Structural study of the low-temperature phase of TlH2PO4 at 180 K

The low-temperature phase of TlH2PO4 has been studied by X-ray diffraction. A structural model is proposed with space group P (1) over bar. This model is compared with the structure of the antiferroelectric phase of TlD2PO4 at room temperature to analyze the expected isomorphism at low temperature. Given the structural distortion of TlH2PO4, such isomorphism present in the common high-temperature phase is not recovered in this phase. Through the analysis of the displacements of the PO4 groups there is some evidence that the ordering of the H atoms responsible for the appearance of antiferroelectricity seems to be incomplete. An exhaustive study of the detected ferroelastic domains is also performed.