Neutron Single Crystal Diffraction on KDP at 20K and 1.7GPa.

In-situ neutron single crystal diffraction experiments on KH2PO4 (KDP) are reported. They were performed in a clamped pressure cell developed for the use in standard He-flow cryostats. The details of the structure of KDP in the paraelectric phase close to the ferroelectric phase transitionTc at a pressure of 1. 7 GPa and 20K were established. The resulting H. . . H separation δ in the disordered H-bond is 0. 26 (2) Å with a corresponding O. . . O distance of 2R=2. 446 (3) Å. These numbers deviate towards larger values from the established linear relationship of δ and 2R versus Tc.