Infrared Absorption by Hydrogen Bonds in Single Crystal KH2PO4, KD2PO4, and KH2AsO4

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Abstract
The infrared absorption spectrum of KH2PO4, KD2PO4, and KH2AsO4 single crystals has been measured from 3000-1000 cm−1 over a temperature range from 20°-300°K. Three broad absorptions are found at 2670, 2360, and 1720 cm−1 which are associated with the hydrogen bonds. The two upper frequencies are stronger for light polarized perpendicular to the polar axis but are also found for light polarized along the polar axis. These absorptions have counterparts in the deuterated crystal with a frequency ratio νH / νD≃1.35. The lower-frequency absorption is strongest for light parallel to the polar axis and shows intensity changes at the Curie temperature, accompanying the appearance of a sharper absorption at 1850 cm−1 which has a Curie-law behavior above Tc. It is concluded that the uncoupled hydrogen-bond vibrations which give rise to the absorption at 2670 and 2360 cm−1 show no observable splitting of the ground or first vibrational state. This eliminates a double minimum potential with appreciable tunneling between minima as a model for this bond. In addition these motions appear to make an angle of several degrees with the x, y plane of the crystal. The 1720-cm−1 absorption is a hydrogen-bond vibration primarily along the polar or z axis with the 1850-cm−1 line representing coupled motion along z among locally ordered bonds.

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