Structural stability and optical properties of borate-based oxide Sr3Ti3O6(BO3)2

Borate-based oxide materials are an important class of compounds for optical applications due to their wide optical transparency window, optical response, and high laser damage threshold. In this work, the Sr-Ti-B-O quaternary system was investigated computationally for new materials. The crystal structure of Sr 3 Ti 3 O 6 (BO 3 ) 2 and SrTi(BO 3 ) 2 materials were predicted. Their stability, electronic, mechanical, and optical properties were investigated using ab initio methods, to identify future applications. A comparison of Sr 3 Ti 3 O 6 (BO 3 ) 2 with the related Ba 3 Ti 3 O 6 (BO 3 ) 2 compound revealed that both materials possess similar crystal symmetry, while Sr 3 Ti 3 O 6 (BO 3 ) 2 exhibits distinct local structural distortions associated with rotations of the BO 3 units and deformation of the TiO 6 polyhedra. This crystal structure was matched with the experimentally obtained material, which was synthesized by optimized crystallization of glass granules prepared from a melt obtained at 1500 °C, and confirms our calculations. The thermodynamic and dynamical stability of the material were evaluated through convex hull and phonon calculations, while a machine-learned interatomic potential was employed to eliminating imaginary phonon modes and finding stable crystal structure. The mechanical responses of Sr 3 Ti 3 O 6 (BO 3 ) 2 and SrTi(BO 3 ) 2 were analyzed. The calculated HSE band gaps, pronounced optical anisotropy, and relatively high refractive indices, in particular, Sr 3 Ti 3 O 6 (BO 3 ) 2 shows stronger interaction in the visible–near-UV range, whereas the wider-gap SrTi(BO 3 ) 2 exhibits a broader low-energy transparency window.