K4Cd3Ge4O13: A Congruent-Melting Germanate Nonlinear Optical Crystal with a Moderate Second-Harmonic Generation Intensity and Broad Transparent Window

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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, K 4 Cd 3 Ge 4 O 13, using spontaneous crystallization. K 4 Cd 3 Ge 4 O 13 demonstrates a distinct three-dimensional structural framework characterized by twisted [Ge 4 O 13 ] and [Cd 3 O 10 ] clusters, composed of [GeO 4 ], [CdO 4 ], [CdO 5 ], and [CdO 6 ] basic building units, respectively, which represents an unprecedented structural feature. The title compound undergoes a desirable congruent melting behavior at about 727 °C. Notably, K 4 Cd 3 Ge 4 O 13 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 KH 2 PO 4 (KDP) at 1064 nm. Theoretical analyses further elucidate that the remarkable optical performances of K 4 Cd 3 Ge 4 O 13 are predominantly attributed to the cooperative effects of Ge–O and Cd–O bond-based motifs. These desired characteristics underscore the potential of K 4 Cd 3 Ge 4 O 13 as a good candidate material for mid-IR NLO applications.

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