Isotope effect in the phase transition of partially d euterated T1H2As 04

The complex dielectric constant of Tl(H_1-X D_X)₂AsO₄ powder pellets has been measured from 100 to 450 K. It is found that the I11 ← 11 phase transition point (T_L) shifts from 251 K(for hydrogenated crystal) to about 340 K(for partially deuterated crystal), while the I1 ← I phase transition point(T_U) goes from about 390 K to 433 K. The order of the I11 ← I1 phase transition changes from a weakly first-order to a second- order, while that of the I1 ← I phase transition from a second-order to a first-order. From the experimental results, the tentative phase diagrams of Tl(H_1-X D_X)₂AsO₄, and Tl(H_1-X D_X)₂PO₄ are proposed.