Crystal structure of telluric acid ammonium phosphate (TAAP) in the paraelectric phase

Crystal structure of monoclinic telluric acid ammonium phosphate (TAAP) has been studied by an X-ray diffraction method in the paraelectric phase at 333 K and in the ferroelectric phase at room temperature. It is confirmed that the space group in the paraelectric phase is P2/n. Comparison of the results between the two temperatures suggests that the plane which is made by the PO4 tetrahedron linked by hydrogen bonds has an important role for the ferroelectric phase transition. This plane is almost parallel to the direction of spontaneous polarization.