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 mu m 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) circle times 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.