STRUCTURAL PHASE TRANSITIONS IN Rb₂COCl₄ SINGLE CRYSTAL. Novikova Maria Sergeevna, Tamazyan Rafael Arshamovich, Institute of Crystallography RAS, Leninskiy prospekt, 59 Moscow Russia

Temperature dependence of unit cell parameters, q modulation vector and intensities of satellite reflections in the temperature interval 120-305 K were measured and phase transitions from high-temperature phase to incommensurate and from incommensurate to ferroelectric phase established. X-ray diffraction experiments were carried out at the temperature of 348,294,202, 177 K. The structure of the high-temperature phase was refined at T=348 K (R=0,046;wR=0,034) and T= 294 K (R=0,025;wR=0,017) correspondingly. Atomic structure of the following incommensurately modulated phase was refined under T=203 K using 4-dimentional space group P(Pcmn/ss(-1)) ( R-factors R=0,04;wR=0,046 over main reflections and R=0,079;wR=0,096 over satellite reflections). Temperature dependence of the q vector shows a 5 K histeresice under ferroelectric phase transition. The ferroelectric phase was refined at T=177 K in space group P(Pc21n/s(-1)(-1)) (R-factors: over main reflections wR=0,088, wR=0,108 over satellite reflections). The results were compared with those of our previous structural studies of Rb₂ZnBr₄. Reasons for different behavior of the q vectors in these two structures were analyzed.