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), ferroelectric betaine phosphite (BPI), and mixed compounds of BP1−xBPIx. In agreement with previous studies, the phases which derive from a high‐temperature phase with P21/c symmetry are clearly detected. However, in contrast with previous assignments, for 0.90 < x ≦ 1.0 a transition occurs to the P21 ferroelectric phase. For 0 ≦ x < 0.20, the two successive transitions are to a P21, phase and subsequently to a Pc phase with a simultaneous doubling of the unit cell. For 0.9 ≦ x < 1.0 the results confirm previous assignments of a single transition to a P21 phase. In the intermediate region 0.20 < x < 0.90 a clear evidence of an isomorphous transition to a glass phase is obtained, where the relative dominance 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 ordering of the dipoles and the lattice is evidenced. The role of the absent oxygen in the behaviour of the lattice parameters as well as its subsequent influence on the competitive interactions between dipoles are discussed.