Acta Crystallographica Section B

Structural Science

Volume 62, Part 5 (October 2006)

research papers

Acta Cryst. (2006). B62, 719-728 [ doi:10.1107/S0108768106018222 ]

Structural phase transition and hydrogen ordering of TlH₂PO₄ at low temperature

I. H. Oh, M. Merz, S. Mattauch and G. Heger

Abstract: 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-HO 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.

Keywords: phase transition; hydrogen ordering; low temperature; X-ray powder diffraction.