Measurements of the dielectric constants, piezoelectric constants, and elastic constants have been made for the crystal N${\mathrm{D}}_4$${\mathrm{D}}_2$P${\mathrm{O}}_4$ (heavy water ADP). This crystallizes in the same form as normal ammonium dihydrogen phosphate (ADP) but has a transition at -31°C which is 94° higher than that for normal ADP. From the form of the crystal below the transition temperature, it is inferred that the transition is an antiferroelectric one with one of the "$a$" crystallographic axes as the antiferroelectric axis. The crystal has a zero temperature coefficient of frequency at 0°C, and it may be of use in transducers and mechanical filters.

• Received 23 July 1952

DOI:https://doi.org/10.1103/PhysRev.88.477