K<sub>4</sub>Cd<sub>3</sub>Ge<sub>4</sub>O<sub>13</sub>: A Congruent-Melting Germanate\nNonlinear Optical Crystal with a\nModerate Second-Harmonic Generation Intensity and Broad Transparent\nWindow

Mid-infrared (IR) nonlinear optical (NLO) materials have\ngenerated\nextensive research interest because of their crucial role in laser\ntechnology applications. Here, we report the synthesis of a novel\ncadmium germanate NLO crystal, K<sub>4</sub>Cd<sub>3</sub>Ge<sub>4</sub>O<sub>13</sub>, using spontaneous crystallization. K<sub>4</sub>Cd<sub>3</sub>Ge<sub>4</sub>O<sub>13</sub> demonstrates a distinct three-dimensional\nstructural framework characterized by twisted [Ge<sub>4</sub>O<sub>13</sub>] and [Cd<sub>3</sub>O<sub>10</sub>] clusters, composed of\n[GeO<sub>4</sub>], [CdO<sub>4</sub>], [CdO<sub>5</sub>], and [CdO<sub>6</sub>] basic building units, respectively, which represents an\nunprecedented structural feature. The title compound undergoes a desirable\ncongruent melting behavior at about 727 °C. Notably, K<sub>4</sub>Cd<sub>3</sub>Ge<sub>4</sub>O<sub>13</sub> demonstrates a short UV\ncutoff edge at 261 nm, coupled with a wide energy gap of 4.4 eV, and\nmaintains an extended IR transparency window at around 6.0 μm.\nMore importantly, it demonstrates a strong second-harmonic generation\nactivity comparable to that of KH<sub>2</sub>PO<sub>4</sub> (KDP)\nat 1064 nm. Theoretical analyses further elucidate that the remarkable\noptical performances of K<sub>4</sub>Cd<sub>3</sub>Ge<sub>4</sub>O<sub>13</sub> are predominantly attributed to the cooperative effects\nof Ge–O and Cd–O bond-based motifs. These desired characteristics\nunderscore the potential of K<sub>4</sub>Cd<sub>3</sub>Ge<sub>4</sub>O<sub>13</sub> as a good candidate material for mid-IR NLO applications.