The ternary oxides, Li2NpO4 and Li4NpO5, were synthesized under mild hydrothermal conditions using concentrated LiOH solutions containing NpO2(NO3)2. The reactions resulted in the formation of single crystals of both compounds, enabling the determination of their single crystal structures for the first time. Exploration of the synthetic phase space demonstrates that the resulting neptunate phases are dependent on the concentration of LiOH, transitioning from Li2NpO4, containing a typical octahedral neptunyl geometry with two shorter Np≡O bonds, at lower LiOH concentrations to Li4NpO5 with two long and four short Np-O bonds under saturated solution conditions. Reactions exploring the same synthetic conditions are also reported for uranyl(VI) for comparison. Raman spectra of the compounds were collected and analyzed to evaluate the Np-O bonding in these compounds.