Vibration spectra of ferroelectric alkali trihydrogen selenites and the nature of the hydrogen bond potential in these crystals

Abstract

Raman spectra of the ferroelectric LiH₃(SeO₃)₂ and NaH₃(SeO₃)₂ and the anti-ferroelectric KH₃ (SeO₃)₂ have been recorded at room temperature using a He-Ne and also an Ar-ion laser source. The infrared absorption spectra of these crystals and their deuterated analogues have been recorded in the region 400-4000 cm^-1 both below and above the Curie temperature. From an analysis of the spectrum in the region 400-900 cm^-1 it is concluded that (i) in LiH₃ (SeO₃)₂ the protons are ordered in an asymmetric double minimum potential with a low barrier and the spectrum can be interpreted in terms of HSeO₃^- and H₂SeO₃ vibrations, (ii) in NaH₃ (SeO₃)₂ all three protons occupy a single minimum potential at room temperature and below the transition temperature the groups HSeO₃^- and H₂SeO₃ are present, (iii) the proton at the inversion centre in KH₃(SeO₃)₂ is in a broad troughed potential well and the low temperature spectrum is more likely to be due to H₃SeO₃⁺ and SeO₃^2- species. This deviation of the spectrum from that of the previous two crystals is attributed to the difference in H-bond scheme and hence the absence of any cooperative motion of protons in this crystal.