The Raman spectra and the antiferroelectric phase transition of copper formate tetrahydrate

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Abstract
The Raman spectra of Cu(HCOO)24H2O and Cu(DCOO)24D2O have been compared in the temperature range 170-290K in connection with the paraelectric-antiferroelectric phase change at 235.3K. The transition is due to the ordering of the water layers of hydrogen atoms, the role of the water molecules in the transition is shown by the splitting of a broad Raman band, assigned to translational vibrations of H2O or D2O, into five lines below 235K. Librational modes of formate ions are assigned and it is shown that the doubling of the unit cell is accompanied by a splitting of the Raman lines assigned to the internal modes nu 2, nu 3 and nu 6 of the formate ion.