CRYSTAL STRUCTURAL STUDIES OF THERMAL-DECOMPOSITION PROCESS OF PBMG(1/3)NB(2/3)O-3 SINGLE-CRYSTAL INTO PYROCHLORE TYPE COMPOUND

Crystal structure parameters including B thermal parameters of each constituent atom were determined for PbMg1/3Nb2/3O3 (PMN) between 27 and 700 degrees C by the single crystal high temperature X-ray diffraction technique. For lead, and magnesium and niobium atoms, the optimum shift directions from the special positions were not a single direct-ion but ''spherical'' shifts were observed. For oxygen atom, it was confirmed 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 polycrystalline pyrochlore type compound at 600 degrees C. The mechanism of decomposition was discussed from the view point of crystal structure.