Experimental and ab initio study of Ba₂Na₃(B₃O₆)₂F stability in the pressure range of 0–10 GPa

Both numerical and experimental studies of the stability and electronic properties of barium–sodium metaborate Ba₂Na₃(B₃O₆)₂F (P6₃/m) at pressures up to 10 GPa have been carried out. Electronic-structure calculations with HSE06 hybrid functional showed that Ba₂Na₃(B₃O₆)₂F has an indirect band gap of 6.289 eV. A numerical study revealed the decomposition of Ba₂Na₃(B₃O₆)₂F into the BaB₂O₄, NaBO₂, and NaF phases above 3.4 GPa at 300 K. Subsequent high-pressure high-temperature experiments performed using ‘Discoverer-1500’ DIA-type apparatus at pressures of 3 and 6 GPa and temperature of 1173 K confirmed the stability of Ba₂Na₃(B₃O₆)₂F at 3 GPa and its decomposition into BaB₂O₄, NaBO₂, and NaF at 6 GPa, which was verified by energy-dispersive X-ray analysis and Raman spectroscopy. The observed Raman bands of the Ba₂Na₃(B₃O₆)₂F phase were assigned by comparing the experimental and calculated spectra. The experimental Raman spectra of decomposition reaction products obtained at 6 GPa suggest the origin of a new high-pressure modification of barium metaborate BaB₂O₄.