Phase Transition Scheme of Isolated Hydrogen-bonded Material h-MeHPLN Studied by Neutron and X-ray Diffraction

Abstract

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 (T_c=42 K). The formation of a superlattice of 2×b was found below T_c, 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 T_c. 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.