Preparation of multiple-cation α-SiAlON ceramics containing lanthanum

Until recently, it was accepted that Ce³⁺ cations, with an ionic radius (r) of 1.03 Å, 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 Å. 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 β-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.