Two-mode nature of the Raman spectrum of lithium niobate crystals

The ordering of the host and impurity cations in nominally pure (with different values of Li/Nbratio) and doped lithium niobate crystals is studied using Raman spectroscopy. It is shown that depending on the composition-controlled ordering of structural units of the cationic sublattice the crystal may exhibit, in the region of bridge stretching vibrations of the oxygen ions, either single-mode or two-mode behavior. The nominally pure lithium niobate crystals show, within the homogeneity region, single-mode behavior, while the crystals doped with divalent or trivalent cations show single-mode behavior at low concentrations of the dopant and two-mode behavior at higher concentrations.