Frequencies of Stretching and Bending OH (OD) Vibrations in KDP (DKDP) Crystals, According to the Temperature Dependence of Their Raman Spectrum
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The temperature dependence of OH (OD) vibrations in KDP (DKDP) crystals is studied by Raman spectroscopy in different scattering geometries at temperatures from 30 to 299 K. The three lower frequencies from the five well-known high-frequency bands of OH (OD) vibrations soften upon an increase in the paraelectric phase temperature. This results from the softening of the corresponding harmonic potential upon an increase in the interatomic distance, and these frequencies are attributed to bending vibrations. The two higher frequencies of OH (OD) vibrations are virtually independent of temperature with a slight tendency to grow upon an increase in the paraelectric phase temperature. This is in better agreement with the complicated temperature dependence of the energy levels of the double-well potential along a hydrogen bond and allows these bands to be attributed to stretching OH (OD) vibrations.
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