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Raman investigation of H₂O molecule and hydroxyl groups in the channels of hemimorphite

Institute of Inorganic Chemistry, Russian Academy of Sciences, Lavrentiev av. 3, Novosibirsk 630090, Russia

Correspondence: ^* E-mail: kolesov{at}che.nsk.su

Single-crystal polarized Raman spectra (40 to 4000 cm^–1 at 4 T 860 K) were measured for hemimorphite, Zn₄Si₂O₇(OH)₂·H₂O, to determine the behavior of H₂O molecules and OH groups in the channels. All observed stretching modes of the O-H vibrations could be assigned. Low-temperature Raman spectra show the presence of two different spatially divided regions in the crystal where the H₂O molecules and hydroxyl groups have slightly different positions in the channel. Two structural phase transitions were observed at 20–30 and 80–100 K, respectively, which reflect temperature replacement and reorientation of the H₂O molecules in the cavity. At T > 100 K, H₂O molecules are rotationally disordered around the c axis. An analysis of combined modes consisting of external and internal H₂O vibrations permits the frequency of translational T(H₂O) modes at 30, 53, 71, 90, 118, and 148 cm^–1 to be determined. The T(H₂O) modes also can be observed directly at low frequencies in the Raman spectra. The dehydration process of hemimorphite was investigated by Raman measurements at elevated temperatures of the crystal. One of two hydroxyl groups in the dehydrated crystal is hydrogen-bonded to neighbor oxygen and disordered statistically.

Key Words: Hemimorphite • H₂O molecules • microporous minerals • Raman spectroscopy

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