X-ray and Neutron Studies of Antiferroelectric Phase Transition in Cu(HCOO) 2 ·4H 2 O

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.