SPECTRA-STRUCTURE CORRELATION OF SOFT FERROELECTRICS

Author Institution: Department of Physics, Indian Institute of Science Bangalore 12, India (added at galley stage)It is known that the hydrogen motions are fundamental to the ferroelectric or antiferroelectric behaviour of the `soft' ferroelectrics. Since a spectroscopic study of such crystals may reveal the difference between ordered and disordered structures and the shifts of frequency due to the internal electric fields reveal the transition to a polarized phase, the infrared spectra of lithium hydrazonium sulphate, hydrazonium sulphate $(N_{2}H_{6}SO_{4})$ and dihydrazinium sulphate $(N_{2}H_{5})_{2}SO_{4}$ were investigated. Of these, lithium hydrazinium sulphate which is known to be ferroelectric from approximately $-15^{\circ} C$ to $80^{\circ} C$, has a new type of arrangement which would allow the protons to migrate along a chain of hydrogen bond system. The position of the 21 well defined infrared absorption maxima agree with the known crystal structure containing hydrazinium ions $NH_{2}-NH_{3}^{+}$ and $N-H \cdots N$ and $N-H \cdots O$ bonds in accordance with the correlation curve given by Krishnan, R.S. and Krishnan, K. The infrared spectra of $N_{2}H_{6}SO_{4}$ and $(N_{2}H_{5})_{2}SO_{4}$ show a more pronounced hydrogen bonding in the former case and help us in identifying the band at about $2100 cm^{-1}$ in $Li(N_{2}H_{5})SO_{4}$ as due to a combination of the $NH_{3}^{+}$ deformation and a torsional mode around $500 cm^{-1}$. The observed spectral changes in the soft ferroelectrics in the neighbourhood of the Curie point will be discussed in relation to the phenomenological theory of Ginsburg and the lattice dynamical approach of Cochron with particular reference to Triglycine sulphate, ammonium dihydrogen phosphate and lithium hydrazinium sulphate