Composition dependence of the transition temperature in mixed ferroelectric-ferroelectric systems

The composition dependence of the transition temperature T-C(x) for mixed c rystals of triglycine sulfate (TGS) and deuterated triglycine sulfate (DTGS ) has been investigated experimentally and has been analyzed theoretically. A similar investigation has been carried out for triglycine selenate (TGSe ) and deuterated triglycine selenate (DTGSe). The experimental results agre e well with previous data by Brezina (TGS/DTGS) and Gesi (TGSe/DTGSe). Clas sically we would expect a linear dependence of temperature T-C(x)=T-C(0) +A x with composition x. However T-C(x) is clearly below the straight line, es pecially at x similar to 0.5. We show that an effective field approach with quantum effects can justify well this lowering of T-C(x). The experimental data are also compared with classical and quantum Monte Carlo simulations for mixed Ising systems. It is shown that there is a good qualitative agree ment with the observed behavior in mixed crystals of the TGS family.