Low Temperature Structural Phase Transitions in Novel Oxides

Analysis of the Raman active phonon modes offers a symmetry dependent determination of structural phase transitions. We have performed polarized Raman scattering measurements on oriented single crystals of the superconducting pyrochlore Cd$_{2}$Re$_{2}$O$_{7}$ and the layered ruthenate La$_{4}$Ru$_{2}$O$_{10}$ as a function of temperature. In Cd$_{2}$Re$_{2}$O$_{7}$ we resolve and assign each of the six Raman-active (A$_{1g}$ + E$_{g}$ + 4F$_{2g}$) modes of the room temperature cubic phase. Below the structural phase transition at 200K (and 120K) we observe new symmetry dependent Raman-active vibrations associated with a cubic-tetragonal (tetragonal-tetragonal) phase transition. We identify two ``soft" modes and discuss a structural order parameter with E$_{u}$ symmetry. We measure La$_{4}$Ru$_{2}$O$_{10}$ through the monoclinic-triclinic phase transition @ 150K and compare the symmetry dependent results with expectations based on x-ray structural analysis.