NaMoO 3 (IO 3 )(H 2 O): 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 3 (IO 3 )(H 2 O), was designed based on the CS matrix NaMoO 3 (IO 3 ) via introducing a water molecule into the structure. The introduction of one crystalline water molecule results in the rearrangement of Λ-shaped cis -[MoO 4 (IO 3 ) 2 ] units, and the proper array of the cis -[MoO 4 (IO 3 ) 2 ] units in NaMoO 3 (IO 3 )(H 2 O) results in its strong SHG response of 4.6 × KH 2 PO 4 . In addition, NaMoO 3 (IO 3 )(H 2 O) exhibits a wider optical bandgap of 3.44 eV and a larger birefringence of 0.231 than its matrix. Furthermore, the framework of NaMoO 3 (IO 3 )(H 2 O) is highly similar to that of α-KMoO 3 (IO 3 ), with water molecules assisting Na + cations in occupying the position of K + . However, due to the extra hydrogen bond of water molecules, the [MoO 3 (IO 3 )] ∞ layers in NaMoO 3 (IO 3 )(H 2 O) retain a parallel-stacking arrangement, different from the antiparallel arrangement of layers in α-KMoO 3 (IO 3 ) 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.