We study structural phase transitions such as ferroelectric one from structural aspects.

Some of crystals like KH₂PO₄ (KDP) exhibit phase transition accompanying structural change (structural phase transition), if we change temperature and/or pressure. An ordered phase with spontaneous polarization (ferroelectric phase) appears either above or below this point (transition point, Tc). It is well known that Tc in KDP etc. rises more than 100K if deuterium is substituted for hydrogen. This large isotope effect has attracted much interest, since this is something like that the icing point in heavy water is 100^oC! Recently the new phenomenon that the phase transition is induced by deuteration have been discovered (deuteration-induced phase transition).

Our aim is in understanding these phenomena from microscopic viewpoint. Precise X-ray diffraction focused on bonding electron and neutron diffraction are used. We collaborate with the group of Uppsala University, Sweden, where the data collection is done. Mainly crystal preparation, preliminary measurements, and the data analysis are performed in our laboratory. Some work in progress are: ``Deuteration-induced structural phase transition in M<sub>3</sub>H(XO<sub>4</sub>)<sub>2</sub> (M=K, Rb, Cs; X=S, Se) with 0-dimensional H-bond network'' and ``Deformation-density study on NaH₂PO₄ as a reference material of KDP'' etc.

   ICHIKAWA, Mizuhiko (Associate Professor)  011-706-4416

   OZAKI, Yoshiki

   NAMBU, Hiroki