The crystal structure of tetragonal KH2PO4and KD2PO4as a function of temperature and pressure

The structures of the paraelectric (tetragonal) phase of KH 2 PO 4 (KDP) and 95% deuterated KD 2 PO 4 (DKDP) have been investigated as a function of pressure and temperature using high-resolution (sin theta max / lambda >1 AA -1 ) single-crystal neutron-diffraction techniques. Data have been collected for both KDP and DKDP at approximately 16 kbar at room temperature, and at approximately 12 kbar a few degrees above the pressure-reduced transition temperature, T c . Refinements yield good determinations of the P-T dependence of the disordered hydrogen (deuteron) site separation, delta , and of the O...O bondlength, 2R. Constrained refinements and statistical tests show that the pressure derivatives of these parameters are significantly temperature dependent. For KDP this information, together with the results of a separate study of the temperature dependence of the structure, has been used to extrapolate the values of delta , 2R and the along-bond thermal amplitude, mu 22 (H), to the P-T conditions at which the ordered phase disappears: T c to 0K at 17 kbar. The structural results are also used to examine critically current double-minimum potential-well models for the proton dynamics of the tetragonal phase.