Isotope Effect of Monoclinic Rb(D1-xHx)2PO4 on the Phase Transitions

Dielectric constant along the b axis of crystals in monoclinic Rb(D 1- x H x ) 2 PO 4 system was measured in a temperature range from -190°C to 160°C. Phase diagram of the system obtained in a range between x =0 and 0.8 shows a relatively large isotope effect on the III-II phase transition with a shift of its transition temperature to higher temperature for the decrease of x , and a small isotope effect on the II-I phase transition but with a shift of its transition temperature to lower temperature for the decrease of x .