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 Angstrom, were too large to form an alpha-SiAlON structure.
However, more-recent studies have shown that cerium cations can be
incorporated into alpha-SiAlON via quenching at a rate of 600 degrees
C/min, after sintering at 1800 degrees C. Thus far, no alpha-SiAlON
formation has been observed for La3+ cations with r = 1.06 Angstrom. In
the present work, the possibility of having the La3+ species as a
dopant cation in alpha-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 degrees C for
up to 720 h to devitrify the grain-boundary glass into crystalline
phases and also to observe the alpha --> beta SiAlON transformation.
X-ray diffractometry on samples that were densified with single cations
revealed that the La3+ cation alone does not form an alpha-SiAlON;
rather, it forms the N-phase (La3Si8O4N11) with a beta-SiAlON phase. In
the case of multiple cations, alpha-SiAlON was observed only as a
matrix phase. Energy-dispersive X-ray measurements have proven that
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La3+ cations can be accommodated into the alpha-SiAlON structure and
this structure also does not transform to beta-SiAlON at lower
temperatures.