Phase transition and X-ray radiation damage in betaine phosphate/arsenate mixed crystal

Betaine phosphate (BP) undergoes the antiferroelectric transition at low-te mperature, while betaine arsenate (BA) undergoes the ferroelectric one. The mixed crystal system has been interested whether it is a dipole-glass syst em or not. Recently a peculiar behavior of dielectric susceptibility was re ported in the mixed system. In order to declare the structural transformati on at low-temperature, we have performed X-ray diffraction study in BP(1-x) BA(x) system with x = 0.2 (nominally 0.4) by the use of a four-circle diffr actometer. In the antiferroelectric phase (T < 75 K), the superlattice refl ections appeared, but they were affected by X-ray radiation damage. The int ensity decreased with increasing dose time. After a long time dose, the tra nsition temperature became very low. The damage could be recovered by annea ling the sample at T > 100 K.