Metastable forms of the tetragonal phase in compositionally homogeneous ZrO2-CaO nanopowers

I.O. FÁBREGAS; D.G. LAMAS; R.O. FUENTES; M.E. FERNÁNDEZ DE RAPP; N.E. WALSÖE DE RECA; A.F. CRAIEVICH

Activity Report of the Brazilian Synchrotron Light Laboratory

Año: 2005 p. 396 - 397

Zirconia-based ceramics have been widely investigated due to their excellent mechanical and electrical properties, which strongly depend on their crystal structures and phase changes.     One of the most notable characteristics of several zirconia-based solid solutions is the existence of three tetragonal forms, all belonging to the P42/nmc space group. The stable tetragonal form is called the t-form, which is restricted to the solubility limit predicted by the equilibrium phase diagram. There is also a t´-form with a wider solubility, but unstable in comparison with the mixture of the t-form and cubic phase. Finally, the t"-form has an axial ratio c/a of unity, but with the oxygen atoms displaced along the c axis from their ideal sites of the cubic phase (8c sites O2- anion in the asymmetric unit of the tetragonal of the Fm3m space group). The metastable forms of the tetragonal phase have been investigated in several systems, including ZrO2-CaO.    In this work, the crystal structure of nanocrystalline ZrO2-CaO solid solutions, synthesized by a nitrate-glycine gel-combustion route, was investigated by means of synchrotron powder X-ray diffraction (XRD). The tetragonal/cubic compositional boundary was determined by monitoring the variation of weak reflections of the tetragonal phase related to the displacement of oxygen atoms from their ideal positions in the cubic phase. The most intense of these "forbidden" reflections is the (112).