Semiconduction in non-stoichiometric rare earth oxides

Abstract

The electrical conductivities of the sesquioxides, dioxides and a few non-stoichiometric oxides of Pr and Tb have been measured at various temperatures and the activation energies for the conduction process have been reported. The conductivity passes through a maximum in the composition range LnO1.70-LnO1.83 (Ln = Pr or Tb). The conductivity data are consistent with the hopping model for these semiconductors. Calculation of the Frohlich's coupling constant and transition probability in Pr6011 has shown that the intermediate polaron theory is applicable to these systems. The electrical conductivities of CeOzfY203 and CeOzfNd203 systems show a maximum in conductivity at a composition of about 30 mole % of Ln0l.s (Ln = Nd or Y). The reflectance spectra of Pr and Tb oxides show a minimum around a composition of LnO1.75.Recent studies have established the phase equilibria in the praseodymium + oxygen 1 . 2 and the terbium+ oxygen 39 4 systems and the crystallography of the non-stoichiometric phases between Ln2O3 and LnO2 have been described.5 There have been a few reports of the electrical properties of the rare earth oxides.6-8 Honig and coworkers 9 have reported conductivity and thermoelectric power (T.E.P.) data of a few non-stoichiometric praseodymium oxides and shown that the charge carriers responsible for the conductivity are the 4felectrons of the lanthanide ions. The electrical conductivities of the non-stoichiometric oxides of praseodymium and terbium as well as their sesquioxides and dioxides have presently been examined with a view to study the variation of the conductivity with the oxygen-to-metal ratio and establish the mechanism of the conduction process in these oxides.Since the non-stoichioinetric rare-earth oxides are bivalent oxides with the lanthanide ions in the + 3 and the + 4 valence states, it was considered interesting to study the conductivities of Ce02 + Y2Q3 and CeOz + Nd203 systems of varying composition and compare the results with those of the non-stoichiometric oxides. The systems were chosen since the radii of Ce4f and Y3+ (or Nd3+) ions correspond closely to those of Pr4f and Pr3f ions.The reflectance spectra of the praseodymium and terbium oxides have been studied since the reflectance maximum should depend on the proportion of Ln3f and Ln4f ions in the oxides as does the electrical conductivity. It was expected that the reflectance spectra would also show variation with the oxygen-to-metal ratio. EXPERIMENTAL Lindsay 9999 % Pr6011 and Tb4O7, ignited to coiistant weight, were used. C-Pr203 and C-Tb203 were prepared by the reduction of Pr6011 and Tb407 in a stream of dry hydrogen for 6 h at 6QO and 900°C respectively.3~ 10 PrO1.71 was prepared by heating Pr6O11 in vacuum (-1O-5rnmHg) at 490°C for 4h.11 Pro2 and TbO2 were prepared by the methods described earlier. 11

Layer-by-layer deposition of praseodymium oxide on tin-doped indium oxide (ITO) surface

“…is deposited (10-50 layers). Such result is consistent with the literature that the Pr 6 O 11 particles exhibit a superior electrical conductivity due to the electron hopping mechanism between Pr 3+ and Pr 4+ within the semi-conductive oxide structure [16,23]. In addition, the increasing capacitance value (C 2 ) is clearly denoted at higher deposition cycles from 10 to 50 layers, which is mainly attributed to increase in the semiconductive properties of the overlayers on the ITO surface.…”

Boosting the oxygen reduction reaction mechanisms in IT-SOFC cathodes by catalytic functionalization

“…12,13 Pr oxide undergoes a phase transition from the ordered PrO 1.833 at room temperature to disordered PrO 1.83 at increasing temperatures, which is reected in the electrical conductivity behaviour. [14][15][16] It seems that the variation of E a in the R p coincides with this phase transition.…”

Fundamentals and Catalytic Applications of CeO2-Based Materials

“…Increasing the temperature treatment to 1600 °C, the maximum amounts of M 2 O 3 miscible into CeO 2 lattice significantly decreases for all the trivalent dopants considered . The anion vacancy model has been confirmed for these solid solutions by both density and X-ray intensity analysis. , In some cases, a certain long-range ordering of the anion vacancies, resulting in the formation of C-type rare earth oxide structure, has been observed, with ordering favored for the smaller trivalent metal cations. , The formation of oxygen vacancies within the structure of M 2 O 3 -doped CeO 2 is a key factor to promote oxygen diffusion in the materials, allowing hopping of O 2– anions from one position to the next empty one . Among the ceria-rare earth oxide systems, Sm­(III) and Gd­(III) have been shown to cause the highest increase of electrical conductivity given their ionic radii, which are very close to that of Ce­(IV).…”