Specific heat properties of KH2PO4

Calorimetric studies of KDP crystals are reviewed, and two categories of studies are involved: (1) Specific heat measurements near the transition temperature, and (2) specific heat measurements at low temperatures (0.1-30 K). Measurements near T c confirm the first-order transition in KDP and correlate very well with dielectric data via the predictions of the free-energy expansion. Similar measurements on KDP-type isomorphous crystals confirm the order-disorder nature of the transition. At low temperatures KDP displays a maximum in c/T 3 at 17 K associated with proton motions which may be the residual of the soft mode. An anomalous T 3/2 contribution to the specific heat appears below 5 K and depends on the size and purity of the crystal. This term is missing if the hydrogen bonds are disordered [i.e., as in the frustrated system Rb 0.52 (ND 4 ) 0.48 D 2 PO 4 ] and a glasslike linear term appears instead. An average calorimetric Debye temperature of 362 K for KDP is recommended. Suggestions are made for future calorimetric studies on KDP-type crystals.