COMPARISON OF 158 K THERMAL PARAMETERS FOR
(NH4)H2PO4 OBTAINED FROM NEUTRON AND FROM HIGH-ORDER CONVENTIONAL AND SYNCHROTRON X-RAY DATA. M. Souhassou, N. Peres, C.Lecomte, H. Graafsma, and E. Espinosa, Universite de Nancy, LCM3B, URA CNRS 809, BP 239, BP 239, 54506 Vandoeouvre-les-Nancy, France
Ammonium dihydrogen phosphate undergoes a phase tansition from a high-temperature paraelectric to a low-temperature antiferroelectric phase at 148 K. In order to do experimental electron density studies, we have measured three diffraction data sets at 10 K above the transition temperature:
(1) neutron data at LLB Saclay,
(2) conventionl Mo K-( X-ray data in Nancy, and
(3) short-wavelength synchrotron X-ray data at ESRF in Grenoble.
The neutron data, which were measured in order to accurately determine the positions and mean-square displacements of the hydrogen atoms, suffer from anisotropic extinction. The synchrotron X-ray measurements at a wavelength (= 0.2 Å yielded data to the very high resolution sin(theta)/( = 1.7 reciprocal Å. Results of refinements that fitted positional and thermal parameters to the neutron data and to the high-order synchrotron data, and multipolar electron density parameters to the conventional X-ray data, will be presented.