Isotope effect and phase diagram in Tl(H1-xDx)2PO4

TlH2PO4 crystal undergoes a ferroelastic phase transition at 357 K (T-cl) and a low-temperature transition at 230K (T-c2). However, light scattering, x-ray diffraction and neutron scattering suggested that TlD2PO4 undergoes a different kind of phase transition sequence compared to that of TlH2PO4. The dielectric constant of deuterated crystal of Tl(H1-xDx)(2)PO4 shows an upward shift of T-c2, while T-c1 is not significantly affected by deuteration. The high-temperature transition seems to be impeded by thermal decomposition at the surface.