POLYMORPHISM OF TlH₂PO₄ AND TlD₂PO₄. G. Heger¹, S. Rios², W. Paulus^1,2, A. Cousson², M. Quilichini², M. Becker¹, J. Glinnemann¹. ¹RWTH Aachen, Institut für Kristallographie, D-52056 Aachen, Germany; ²Laboratoire Léon Brillouin, CE Saclay, F-91191 Gif-sur-Yvette, France

The crystal structures of TlH₂PO₄ (TDP) and TlD₂PO₄ (DTDP) are principally characterized by hydrogen bonded PO₄ groups which form a double-layer arrangement instead of the 3 dim. network known for the related KH₂PO₄ (KDP) phases. We have reexamined the crystal structures and phase transitions of TDP and DTDP as a function of temperature by X-ray and neutron diffraction on powders and single crystals in order to clearify contradictions in the literature[1]. The gradual ordering of the protons/deuterons in the hydrogen bonds leads to two different branches of symmetrically distinct phases. The common orthorhombic high temperature structure of space group type P2/c2₁/a2₁/n with Z = 4 shows a dynamical hydrogen disorder in a double-well potential along all the O-H/D-O bonds. The two monoclinic structures at room temperature with partial hydrogen ordering (TDP-II: P12₁/a1, Z = 4; DTDP-[[gamma]]: P112₁/a, Z = 8) differ mainly due to the orientation of their remaining two-fold axes. Therefore, the almost equal phase transition temperatures for TDP (T_I-II = 350 K) and DTDP (T_{[[beta]]-[[gamma]]} = 357 K) cannot be interpreted in terms of an isotope effect. For TDP-III below 230 K, there is a triclinic low temperature phase (P`1, Z = 8) with complete H ordering. Since all of the polymorphous TDP/DTDP phases are centrosymmetric ferroelectric behaviour is ruled out. For [[gamma]]-DTDP antiferroelectricity was found [2].

[1] S.Rios, W. Paulus, A. Cousson, M. Quilichini, G. Heger, N. Le Calvé and B. Pasquier, J. Phys. I France 5, 763 (1995)

[2] N. Yasuda, S. Fujimoto and T. Asano, Phys. Lett. A76, 174 (1980)