TlH₂PO₄ (TDP) belongs to the family of hydrogen-bonded (anti)ferroelectric materials of KH₂PO₄ (KDP) type. This compound shows two phase transitions at T₁ = 357K and T₂ = 230K. The transition at 357K is known to be ferroelastic. In the ferroelastic phase of TlH₂PO₄ there exists a partial proton ordering in hydrogen bonds before the PO₄ groups : one of three hydrogen bonds is asymmetric (ordered), whereas the other symmetric hydrogen bonds are disordered. Single crystals of TDP were investigated at room temperature, 240K and 210K by neutron diffraction. At room temperature a detailed structure analysis shows the partial proton ordering in space group P 1 2₁/a 1 . At 240K, slightly above the structural phase transition the asymmetry of the one ordered hydrogen bond is pronounced. At this temperature the lattice parameters are a = 14.26(2) Å, b = 4.517(9) Å, c = 6.497(9) Å and  β = 92.18(13)°. In order to solve the crystal structure of low-temperature phase measurements at 210K were carried out. From this structure analysis a complete proton ordering in all hydrogen bonds was obtained. This means that all hydrogen bonds are asymmetrical. However, due to crystal twinning it was impossible to refine anisotropic displacement parameters. Following lattice parameters are obtained : a = 28.483(12) Å, b = 9.016(5) Å, c = 6.502(3) Å, α = 90.09(5)°,  β = 92.21(3)° and γ = 90.49(3)°.To identify the character of the phase transition the (802)-reflection was investigated with ω – Scan technique on cooling and heating. Below the phase transition temperature a broadening and finally splitting of the reflection was observed. This effect in related to the twinning of the crystal according to the symmetry reduction (P 1 2₁/a 1→P -1 ) at the phase transition.

In addition a huge increase of the integral intensity of the reflection occurs at the phase transition temperature. This effect can be explained by the sudden reduction of extinction related to the appearance of the twin domains at the phase transition of first order type.

Both observations, the broadening / splitting and the intensity increase of the reflection, are completely reversible without any significant hysteresis. The excellent quality of the sample crystal grown from aqueous solution is maintained in the room temperature phase. Also the rapid change of the reflex was observed, which indicates that this phase transition is first order in character. But no hysteresis was observed.

According to the above mentioned fact it could be conclude that the crystal system of TDP at low-temperature seems to be triclinic. But this crystal shows the monoclinic system at the room-temperature. The phase transition between the room- temperature phase and the low-temperature phase is first order.

Fig 1. Crystal structure of TDP at 240K (room-temperature phase)

Fig.2 Crystal structure of TDP at 210K (low-temperature phase)

Fig.3 Temperature dependent change of the FWHM of  reflection (802)

(red : heating, blue : cooling)

Fig.4 Temperature dependent change of the integral intensity of reflection (802)

(red : heating, blue : cooling)

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