STRUCTURE REFINEMENTS OF BETA-SI3N4 AT TEMPERATURES UP TO 1360-DEGREES-C BY X-RAY-POWDER INVESTIGATION

The structure of beta-Si3N4 was reinvestigated with X-ray powder diffr action and Rietveld refinement to determine if the space group should be centrosymmetric, P6(3)/M, or acentric, P6(3). Displacement for one sort of the nitrogen atoms N(1) away. from the high-symmetry positions are found to be even larger than reported before and this behaviour i s retained to higher temperatures. R-factor ratio tests favour P6(3) a t a 95% significance level. No exceptional axial thermal expansion was observed. Above 900-degrees-C a significant growth of a disordered hi gh-cristobalite phase was observed. It is shown that structure refinem ents with the X-ray powder technique can be done even at high temperat ure with high confidence. A newly developed X-ray mirror furnace reach ing temperatures up to 1360-degrees-C is described.