Temperature dependence of the OH-stretching frequencies in topaz-OH

Hydrogen bonding in topaz-OH, Al₂SiO₄(OH)₂, was investigated by IR spectroscopic analysis of the temperature dependence of the OH-stretching frequencies. Low-temperature spectra ranging from −196 to −160°C prove the existence of four non-equivalent H-positions in the crystal structure from the occurrence of four different OH-bands. With increasing temperature, these bands merge first, above −160°C, into two OH-bands and then above 400°C into one asymmetric broad band. Shifting of the OH-bands is caused by thermally induced hydrogen order–disorder. Low temperature fixes the protons in their positions; increasing temperature induces proton movement and allows switching between the different positions. Autocorrelation analysis of the IR spectra reveals two phase transitions, one at about −155°C from P1 to Pbn2 ₁ characterized as static–dynamic change and the second at about 380°C from Pbn2 ₁ to Pbnm caused by disordering of the protons. The increasing symmetry with temperature is due to advanced proton movement and dynamical averages over the proton distribution densities.