NEUTRON POWDER INVESTIGATION OF THE TETRAGONAL TO MONOCLINIC PHASE-TRANSFORMATION IN UNDOPED ZIRCONIA

The tetragonal (t) to monoclinic (m) transformation in pure ZrO2 was investigated by neutron powder diffraction at temperatures between 1900 K and room temperature. The results of a Rietveld analysis are compared with a previous investigation of the m --> t transformation. The t --> m transformation takes place near 1200 K (implying a hysteresis of 300 K) and in a much smaller interval (about 150 K compared with about 600 K in the m --> t case). There are no indications of a two-stage process as found for the m --> t transformation. The structural parameters of the m phase depend only on temperature while those of the t phase differ at the same temperatures for the forward and reverse transformation. The temperature dependence of the lattice constants suggests an orientational relationship a(t) parallel-to a(m)* and c(t) parallel-to b(m). There are no macrostrains whereas the overall microstrain behaviour is similar in both cases, viz. the large microstrains present in both phases are released within the transformation regime. An analysis of temperature factors and diffuse background suggest dynamical disorder in the t phase and static disorder in the m phase.