K₄Cd₃Ge₄O₁₃: A Congruent-Melting Germanate Nonlinear Optical Crystal with a Moderate Second-Harmonic Generation...

Mid-infrared (IR) nonlinear optical (NLO) materials have generated extensive research interest because of their crucial role in laser technology applications. Here, we report the synthesis of a novel cadmium germanate NLO crystal, K₄Cd₃Ge₄O₁₃, using spontaneous crystallization. K₄Cd₃Ge₄O₁₃ demonstrates a distinct three-dimensional structural framework characterized by twisted [Ge₄O₁₃] and [Cd₃O₁₀] clusters, composed of [GeO₄], [CdO₄], [CdO₅], and [CdO₆] basic building units, respectively, which represents an unprecedented structural feature. The title compound undergoes a desirable congruent melting behavior at about 727 °C. Notably, K₄Cd₃Ge₄O₁₃ demonstrates a short UV cutoff edge at 261 nm, coupled with a wide energy gap of 4.4 eV, and maintains an extended IR transparency window at around 6.0 μm. More importantly, it demonstrates a strong second-harmonic generation activity comparable to that of KH₂PO₄ (KDP) at 1064 nm. Theoretical analyses further elucidate that the remarkable optical performances of K₄Cd₃Ge₄O₁₃ are predominantly attributed to the cooperative effects of Ge-O and Cd-O bond-based motifs. These desired characteristics underscore the potential of K₄Cd₃Ge₄O₁₃ as a good candidate material for mid-IR NLO applications.