Electron paramagnetic resonance of ${\mathrm{Gd}}^{3+}$ in $C_S$ symmetry and crystallographic study of rare-earth ultraphosphates. Case of ${\mathrm{Gd}}^{3+}$, ${\mathrm{Eu}}^{3+}$, and ${\mathrm{Gd}}_x{\mathrm{Eu}}_{1-x}$ ultraphosphates

The electron paramagnetic resonance of Gd${\mathrm{P}}_5$ ${\mathrm{O}}_{14}$ and of mixed ${\mathrm{Gd}}_x{\mathrm{Eu}}_{1-x}$ ultraphosphates has been studied. The Gd ultraphosphate shows a strong anisotropic structure, but the observed linewidths of the order of several-hundred gauss forbid a detailed analysis of the local symmetry of gadolinium. However, mixed samples containing predominantly ${\mathrm{Eu}}^{3+}$ and 5 × ${10}^{-2\mathrm{}}$ at.% of gadolinium show a number of relatively narrow lines (linewidths: 5-10 G). The angular variations of the allowed and forbidden absorption lines have been fitted to the spin Hamiltonian corresponding to $C_S$ symmetry. Eight different centers have been found, four of them being related to the others by two reflexion planes. A refinement of the crystal structure of europium ultraphosphate Eu${\mathrm{P}}_5$ ${\mathrm{O}}_{14}$ (monoclinic space group $P{2}_{\frac{1}{b}}-C_{2h}^5$) has been carried out by application of the least-squares method of calculation to single-crystal x-ray data ($R=0.040\mathrm{}$). The cell parameters are $a=8.744\mathrm{}$, $b=12.949\mathrm{}$ and $c=8.925\mathrm{}$ Å, $\gamma =90.46\mathrm{}°$, $Z=4\mathrm{}$. Crystal-structure studies show that the structure consists of P${\mathrm{O}}_4$ tetrahedra sharing tops in zig-zag chains running parallel to the $b$ axis and P${\mathrm{O}}_4$ tetrahedra loops connecting the chains. The oxygen atoms around the Eu atom form an eight-coordinate polyhedron. The correlations between the crystallographic structure and the EPR results for rare-earth ultraphosphates are discussed in detail.