UNIT-CELL PARAMETERS AND OXYGEN DISPLACEMENT OF METASTABLE HFO2-YO1.5 SOLID-SOLUTIONS

HfO2-X mol% YO1.5, solid solutions (X=0-22) were prepared by are 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, Unit-cell parameters of monoclinic and tetragonal phases of HfO2-X mol% 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 less than or equal to X less than or equal to 7), the lattice parameter a increased, while the b and the beta angle decreased with an increase of YO1.5 content, In the tetragonal phase, the lattice parameter a increased (6 less than or equal to X<13), while the c and the axial ratio c/a(f) decreased (7 less than or equal to X<13) with increasing of YO1.5 content, and then a(f) and c axes values coincided each other at X=13, where the suffix f denotes the distorted psendo-fluorite cell, In the tetragonal hafnia (10 less than or equal to X less than or equal to 16), the oxygen displacement from the ideal fluorite site decreased with increasing of YO1.5 content, The HfO2-13 mol% YO1.5 sample exhibited an axial ratio of unity within the experimental resolution, but showed 112(f) reflection peak, strongly suggesting the existence of the t ''-form, which is defined as a tetragonal phase with a c/a(f) of unity, The t ''-form is interpreted to appear in the compositional region of 13 to 16 mol% YO1.5.