Elucidating the structure-nonlinear optical property relationship of Te2O4(OH)2

A mixed-valent tellurium oxide hydroxide, Te2O4(OH)2, containing both tetravalent (IV) and hexavalent (VI) tellurium cations has been synthesized using a hydrothermal reaction method. Single crystal X-ray diffraction reveals that the compound crystallizes in the polar noncentrosymmetric space group, Pna21 (no. 33). The re-ported compound has a layered structure consisting of Te(VI)O4(OH)2 octahedra and Te(IV)O4 seesaw, in which each layer is aligned through interlayer hydrogen bonds. In addition to the structural refinements, the structure and optical properties of the material have been studied and compared with Te2O5, which is dehydrated residue of the title compound. Second harmonic generation (SHG) measurements indicate that the Te2O4(OH)2 exhibits a large SHG efficiency of ca. 4.7 times that of KH2PO4. The ultraviolet-visible diffuse reflectance spectrum shows that the material has a band gap of ca. 3.5 eV, corresponding to a UV cutoff edge of 354 nm. These values suggest that the compound has expanded its band gap while maintaining its SHG response, compared to Te2O5. Theo-retical calculations using the density functional theory have been also performed to clarify the structure-property relationship of the reported material more effectively.