NaMoO(IO)(HO): water molecule introduction induces strong second harmonic generation response, widened band gap and large anisotropy.

Exploring feasible tactics to induce the formation of non-centrosymmetric (NCS) structures, especially from centrosymmetric (CS) structures, is essential for the development of nonlinear optical crystals with more potential. An NCS alkali metal-containing molybdenum iodate hydrate, namely, NaMoO(IO)(HO), was designed based on the CS matrix NaMoO(IO) introducing a water molecule into the structure. The introduction of one crystalline water molecule results in the rearrangement of Λ-shaped -[MoO(IO)] units, and the proper array of the -[MoO(IO)] units in NaMoO(IO)(HO) results in its strong SHG response of 4.6 × KHPO. In addition, NaMoO(IO)(HO) exhibits a wider optical bandgap of 3.44 eV and a larger birefringence of 0.231 than its matrix. Furthermore, the framework of NaMoO(IO)(HO) is highly similar to that of α-KMoO(IO), with water molecules assisting Na cations in occupying the position of K. However, due to the extra hydrogen bond of water molecules, the [MoO(IO)] layers in NaMoO(IO)(HO) retain a parallel-stacking arrangement, different from the antiparallel arrangement of layers in α-KMoO(IO) with a centric structure. This study confirms the feasibility of applying a water molecule to adjust the orientation of basic building block units to assemble an NCS structure based on CS crystals.