NaMoO3(IO3)(H2O): water molecule introduction induces strong second harmonic generation response, widened band gap and large anisotropy

Abstract

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, NaMoO₃(IO₃)(H₂O), was designed based on the CS matrix NaMoO₃(IO₃) via introducing a water molecule into the structure. The introduction of one crystalline water molecule results in the rearrangement of Λ-shaped cis-[MoO₄(IO₃)₂] units, and the proper array of the cis-[MoO₄(IO₃)₂] units in NaMoO₃(IO₃)(H₂O) results in its strong SHG response of 4.6 × KH₂PO₄. In addition, NaMoO₃(IO₃)(H₂O) exhibits a wider optical bandgap of 3.44 eV and a larger birefringence of 0.231 than its matrix. Furthermore, the framework of NaMoO₃(IO₃)(H₂O) is highly similar to that of α-KMoO₃(IO₃), with water molecules assisting Na⁺ cations in occupying the position of K⁺. However, due to the extra hydrogen bond of water molecules, the [MoO₃(IO₃)]_∞ layers in NaMoO₃(IO₃)(H₂O) retain a parallel-stacking arrangement, different from the antiparallel arrangement of layers in α-KMoO₃(IO₃) 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.