Structural studies of KDP and the KDP-type transition by neutron and x-ray diffraction: 1970–1985

Structural work on KH2PO4, KD2PO4, RbH2PO4 and partially-deuterated CsH2AsO4 and RbH2PO4 over the period 1970 to 1985, is fully reviewed. The content is predominantly the Edinburgh-based programme of neutron-diffraction studies, but other X-ray and neutron-diffraction work on these isomorphs is also included. Following a general presentation of the period 1970 to 1980, the most recent results, conclusions and current work are discussed in terms of four main topics: the hydrogen bond, the onset of structural order below Tc , pressure effects, and the soft mode. Finally, some suggestions are made for further structural studies and closely-related theoretical and experimental work.