Perturbed-angular-correlation study of phase transformations in nanoscaled Al₂O₃-coated and noncoated ZrO₂ particles synthesized in a microwave plasma

The phase transformations of nanoscaled (n-) Al$_{2}$O$_{3}$-coated and noncoated ZrO$_{2}$ particles synthesized in a microwave plasma have been investigated by perturbed-angular-correlation (PAC) measurements of the electric quadrupole interaction (QI) of $^{181}$Ta on Zr sites between 290 and 1600 K. For the phase identification and structural characterization the QI parameters of the nanoscaled particles are compared to those of coarsegrained ZrO$_{2}$ which were measured between 290 and 2160 K. The PAC spectra of the nanoscaled particles in the as-prepared state are characterized by a broad distribution of strong, axially asymmetric QI’s, which reflects a highly disordered oxygen environment 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 noncoated n-ZrO2, in contrast to the previously reported annealing reaction of n-ZrO$_{2}$ synthesized by gas-phase condensation in which the tetragonal phase has not been observed. This disorder-order transformation is partially reversible upon cooling. In n-ZrO$_{2}$ /Al$_{2}$O$_{3}$ synthesized in a microwave plasma the monoclinic phase can be completely suppressed up to 1600 K. ... mehrIn the noncoated particles monoclinic ZrO$_{2}$ starts to appear at 600 K. At 1400 K the transformation 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)x10$^{-5}$ s$^{-1}$. In nanoscaled coated and noncoated tetragonal ZrO$_{2}$ the T$^{3/2}$-temperature dependence of the quadrupole frequency v$_{q}$ is weaker than in the same phase of the coarse-grained compound, suggesting a decrease of the mean-square vibrational amplitudes with decreasing particle size.