Abstract
An approximate structure of the antiferroelectric phase of copper formate tetrahydrate is reported based on neutron diffraction data. The shift from the paraelectric phase is due to a doubling of the c axis and a change in space group from P21/a to P21/n or P21 P21/n is shown to be a good approximation and is used. Most of the positional parameter change through the transition is due to ordering of the water layer hydrogen atoms and some shift in oxygen atom position. Hydrogen atom occupancies in the paraelectric phase have been re-refined and some new conjectures of the structures underlying the disorder made.