Formation of metastable forms by quenching of the HfO2-RO(1.5) melts (R=Gd,Y and Yb)

The existing phases in are-melted HfO2-RO(1.5) samples (R = Gd, Y and Yb) were investigated by Raman spectroscopy and X-ray diffraction. The compositional regions of the monoclinic phase, metastable gamma(2)- and gamma(1)-forms, were 0-6, 4-9 and 8-10 mol.% YO1.5 respectively, in the HfO2-YO1.5 system. A metastable tetragonal (t') form is obtained at or below 12 mol.% RO(1.5) compositions. The lattice parameters a and c of the t'-HfO2-RO(1.5) solid solution increases and decreases with increasing RO(1.5) content, and coincided with each other at 14 mol.% RO(1.5) regardless of dopant species R. The lattice parameter of tetragonal HfO2-x mol.% RO(1.5) with a fixed dopant concentration x increases with an increase of the ionic radius of dopant R(3+) ion. The Raman band intensity at 450-500 cm(-1), which is characteristic of the tetragonal phase, decreases with an increase of RO(1.5) content and seems to disappear at an RO(1.5) content of approximately 18-20 mol.%. It can be interpreted that the t ''-form, which is a tetragonal phase with an axial ratio of unity, appears in the compositional region of 14-18 mol.% RO(1.5).