Structural changes and pressure-induced chemical decomposition of boric acid

X-ray powder diffraction studies along with complementary Raman and infrared spectroscopy measurements disclosed the pressure-induced chemical decomposition of boric acid, H3BO3. The layered triclinic structure of H3BO3 suffers a highly anisotropic compression associated to a strong reduction of the interlayer spacing. The starting structure becomes unstable at about 2 GPa and decomposes on a cubic polymorph of the metaboric acid, HBO2, and ice. Two structural modifications of ice, ice VI and ice VII, are formed in the course of the chemical decomposition of boric acid. X-ray diffraction data suggest a new polymorph of H3BO3 as a precursor structure of the decomposition process. The triclinic lattice of the new phase has a translation period corresponding to 3 layers of hydrogen-bonded B(OH)3 molecules in place of the 2 layers period observed in the initial phase. The large volume variation and the important structural changes associated to the decomposition suggest a high free energy of activation. This yields a slow kinetics at room temperature, and a phase composition very dependent of the specific pressure-time path followed by the sample. Based on the obtained results a mechanism for the chemical decomposition of H3BO3 is proposed and the possible role of hydrogen bonds in the observed transformations is discussed.