The single-crystal structure of TlTiOPO4 (TTP) is studied by X-ray dif
fraction at 11 K. The data obtained are compared with the literature d
ata on thallium-containing compounds. The results allowed us to clarif
y the dynamics of the Tl-cation behavior in the TTP structure and to g
ain a better understanding of the mechanism of the ferroelectric phase
transition observed in crystals of the KTiOPO4 (KTP) family.