High pressure Raman spectroscopy of nitric acid

New high pressure Raman spectroscopy measurements on pure anhydrous HNO3 and HNO3-H2O mixtures up to 38 mol % in water (commercial grade concentration) are reported up to 50 GPa. The main feature is the reversible and progressive transformation of pure solid nitric acid at pressures between 10 and 17 GPa, evidenced by an enhancement of the 1057 cm(-1) peak assigned to the nu(1) vibrational stretching mode of the NO3- group and by the softening of the symmetric stretching NO2 mode with pressure. The formation of the H bonding by a charge transfer due to a strong interaction between the surrounding molecules is discussed. Two hypotheses are discussed: nitric acid might be pressure autoionized or cross-linked as a reversible polymerlike compound. These two hypotheses are not mutually exclusive. An increase of water concentration in nitric acid shifts the limit of the appearance of the NO3- vibrational mode to lower values of the pressure at constant temperature.