Studies on the thermal treatment of amorphous zirconia (gel, precipitate, etc.) have shown that its conversion to crystalline forms usually starts around 400±500 8C [1±4], and that the precise temperature of conversion may vary with the composition (e.g. the type and amount of cationic dopants, hydroxyl content). It has been reported by various workers [2± 4] that the product of this initial crystallization is a metastable tetragonal (t) phase, not the thermodynamically stable monoclinic (m) phase. Further, it has been shown that this metastable t-phase persists, even at higher temperatures within the stability range of the m-phase before ®nally converting to the latter. Thus, for an undoped zirconia gel, crystallization mainly to the t-phase was observed at 400 8C, and t! m conversion at 1000 8C [4]. Generally speaking, this follows Ostwald's step rule [5]. In addition, the apparent stabilization of t-ZrO2 at low temperatures has been explained to be a function of structural similarities of an amorphous phase with the t-phase [3], crystallite size [6], effect of strain [5] etc., apart from the presence of ionic impurities (as mentioned previously). Naskar and Ganguli [7] recently studied the crystallization behavior of rare earth (RE) doped zirconia gel ®bers that were obtained by spinning doped zirconium acetate sols. They showed (Fig. 1) that the metastable t-phase obtained from ZrO2RE2O3 gel ®bers yields partially or fully to the mphase at higher temperatures as a function of the size of the cationic dopant (2 mol % rare earth oxide in each case). Fig. 1 indicates that although all the dopant cations yielding t-ZrO2 were larger in size than Zr4 (La . Pr . Nd . Sm . Gd . Dy . Zr), the larger cations among them yielded the phase with progressively shorter thermal ` stability'' ranges. On
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