Abstract
Incoherent inelastic neutron scattering spectra of antiferroelectric Cu(HCOO)2·4H2O and its isomorphous Cu(HCOO)2·4D2O were measured. Comparing the spectra between these crystals, we extracted the H2O-related modes from many other modes. We find new peaks in the energy range of the H2O 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 H2O molecules induced by the out-of-plane hydrogen motion.