Raman spectroscopic study of H2O in bikitaite: “One-dimensional ice”

Abstract

The zeolite bikitaite, Li 2 [Al 2 Si 4 O 12 ]·2H 2 O, has structural channels containing infinite chains of H 2 O molecules running parallel to [010]. One hydrogen atom of an H 2 O molecule is weakly hydrogen bonded to an O atom of a neighboring molecule, while the other hydrogen atom is unbonded. The molecules are ordered and the chains they form have been called “one-dimensional ice.” Polarized Raman spectra of single crystals in the wavenumber range 40-4000 cm -1 were measured from 5 to 625 K. At low temperatures, four different O-H stretching vibrations can be observed between 3330 and 3600 cm -1 , as well as H 2 O bending vibrations at about 1640-1650 cm -1 . The two lower wavenumber hydrogen-bonded O-H stretching modes increase in wavenumber with increasing temperature, while the higher wavenumber non-hydrogen-bonded OH modes decrease in wavenumber. The temperature dependence of the linewidths of the O-H stretching modes and the degree of hydrogen bonding between neighboring H 2 O molecules show that the main cause of line broadening is modulation of the OH potential from low-energy thermal O···O vibrations in the H 2 O chains. At elevated temperatures, the different O-H stretching modes become similar in energy and only a single symmetric H 2 O stretching band is observed above 520 K. At these temperatures the H 2 O molecules lose their hydrogen bonding and are only bonded to Li cations at the walls of the channels.