Distinct near-surface states and polishing responses of LiNbO₃ to chemically distinct alkaline additives in magnetic-chemo-mechanical polishing

The role of alkaline additives in magnetic-chemo-mechanical polishing (MCMP) of lithium niobate (LiNbO₃) cannot be evaluated by initial slurry pH alone. In this study, NaOH was used as a hydroxide-controlled alkaline reference. Sodium metasilicate nonahydrate was used as an alkaline silicate-containing additive. The two systems were compared at matched initial pH values. Additional base-slurry and NaNO₃-containing NaOH controls were introduced to separate the effects of alkalinity, sodium ions/ionic strength, and silicate species. The NaOH reference slurry showed a gradual pH-dependent response. In contrast, sodium metasilicate produced a stronger and non-monotonic response. The optimal result was obtained at pH 10.5 in the sodium metasilicate slurry, with a material removal rate of 3.68 μm/h and a surface roughness of Ra = 0.16 nm. Slurry characterization showed that the sodium metasilicate system provided better pH retention and abrasive dispersion stability. XPS analysis indicated silicate-influenced Li-, Nb-, O-, and Si-related near-surface components on the sodium metasilicate-polished surface. DFT and adsorption MD simulations further suggested more spatially distributed O-related interaction sites and a broader interfacial adsorption state. These coupled slurry-state and interfacial effects were associated with improved surface integrity, reduced subsurface damage, and better optical response.