A high-resolution neutron-diffraction study of the effects of deuteration on the crystal structure of KH2PO4

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Abstract
Previously collected high-resolution neutron-diffraction data from KH2PO4 (KDP) at room temperature have been re-analysed, including careful correction for thermal diffuse scattering, absorption and extinction and similar, new data have been obtained from 95%-deuterated KH2PO4 (DKDP). This has made possible, for the first time, a detailed study of the effects of deuteration alone on the KDP crystal structure. The main structural change on deuteration is a lengthening of the H(D) bonds, in response to which the PO4 groups rotate in such a way that the oxygen polyhedron around each K atom remains unaltered in size and shape. The (small) changes in the thermal motion of the K, P and O atoms can be attributed to an expansion of the unit cell on deuteration, and to differences between the ferroelectric-mode amplitudes of KDP and DKDP. A comparison of the H and D thermal motion (i) reveals that it is inadequate to treat each minimum of the H(D) double-well potential as 'riding' on the nearer O atom alone, and (ii) underlines the large and important effects on the H(D) potential of thermal fluctuations in the H(D)-bond length.