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Preparation of Multiple-Cation alpha-SiAlON Ceramics Containing Lanthanum

Until recently, it was accepted that Ce3+ cations, with an ionic radius (r) of 1.03 A, were too large to form an α-SiAlON structure. However, more-recent studies have shown that cerium cations can be incorporated into α-SiAlON via quenching at a rate of 600°C/min, after sintering at 1800°C. Thus far, no α-SiAlON formation has been observed for La3+ cations with r= 1.06 A. In the present work, the possibility of having the La3+ species as a dopant cation in α-SiAlON has been investigated by using La2O3 alone or in equimolar mixtures with CaO or Yb2O3. The resulting materials have been heat-treated at a temperature of 1450°C for up to 720 h to devitrify the grain-boundary glass into crystalline phases and also to observe the α→β SiAlON transformation. X-ray diffractometry on samples that were densified with single cations revealed that the La3+ cation alone does not form an α-SiAlON; rather, it forms the N-phase (La3Si8O4N11) with a s-SiAlON phase. In the case of multiple cations, α-SiAlON was observed only as a matrix phase. Energy-dispersive X-ray measurements have proven that La3+ cations can be accommodated into the α-SiAlON structure and this structure also does not transform to β-SiAlON at lower temperatures.