CRYSTAL STRUCTURAL STUDIES OF THERMAL-DECOMPOSITION PROCESS OF PBMG(13)NB(2/3)O-3 SINGLE-CRYSTAL INTO PYROCHLORE TYPE COMPOUND/

Crystal structure parameters including B thermal parameters of each co nstituent atom were determined for PbMg1/3Nb2/3O3 (PMN) between 27 and 700 degrees C by the single crystal high temperature X-ray diffractio n technique. For lead, and magnesium and niobium atoms, the optimum sh ift directions from the special positions were not a single direct-ion but ''spherical'' shifts were observed. For oxygen atom, it was confi rmed that the optimum shift direction was not one ring on XY plane but two parallel rings away from XY plane. PMN began to decompose into po lycrystalline pyrochlore type compound at 600 degrees C. The mechanism of decomposition was discussed from the view point of crystal structu re.