TlH₂PO₄ (TDP) and RbH₂PO₄ (RDP) belong to the family of hydrogen-bonded (anti)ferroelectric materials of KH₂PO₄ (KDP) type. Although Rb⁺ and Tl⁺ have almost same ionic radii (1.47 Å), the crystal structures of TDP and RDP are different.

To investigate the stability of these different phases, mixed crystals were synthesized and investigated by neutron diffraction on single crystal. TDP undergoes an antiferroelectric phase transition at 230K and it is monoclinic at room temperature in contrast to RDP, which has a tetragonal symmetry. RDP undergoes a ferroelectric phase transition at 147K. Both low temperature phases (RDP : ferroelectric and TDP : antiferroelectric) are characterized by complete proton ordering in the hydrogen bonds before the PO4 groups.

We have been able to show that (Rb_0.5Tl_0.5)H₂PO₄ crystallizes in the TDP structure type(space group  P 1 2₁/a 1  ) with lattice parameters a  = 14.36(2) Å, b = 4.524(3) Å, c = 6.50(1) Å, β = 91.8(1)°. As for TDP a partial proton ordering with one asymmetric and two symmetric hydrogen bonds In order to calculate the exact content of Rb and Tl following relationships were used.

x = (b - b_Rb) /(b_Tl - b_Rb)
　

 x :Tl – content and  b  : the refined scattering length of the sample from the diffraction data.

From above relationships we obtained Rb_0.46Tl₀₅₄H₂PO₄.

For the (Rb_1-xTl_x)H₂PO₄ the following principal results are obtained :

1.     In the range of 0.2 < x < 1, single phase crystals of the TDP structure type can be synthesized from aqueous solution.

2.     The dominant stability of the TDP structure type is probably caused by the lone pair electrons of Tl⁺.

3.     Rb_0.46Tl₀₅₄H₂PO₄ shows no structural phase transition at low temperature.

Fig. 1 Crystal structure of the mixed Rb-Tl crystal. The H3 proton belongs to the asymmetric hydrogen bond.

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