Infrared Absorption Spectrum of Lithium Hydroxide

Abstract

The polarized infrared absorption spectrum of crystalline lithium hydroxide in the region between 2500 cm—1 and 8000 cm—1 has been obtained with a grating spectrometer. The spectrum consists in part of a strong band at 3678 cm—1 polarized parallel to the c axis and bracketed in a nearly symmetric manner by five pairs of side bands polarized perpendicular to the c axis and by two pairs of side bands polarized parallel to the c axis. In addition an isolated band at 7172 cm—1 is observed polarized parallel to the c axis. Changes in the spectrum as the sample temperature is increased from room temperature to 400°C are described. The hydrogen positions in lithium hydroxide have been deduced. It is shown, using the factor group selection rules as a guide, that the spectrum can be explained as consisting of an infrared active OH stretching fundamental surrounded by binary combinations of lattice fundamentals with the infrared and Raman OH fundamentals. The band at 7172 cm—1 is assigned as the binary combination of the two OH fundamentals. The vibrational modes of the crystal belonging to irreducible representations of the factor group have been constructed to within a good approximation. Tentative assignments of deduced lattice frequencies to constructed modes are presented. The present explanation of the lithium hydroxide spectrum is contrasted with the rotation-vibration explanation of Hexter.