Formation of metastable forms by quenching of the HfO2-RO1.5 melts (R = Gd, Y and Yb)

Article abstract of DOI:10.1016/0022-3697(95)00268-5

The existing phases in arc-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 72-and γ₁-forms, were 0-6, 4-9 and 8-10 mol.% YO_1.5 respectively, in the HfO₂-YO_1.5 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 R³⁺ 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.