Isotope effect in TIH2PO4 and TID2PO4

The crystal structure of the antiferroelectric phase of TlD2PO4, deuterated thallium dihydrogenphosphate, has been determined from single-crystal neut ron diffraction data collected at room temperature. The para-antiferroelect ric transition (T-c(d) - 353 K) of TlD2PO4 is analysed from a structural po int of view and compared with the phase transition of TlH2PO4 at T-I = 357 K, already characterized. The distinct phase sequences observed in the two compounds when decreasing temperature from that of the high-temperature pro totype phase (prototype phase/room-temperature phase/low-temperature phase) are discussed and associated with the different ordering of the two crysta llographically inequivalent H (D) atoms existing in the prototype phase.