Insights into the Defect Structure of NaBa₁₂(BO₃)₇F₄ (NBBF) Crystals Using Raman Spectroscopy: Numerical and Experimental Study

In spite of the fact that the NaBa12(BO3)7F4 (NBBF) crystals do not contain any chromophore ions and should be nominally colorless, their color changes from dark purple to light pink depending on the ratio of BaO, BaF2, B2O3, and Na2O compounds in the initial high-temperature solutions. The study of optical transparency, electronic properties, and Raman spectra with 532 and 325 nm excitation of differently colored NaBa12(BO3)7F4 (NBBF) crystals has been conducted. All Raman-active modes determined for the pristine NBBF structure by ab initio density functional calculations coincide perfectly well with the observed modes in the experimental spectra. However, apart from the modes typical of orthoborates, additional strong modes and their overtones appear in the resonant Raman spectra of intensively colored crystals. These modes are accounted for by the presence of an additional borate group in the channels of the structure.