Structural phase transition and hydrogen ordering of TlH₂PO₄at low temperature

The crystal structure of TlH2PO4 (TDP) has been studied at low temperature. The lattice parameters were derived from high-resolution X-ray powder diffraction in the temperature range between 8 and 300 K. A detailed crystal structure analysis of the antiferroelectric low-temperature phase TDP-III has been performed based on neutron diffraction data measured at 210 K on a twinned crystal consisting of two domain states. The structure model in the triclinic space group P\bar 1 is characterized by a complete ordering of all the H atoms in the asymmetric O—H...O hydrogen bonds. The phase transition from the ferroelastic TDP-II to the antiferroelectric TDP-III phase at 229.5 ± 0.5 K is only slightly of first order and shows no detectable hysteresis effects. Its mechanism is driven by the hydrogen ordering between the partially ordered TDP-II state and the completely ordered TDP-III state. The polymorphism of TDP and the fully deuterated TlD2PO4 (DTDP) is presented in the form of group–subgroup relations between the different space groups.