On the praseodymium + oxygen system

Abstract

An isobaric survey of the system praseodymium oxide + oxygen has been made at oxygen pressures between 0 and 1 atm and in the temperature range 200 to 1150 °C. The derived isobaric sections enable a three-dimensional phase diagram (pressure, temperature, composition) to be constructed with considerable certainty and detail for the composition range PrOj.gg to PrOj.g*. Reasonable extensions to cover the complete range between the sesquioxide and dioxide are proposed, and a projection of the diagram on to the temperature-composition plane is presented. At lower temperatures several discrete, ordered phases of narrow homogeneity range exist. These constitute an homologous series P r„02n_2, with n = 4, 7, 9 ,1 0 ,1 1 ,1 2 , oo. At higher temperatures two wide-range solid solutions obtain: <r, a body-centred cubic phase with a maximum composition range ca. PrOj.gQ to PrOj.yo; and a, a face-centred cubic phase of composition P r O ^ to P r 0 2. The fields of stability of the various phases are defined and the ambient conditions at many invariant axes (peritectoid and eutectoid) enumerated. Miscibility gaps with upper consolute points are exceptional; order-disorder peritectoid transformations are common. Hysteresis in phase transformations is confirmed, and the results further demonstrate the existence of metastable states in phase reactions involving an increase in structural order. The appearance of these pseudo-phases and the nature of non-stoichiometry is explained in terms of a plausible model invoking microdomain texture in defect solids. This model is believed to be appropriate for other non-stoichiometric systems also. Earlier experimental data on the system are examined, and found to be consistent with the present results.

Post-deposition annealing of praseodymia films on Si(111) at low temperatures

“…In this work, it was also shown that the temperatures necessary to obtain PrO 1.5 decrease with decreasing oxygen pressure. Therefore, the low values obtained in the present work can be explained since we used a base pressure of 1 · 10 −10 mbar compared to several 10 mbars used by Hyde et al [22].…”

“…Whereas one species can be allocated to PrO [22]. In particular, during reduction additional oxidation states may be formed.…”

Photoemission study of praseodymia in its highest oxidation state: The necessity of in situ plasma treatment

“…Thus, the vertical lattice constant decreases to 3.09 Å. Since the bulk lattice constant of cubic praseodymia phases of different stoichiometry decreases as the oxygen content increases 25 (cf. the smaller lattice constant of PrO 2 compared to Pr 6 O 11 which can formally be conceived as mixture of PrO 2 and cubic Pr 2 O 3 where the latter has a 3.2% larger lattice constant than PrO 2 ) we conclude that the plasma-treated praseodymia film possesses more oxygen than the untreated film.…”

Epitaxy of single crystalline PrO2 films on Si(111)

“…The bixbyite structure of c-Pr 2 O 3 can be considered as fluorite structure with periodically arranged oxygen vacancies. 9 The anion sublattice of c-Pr 2 O 3 ͑111͒ films forms antiphase boundaries ͑APBs͒ on Si͑111͒ since the size of the surface unit cell of c-Pr 2 O 3 ͑111͒ is four times the size of Si͑111͒ surface unit cell. APBs are disadvantageous in the field of heteroepitaxy, as reported for other oxide films.…”

“…9 Therefore we associate the phase with d ␦ with understoichiometric PrO 2−␦ . At high temperatures praseodymium can only completely be oxidized at high O 2 pressure so that bulk PrO 2 usually has oxygen vacancies.…”