Crystal structure of rare-earth silicon-oxynitrides RE4Si2O7N2

Difference between the crystal structure of La-J-phase and Lu-J-phase and some problems of estimating the lattice constants of RE-J-phase from the X-ray diffraction data is discussed. The crystal structure of RE-J-phase is classified two groups, one of them is La2Si2O7N2 type from La to Sm and the other Lu4Si2O7N2 type Gd to Lu and Eu exists between them. In each group, the lattice constants and unit cell volume increases with increasing rare-earth ion radius. By advanced measurement techniques such as neutron diffraction and Rietveld analysis, it was found at this century that the crystal structure of RE-J-phase was different from the ordinary one. Moreover, phase transition, effects of RE-J-phase on high temperature mechanical strength of Si3N4 sintering and the mechanism, order-disorder arrangement of oxygen and nitrogen of oxynitrides and Si-O/N bond come into question. The crystal structure and lattice constants of RE-J-phase were discussed. (S.Y.)