Perturbed-angular-correlation study of phase transformations in nanoscaledAl2O3-coated and noncoated ZrO2 particles synthesized in a microwave plasma

The phase transformations of nanoscaled (n-) Al2O3-coated and noncoated ZrO 2 particles synthesized in a microwave plasma have been investigated by per turbed-angular-correlation (PAC) measurements of the electric quadrupole in teraction (QI) of Ta-181 on Zr sites between 290 and 1600 K. For the phase identification and structural characterization the QI parameters of the nan oscaled particles are compared to those of coarse-grained ZrO2 which were m easured between 290 and 2160 K. The PAC spectra of the nanoscaled particles in the as-prepared state are characterized by a broad distribution of stro ng, axially asymmetric QI's, which reflects a highly disordered oxygen envi ronment of the Zr sites. Upon annealing, the tetragonal phase is the first well-crystallized structure to emerge at about 500 K, both in coated and no ncoated n-ZrO2, in contrast to the previously reported annealing reaction o f n-ZrO2 synthesized by gas-phase condensation in which the tetragonal phas e has not been observed. This disorder-order transformation is partially re versible upon cooling. In n-ZrO2/Al2O3 synthesized in a microwave plasma th e monoclinic phase can be completely suppressed up to 1600 K. In the noncoa ted particles monoclinic ZrO2 starts to appear at 600 K. At 1400 K the tran sformation from the monoclinic to the tetragonal phase of noncoated n-ZrO2 was observed to occur with a transformation rate of lambda = 6.9(1.2) x 10( -5) s(-1). In nanoscaled coated and noncoated tetragonal ZrO2 the T-3/2-tem perature dependence of the quadrupole frequency v(q) is weaker than in the same phase of the coarse-grained compound, suggesting a decrease of the mea n-square Vibrational amplitudes with decreasing particle size.