The P-T dependence of the crystal structure of KDP and DKDP above Tc

Neutron-diffraction data in the h0l plane have been collected from KH 2 PO 4 at (8.3 kbar, 89K) and from K(H 0.05 D 0.95 ) 2 PO 4 at (16.5 kbar, 183K) and (16.5 kbar, 218K) to determine the high-P/low-T crystal structure in three dimensions for the first time. These and earlier results together yield a good description of the P-T dependence of the structure above T c , without the approximations of previous studies. A correspondingly improved estimate of structural dimensions at (P c , 0K) is then obtained for KH 2 PO 4 , where T c to 0K at P c .