The following nanocrystalline binary oxide systems were studied: Mg-Ti, Ni-Ti, Zr-Al, as
well as some pure and doped unary oxides. The xerogels were heated at a constant T (200 to
1600°C) for 3 to 6 hours. There was a threshold tempearture for oxide formation and in many cases
the products were metastable nanocrystalline phases, depending on the grain size and composition,
including doping. The oxide phases of Ni-Ti, Mg-Ti, and Zr-Al, formed at 900 °C are different from
those formed at higher temperature. New ranges of solid solutions and the formations of higher
temperature structures were found. A transition phase can be defined as a structure formed at
relative low tempearture, irreversibly transforming at higher temperature into an equilibrium phase
of the same elemental composition. Some low temperature transition phases have a structure
similar to that of a high temperature equilibrium phase, e.g., (the equilibrium phase is given in
parentheses) tetragonal ZrO2 (monoclinic) and low-T qandilite-like solid solutions (qandilite +
geikielite). Others are unique with no representation in the equilibrium phase diagram, e.g.,
gamma-like alumina (corundum) and anatase (rutile), which are formed as nanocrystalline oxides
due to a low growth rate caused either by a low temperature of calcination or due to additives.
To asses the importance of crystal size in the stabilization of transition phases, the following studies
were undertaken: (a) XRPD analysis of all unary, doped and binary compositions; (b) the evolution
of transition phases in HT XRPD of the Mg titanates; (c) the phase evolution was studied with time
at temperatures were mixtures of transition and equilibrium phases were found; (d) the retention of
pure tetragonal ZrO2 on quenching Al-Zr oxides after calcinations at high tempetature; (e)
additional evidence from HRTEM, SEM and DTA experiments was also collected. A model,
correlating the size effect with the unusual phases and structures is proposed.
Solid Solutions in Silicate and Oxide Systems
