BaGeO₃: A Mid-IR Transparent Crystal with Superstrong Raman Response

Stimulated Raman scattering processes based on Raman crystals offer a simple and effective method to generate mid-IR lasers. However, currently available mid-IR Raman crystals are extremely scarce. Herein, a new type of mid-IR Raman crystal, BaGeO₃, is reported. It crystallizes in the monoclinic space group C2/c with Ba²⁺ cations and [Ge₃O₉]^6- rings as basic building units and features high transparency from 2.5 to 5.5 μm and a Raman response larger than that of diamond. The BaGeO₃ crystal has 45 IR-active modes (22A_u + 23B_u) and 42 Raman-active modes (20A_g + 22B_g). The wide mid-IR transparent window is attributed to the low phonon energy of the second-order IR-active A_u ⊗ B_g mode. The strongest Raman band, located at 799 cm^-1, arises from the symmetrically stretching vibration of the [Ge₃O₉]^6- extra-ring Ge-O bonds. The findings provide new insights into the crystallographic and Raman spectroscopic characteristics of high-performance mid-IR Raman crystals.