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X-ray diffraction and incoherent inelastic neutron scattering studies are performed to clarify the phase transition mechanism of Cu(HCOO) 2 ·4H 2 O. Contrary to the previous electrical study, we find no evidence of the existence of an intermediate ferrielectric phase. We determine the accurate antiferroelectric structure. It turns out that the atomic arrangement of the Cu(HCOO) 2 layers remains unchanged through the phase transition, while the antiferroelectric H 2 O layers take one of two disordered paraelectric arrangements alternately along the c-axis. We discuss that the rotation of the H 2 O molecules induces the structural correlation one-dimensionally along the antiferroelectric b-axis.

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Incoherent inelastic neutron scattering spectra of antiferroelectric Cu(HCOO) 2 ·4H 2 O and its isomorphous Cu(HCOO) 2 ·4D 2 O were measured. Comparing the spectra between these crystals, we extracted the H 2 O-related modes from many other modes. We find new peaks in the energy range of the H 2 O librational modes. The spectra in this range become broad above the phase transition temperature indicating that the antiferroelectric dynamics are dominated by the order-disorder of the H 2 O molecules induced by the out-of-plane hydrogen motion.

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