Direct determination of the intramolecular O–D distance in ice Ih and Ic by neutron diffraction

Abstract

The mean apparent intramolecular O–D distance in ice Ih, as determined by neutron diffraction using standard methods, is well established1–5 as 1.01 or 1.02 ±∼0.01 A. It is enormously greater than the O–D distance in the vapour, which is6 0.9732 A, and Whalley7 has argued that it cannot be the actual distance, and that the actual distance must be about 0.98 A. The difference between these distances is caused by the orientational disorder of the water molecules5. Kuhs and Lehmann have combined accurate neutron single-crystal diffraction with thermal amplitudes obtained from vibrational spectroscopy, or by assuming that the thermal atomic displacements are purely gaussian, and have concluded that the actual O–D distance in ice Ih is 0.983±0.005 and 0.979±0.005 A, respectively, from the two techniques5. This work is certainly a tour de force, but the method is not entirely direct. This letter describes a new method of determining actual interatomic distances in disordered crystals directly from diffraction measurements alone and its application to ice Ih and Ic.