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Crystal structures and phase transitions in the rare earth oxides

Atkinson, SC 2014, Crystal structures and phase transitions in the rare earth oxides , PhD thesis, University of Salford.

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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.

Item Type: Thesis (PhD)
Contributors: Boag, NM (Supervisor) and Kilcoyne, SH (Supervisor)
Schools: Schools > School of Computing, Science and Engineering
Funders: Non funded research
Depositing User: S Rafiq
Date Deposited: 07 Mar 2014 18:05
Last Modified: 30 Nov 2015 23:53
URI: http://usir.salford.ac.uk/id/eprint/30746

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