Abstract
HfO2-Xmol% YO1.5 solid solutions (X=0-22) were prepared by arc melting to investigate the compositional dependence of the unit-cell parameters of diffusionlessly formed metastable phases through X-ray powder diffraction measurements at room temperature. Unitcell parameters of monoclinic and tetragonal phases of HfO2-Xmol% YO1.5 (X=0, 2, 4, 5, 6, 7, 8 and 9) were refined by the Rietveld analysis. The lattice parameter and oxygen displacement were estimated for samples of X=10 to 22 by the profile-fitting technique. In the monoclinic phase (0≤X≤7), the lattice parameter a increased, while the b and the β angle decreased with an increase of YO1.5 content. In the tetragonal phase, the lattice parameter a increased (6≤X≤13), while the c and the axial ratio c/af decreased (7≤X≤13) with increasing of YO1.5 content, and then af and c axes values coincided each other at X=13, where the suffix f denotes the distorted pseudo-fluorite cell. In the tetragonal hafnia (10≤X≤16), the oxygen displacement from the ideal fluorite site decreased with increasing of YO1.5 content. The HfO2-13mol% YO1.5 sample exhibited an axial ratio of unity within the experimental resolution, but showed 112f reflection peak, strongly suggesting the existence of the t″-form, which is defined as a tetragonal phase with a c/af of unity. The t″-form is interpreted to appear in the compositional region of 13 to 16mol% YO1.5.
