Single-crystal neutron structural analyses of potassium dihydrogen phosphate and potassium dideuterium phosphate

Single-crystal neutron structural analyses have been performed on both potassium dihydrogen phosphate (KDP) and potassium dideuterium phosphate (DKDP) in order to discuss the isotope effect from structural viewpoints. The values of spontaneous polarization are calculated from the refined structural parameters by the point-charge method, and the calculated values are almost in good agreement with the experimental values of each compound. The temperature dependences of the anisotropic atomic displacement parameters U₃₃'s and positional shifts of potassium, phosphorus, oxygen, and hydrogen/deuterium atoms along the polar c-axis is compared between KDP and DKDP. It is concluded that the paraelectric-ferroelectric transitions are perfectly of the improper order-disorder type in both KDP and DKDP; the ordering of the hydrogen atom induces the spontaneous displacements of both potassium and phosphorus atoms. The interatomic distances and angles in the paraelectric and ferroelectric phases of KDP and DKDP are also investigated. From the result, all the structural differences seem to be caused by the difference in mass between protons and deuterons.