X-RAY-DIFFRACTION STUDY OF PHASE-TRANSITIONS IN SR0.75BA0.25NB2O6 BETWEEN 20 K AND 400 K

X-ray powder diffraction patterns of the ferroelectric mixed crystal S r0.75Ba0.25Nb2O6 are recorded in Guinier geometry in a temperature ran ge between 20 K and 400 K by means of the film-lift technique, The lat tice parameters were refined for a single tetragonal structure in the whole temperature range ignoring the incommensurate modulation of the structure. The lattice parameter c exhibit strong anomalous temperatur e dependence at 200 K<T<350 K, indicating the diffuse ferroelectric ph ase transition (DPT). The proceeding of the DPT is also detectable by an increase of the diffuse scattering around the (002)-reflection. The temperature dependence of the unit cell volume indicates further PTs below room temperature in this relaxer material.