Synthesis, crystal structures and nonlinear optical properties of β-RbCdI₃·H₂O and CsCdI₃·H₂O

Searching for new IR nonlinear optical (NLO) crystals with a high laser damage threshold (LDT) is still a challenge. In this paper, two new halides, β-RbCdI₃·H₂O and CsCdI₃·H₂O, have been synthesized by a conventional solution reaction. Both of them crystallize in the monoclinic space group Pc (no. 7). In their crystal structures, all the corner-sharing [CdI₄] tetrahedra connect with each other to form 1D chains, which further connect with [Rb(H₂O)]⁺/[Cs(H₂O)]⁺ complexes to form three-dimensional (3D) frameworks. All the polar [CdI₄]^2- groups align in such a way that results in a favorable net polarization. Powder second-harmonic generation (SHG) measurements indicate that β-RbCdI₃·H₂O and CsCdI₃·H₂O show phase-matchable SHG responses of 2.7 and 2.6 times that of KH₂PO₄ (KDP). The infrared transmission range can reach 21 μm. The UV-vis diffuse reflectance spectra indicate that the band gaps of β-RbCdI₃·H₂O and CsCdI₃·H₂O are about 3.26 and 3.54 eV, respectively. A preliminary measurement indicates that the LDT of their powders is about 90 and 100 MW cm^-2, respectively, suggesting that β-RbCdI₃·H₂O and CsCdI₃·H₂O are two new promising IR NLO materials. Their band structure and optical property calculations based on DFT methods are carried out.