NUCLEAR DENSITY DISTRIBUTION OF KDP(KH₂PO₄) OBTAINED BY THE MAXIMUM ENTROPY ANALYSIS. S. Kumazawa, K. Ishida, Department of Physics, Science University of Tokyo, Japan, M. Takata, M. Sakata, Department of Applied Physics, Nagoya University, Japan, Y. Ishii, Y. Morii, Japan Atomic Energy Research Institute, Japa.

The nuclear density distribution of KDP(KH₂PO₄) at room temperature is obtained by the Maximum Entropy Method (MEM) from neutron powder diffraction data. It is not common to carry out neutron powder diffraction experiment without replacing hydrogen to deuterium, because of large incoherent scattering cross section of hydrogen. One of the long term aims of the present study is to clarify the structural difference of KDP and DKDP, which shows huge isotope effects. Therefore it is absolutely necessary to have the nuclear density distribution of KDP. Since Slater proposed the order-disorder type phase transition model of KDP, many structural studies were carried out. By these study, it is confirmed that the hydrogen atom is located on local equilibrium position at room temperature which is paraelectric phase and the occupancy of the hydrogen atom describes the order parameter of phase transition.

The neutron powder diffraction experiment is carried out by HRPD (High Resolution Powder Diffractometer) at JAERI-3M. The powder pattern is recorded at the wave length 1.823Å and 78 reflections up to 2qmax=150 deg. are collected. This corresponds to 0.476Å resolution. The MEM calculation is done by computer program MEND, which can deal with negative scattering length of hydrogen atom. In the obtained density map of (00z) plane, the hydrogen atom is clearly recognized. They are elongated to the direction of the oxygen atom. But the twin local maxima of hydrogen bond are not seen due to the limited resolution of this study. It is highly desirable to perform the same experiment with much higher resolution (approximately 0.3Å resolution). The distribution of oxygen atom shows substantial skewness, which should be attributed to anharmonic thermal vibrations related to hydrogen bond.