Effect of boron oxide on the structure and optical properties of magnesium fluoride

Chem. Met. Alloys 16 (2023) 1-6

Viktor ZINCHENKO, Igor MAGUNOV, Anton BABENKO, Ganna VOLCHAK, Serhii KULESHOV, Oleksandr IVANENKO

https://doi.org/10.30970/cma16.0427

Magnesium fluoride, MgF₂, is a widely used compound in electrical engineering, optoelectronics, etc. Materials based on it have high transparency up to the deep (vacuum) ultraviolet range of the spectrum. Their main disadvantage is the presence of oxide impurities, mainly MgO, which appear during heat treatment and prolonged storage in open air due to hydrolysis or incomplete reaction during synthesis. Addition of boron oxide, B₂O₃, is proposed to bind oxide impurities of basic nature into complex compounds such as fluoroborates. The amount of additive was calculated based on the estimated content of MgO impurities in the material. Excess B₂O₃ was washed with ethanol, which was then removed by prolonged heat treatment. The base substance, MgF₂, and reaction products were identified by infrared transmission spectroscopy and X-ray diffraction. The IR transmittance spectra of the original sample clearly show the bands characteristic of MgF₂, and after the addition of the additive, absorption bands characteristic of B–O bonds are detected, the intensity of which decreases when the additive is washed away. The original MgF₂ preparate is single-phase, but the increase in the width of the peaks and their shift indicate a disturbance in the structure. The average size of the MgF₂ crystallites was calculated according to the well-known Scherrer equation, and is in the range of 14–20 nm (average value 18 nm). After heat treatment with the addition of B₂O₃, the width of the reflections decreases significantly, and after subsequent washing and calcination, the diffractogram of the sample reveals the reflections of magnesium fluoborite, Mg₃(BO₃)F₃. According to a quantitative analysis by the Rietveld method, its content is about 8.7% by volume. The thickness of the MgO surface layer on the surface of MgF₂-particles was calculated (about 0.068 nm), and confirms the X-ray amorphous nature of the impurity.

Infrared transmission spectra in the intervals 3500-200 (a) and 1400-200 (b) cm^-1 of MgF₂ samples: 1 – original sample, 2 – sample after heat treatment with B₂O₃, 3 – sample after heat treatment with B₂O₃, removal of excess B₂O₃ and repeated heat treatment.

Keywords

Magnesium fluoride / Oxide impurity / Boron oxide / Infrared spectroscopy / X-ray phase analysis