High-Pressure Synthesis, Electronic Properties, and Raman Spectroscopy of Barium Tetraborate BaB4O7 Polymorphs

The synthesis of high-pressure modification of barium tetraborate beta-BaB4O7 has been carried out at 3 GPa and 1000 degrees C in a DIA-type apparatus. Single-crystal X-ray analysis shows that beta-BaB4O7 crystallizes in a centrosymmetric group (Pnam (#62), a = 9.0029(3) A, b = 4.3006(7) A, c = 10.9795(8) A, Z = 4). The density of the beta-BaB4O7 phase increases by 22% relative to the ambient-pressure alpha-BaB4O7 phase. The P-T diagrams for the alpha- and beta-BaB4O7 phases have been calculated. The dynamic stability of beta-BaB4O7 at ambient pressure was confirmed by the absence of imaginary modes in the phonon spectra. The band gap values obtained using the HSE06 hybrid functional are 6.821 and 8.075 eV for alpha- and beta-BaB4O7 phases, respectively, with indirect transitions. The value of 8.075 eV indicates that beta-BaB4O7 can transmit to the deep-UV region. For the first time, the Raman spectra of the alpha- and beta-BaB4O7 phases have been studied by both experimental and numerical methods. The vasp_raman.py code was used to calculate the polarizability tensors for each crystal mode. The assignment of vibrations of cations, interconnected triangle, and/or tetrahedral groups (BO3 and BO4) of both modifications was carried out by comparing the experimental and ab initio calculated spectra and the result is in agreement with the interpretation developed for borates, aluminosilicates, and carbonates.