO4)" and "Co3O3(PO4)" have not been obtained in this work.