Dependence of stoichiometry of lithium niobate nanocrystals on initial lithium to niobium ratios in the synthesis step

Ferroelectric nanocrystals show promise for application in forming hybridized nonlinear materials with liquid crystals. It is well known that bulk single crystals of lithium niobate (LiNbO3) are most easily grown in a congruent (lithium-deficient) form but can also be grown in a stoichiometric form. This is controlled by the specific growth conditions and the stoichiometric ratio r = MLi/(MLi + MNb), where M is the molar fraction. This work explores the dependence of the stoichiometry of LiNbO3 nanocrystals on the value of r in the synthesis step. Batches of LiNbO3 nanocrystals were synthesized using a sol-gel method. The nanocrystals were analysed via SEM and Raman spectroscopy to gain information about their morphology, stoichiometry, defect content, and phase. For bulk crystals, previous work has demonstrated that the spectral widths of specific Raman modes strongly depend on r. For the nanocrystals, the Raman spectra indeed reveal that the resultant nanocrystal stoichiometry depends on the initial r used in the synthesis step. In addition, a close examination of the Raman spectra reveals the presence of an extra phase in batches with r > 55%. Somewhat counterintuitively, this phase is identified by its Raman spectra to be LiNb3O8, a relatively lithium-poor phase compared to LiNbO3. Avoiding this extra phase, we find that high quality, roughly spherical LiNbO3 nanocrystals can be synthesized for r between 52 and 54%.