抄録
The antiferroelectric material with an isolated hydrogen-bond, h-MeHPLN (5-methyl-9-hydroxyphenalenon), was structurally investigated by X-ray and neutron diffraction experiments in the low-temperature phase (Tc=42 K). The formation of a superlattice of 2×b was found below Tc, and the space group was identified to be P21/c transformed from C2/c. Accordingly, the number of crystallographically independent molecules became two. The electron density distribution and the nuclear density distribution revealed some significant features below Tc. One of the independent molecules exhibits an ordering of the hydrogen atom in the hydrogen-bond region, a conformational ordering of the methyl group and a molecular rotation around the a-axis. Moreover, a static electronic dipole moment is found in the hydrogen-bond region in this molecule. In contrast, the other molecule shows a disordered hydrogen atom, disordered conformation of the methyl group, no molecular rotation and a disordered electronic dipole moment. These features can be described simply in terms of a modulation wave of an order parameter.
