Insights into high-pressure stability, vibrational and mechanical characteristics of Ba5(BO3)3F

The fluoroborate Ba 5 (BO 3 ) 3 F, a candidate for UV optical applications, is investigated through density functional theory (DFT) and experimental methods to unravel its electronic, vibrational, mechanical characteristics and high-pressure stability limit. DFT calculations with HSE06 functional reveal a direct band gap of 5.46 eV, corroborating experimental transparency in the mid-UV range. Phonon dispersion analysis confirms dynamic stability, and the simulated Raman spectrum are in good agreement with the obtained experimental data, enabling the detailed assignment of 23 observed modes. The two most intense peaks at 902 and 909 cm –1 attributed to symmetric stretching vibrations of two crystallographically different [BO 3 ] groups in the structure. The Ba 5 (BO 3 ) 3 F compounds exhibits strong elastic anisotropy, with bulk modulus ( B ) varying by a factor of 2.7 (38.7–104.7 GPa) across crystallographic directions. The estimated Vickers hardness (2.68 GPa) and fracture toughness (0.556 MPa·m 1/2 ) of Ba 5 (BO 3 ) 3 F classify it as mechanically soft yet more crack-resistant than, for instance, β -BaB 2 O 4 . High-pressure calculations reveal that Ba 5 (BO 3 ) 3 F is stable up to 9 GPa under hydrostatic compression, beyond which shear instability ( C 44 − P < 0) and soft phonon modes occur. • A direct band gap of 5.46 eV was revealed for Ba 5 (BO 3 ) 3 F, corroborating experimental transparency in the mid-UV range. • For the first time, the Raman spectra of Ba 5 (BO 3 ) 3 F has been studied by both experimental and numerical methods. • Ba 5 (BO 3 ) 3 F exhibits strong elastic anisotropy: bulk modulus ranges 38.7–104.7 GPa. • High-pressure stability limit of Ba 5 (BO 3 ) 3 F was determined to be 9 GPa via Born criteria & phonon analysis.