X-RAY STUDY OF THE SYSTEM BP1-XBPIX

Accurate X-ray diffraction measurements are carried out as function of temperature in antiferroelectric betaine phosphate (BP), ferroelectri c betaine phosphite (BPI), and mixed compounds of BP1-xBPIx. In agreem ent with previous studies, the phases which derive from a high-tempera ture phase with P2(1)/c symmetry are clearly detected. However, in con trast with previous assignments, for 0.90 < x less than or equal to 1. 0 a transition occurs to the P2(1) ferroelectric phase. For 0 less tha n or equal to x < 0.20, the two successive transitions are to a P2(1) phase and subsequently to a Pc phase with a simultaneous doubling of t he unit cell. For 0.9 less than or equal to x < 1.0 the results confir m previous assignments of a single transition to a P2(1) phase. In the intermediate region 0.20 < x < 0.90 a clear evidence of an isomorphou s transition to a glass phase is obtained, where the relative dominanc e of the ferroelectric or antiferroelectric interaction depends on the concentration values. Furthermore an orientational glass phase near x = 0.80 is identified. The existence of a strong anisotropy in all the studied elements of the family and of a weak coupling between the ord ering of the dipoles and the lattice is evidenced. The role of the abs ent oxygen in the behaviour of the lattice parameters as well as its s ubsequent influence on the competitive interactions between dipoles ar e discussed.