Temperature-Dependent Raman Spectra Of Rare-Earth Titanates With The Pyrochlore Structure: A Dipolar Order-Disorder Transition
The pyrochlore structure is an oxygen-deficient derivative structure based upon fluorite in which one-eighth of the oxygen atoms are removed forming an array of ordered vacancies. All of the rare-earth titanates crystallize into the pyrochlore structure with the exception of Nd2Ti2O7. Raman spectra of these materials exhibit extensive line broadening in excess of disorder arising from the crystallinity of the materials as determined by x-ray diffraction. Spectra obtained at liquid nitrogen temperature (128 K) exhibit a marked narrowing of Raman lines and an overall enhancement in intensity. The thermal dependence of the Raman linewidth is attributed to a dynamically disordered dipole caused by titanium ions shifting off of their equipoints. Measurements of dielectric permittivity made down to liquid helium temperature (4.25 K) support this hypothesis.