Crystal structures and phase transitions in the rare earth oxides

Atkinson, SC

Abstract

The lanthanoid sesquioxides exhibit a number of distinct structural phases. Below
2000°C these oxides exist in three crystal systems, namely the A-type hexagonal phase,
the B-type monoclinic phase and the C-type cubic phase. With increasing temperature
the stability of these structures is generalised by the order C → B → A, although not
every oxide will exhibit all phases; this general transition is typical of the middle
members of the group. Under ambient conditions, the A phase is preferred for La2O3 to
Pm2O3. Both the C and B phases exist for Sm2O3, Eu2O3 and Gd2O3. The C phase is
stable at room temperature from Sm2O3 onwards, and at the high atomic number end of
the series this phase is preferred.
Traditionally, the structures of the heavier sesquioxides (Er2O3 to Lu2O3) have been
believed to be cubic from ambient temperature all the way up to their melting points.
However, contrary to the current phase diagram, my work has shown that not only are
B-type Sm2O3, Eu2O3 and Gd2O3 very stable at ambient temperature, but it is also
possible to create 1% monoclinic Yb2O3 by heating and then quenching back to
ambient temperature.
Of the lanthanoids, praseodymium and terbium are known for their existence in both
the +3 and +4 oxidation states. The praseodymium-oxygen system is notable for its
multiple stoichiometries. This work presents kinetic data for the phi - beta phase and the sigma - theta
phase transitions in this system, the results obtained via high-temperature X-ray
powder diffraction and differential scanning calorimetry.
The crystal structures of B-type Gd2O3 and Yb2O3 are reported, the former obtained
using both laboratory and synchrotron X-ray data and the latter using laboratory data
alone. It is proposed that this is the first time these two structures have been determined
following the application of temperature alone, without the additional application of
pressure.

Citation

Atkinson, S. Crystal structures and phase transitions in the rare earth oxides. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Mar 7, 2014
• Publicly Available Date: Mar 7, 2014