Structure, oxygen stoichiometry and electrical conductivity of LnPrO3+y (Ln=Y and Lanthanide) oxides

doi:10.1016/S0921-5107(99)00007-0

Materials Science and Engineering: B

Volume 58, Issue 3, 29 March 1999, Pages 215-220

https://doi.org/10.1016/S0921-5107(99)00007-0 Get rights and content

Abstract

The crystal structure and d.c. electrical conductivity of a series of homogeneous mixed rare-earth oxides in LnPrO_3+y (Ln=Y and lanthanide ion) system are reported. It is found that the ionic size of Ln determines the crystal structure of the oxides in this series. LaPrO_3+y and CePrO_3+y crystallise in the fluorite structure(fcc), whereas the remaining oxides adopt the C-type rare-earth oxide structure. The electrical conductivity measurements show that conductivity is structure type dependent with the fluorites (wherein Ln=La) having a lower conductivity than the C-type mixed oxides (wherein Ln=Gd, Sm, Ho, Er and Y). The conductivity of the mixed oxides at 1020 K, is of the order of 10⁻² Ω⁻¹ cm⁻¹ which is about five orders of magnitude higher compared to their room temperature conductivity. Oxygen excess in these compositions has been determined from wet-chemical analysis and temperature programmed oxygen evolution studies. The mixed oxides evolve oxygen in two distinct stages and the conductivity–temperature plot shows discontinuities corresponding to the onset of oxygen evolution.

Introduction

Metal oxide based ceramic conductors exhibiting both electronic and ionic conductivities have been the subject of considerable interest due to their application in fuel cell technology, gas separators and sensors [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Various mixed ionic-electronic conductors have been studied recently [2], [3], [4], [5], [6], [7], [8], [9], [10] and the presence of a mixed valent metal ion in the composition seems to be a prerequisite to impart electronic conduction. Suitable crystal lattice, optimum amount of oxygen vacancies or interstitial sites for ion transport and presence of suitable ions in desired proportion contribute to ionic conductivity. A combination of both these structural and electronic features determines the mixed conduction process in metal oxide systems.

The commonly used fluorite type oxide is zirconia which on appropriate substitution with trivalent metal ion shows high ionic conductivity and hence finds extensive use in emerging technologies [11]. Ceria based ionic conductors have been developed based on the crystallo–chemical aspects governing the stability and stoichiometry of fluorite structure in the Ce–Gd–O system [12], [13]. However, the fluorite compositions in the ZrO₂–CeO₂–Y₂O₃ system show mixed ionic-electronic conductivity [1] and the conductivity depends on the oxygen partial pressure. In these compositions the ionic conductivity arises from the oxygen vacancies while the electronic conductivity occurs as a consequence of electron hopping between Ce³⁺ and Ce⁴⁺ ions. Similarly, yttria stabilised zirconia becomes a mixed conductor by the incorporation of Tb in the fluorite lattice [10].

Fluorite and structurally related oxides having praseodymium are of interest since praseodymium exhibits mixed valence to give electronic conduction while the fluorite structure and oxygen vacancies can contribute to ionic conductivity. Praseodymium forms a wide range of non-stoichiometric oxide phases and also a homologous series of oxides with the generic formula Pr_nO_2n-2 with n=4, 7, 9, 10, 11 and 12. The phases of composition greater than the sesquioxide are fluorite related. The structure of these oxides may be viewed as being formed by the removal of oxygen atoms from normal sites in the fluorite structure accompanied by a relaxation of the remaining atoms proportionate to the extent of oxygen vacancies. Substitution of suitable metal ions for Praseodymium results in the formation of mixed oxides of various stoichiometries. Steele and co-workers [4], [5] have investigated the crystal structure and electrical conductivity of Pr-based mixed oxides and observed predominantly electronic conduction in these materials.

In the present paper, a systematic study of the structure, temperature programmed oxygen evolution and electrical conductivity of a new series of mixed lanthanide oxides in Ln–Pr–O system is reported. The solid solubility of the rare-earth sesquioxides with praseodymium oxide in 1:1 mole ratio of Ln to Pr in static air at 1623 K is studied. Since the synthesis of mixed metal oxides by chemical methods offer the advantage of achieving atomic level mixing of constituent metal ions in the precursor [14], [15], [16], citrate gel method is employed to prepare the mixed oxides.

Section snippets

Experimental

All the chemicals and reagents used were of high purity and obtained from Aldrich (UK, Dorset). La(NO₃)₃.6H₂O (99.9%), Pr(NO₃)₃.6H₂O (99.9%), ceric ammonium nitrate (99.9%), all other lanthanide nitrates and yttrium nitrate were also of 99.9% purity and citric acid monohydrate was of 99%.

A 0.5 M solution of each of these reagents were prepared by dissolving appropriate amount of the reagents in doubly distilled water. These reagents were mixed together to the required lanthanide(Ln) and

Results and discussion

The mixed lanthanide oxides, LnPrO_3+y crystallise in either fluorite or C-type rare-earth oxide structure depending on the size of Ln. The larger lanthanide ions such as La³⁺ and Ce³⁺ stabilise the fluorite structure while other lanthanide ions stabilise the C-type rare-earth oxide structure. The xrd patterns of the representative oxides are given in Fig. 1.

It was observed that the mixed lanthanide oxides will crystallise in fluorite structure, only when the average lanthanide radius is well

Conclusions

A series of mixed lanthanide oxides LnPrO_3+y have been synthesised and structurally characterised. The mixed oxides crystallise in fluorite structure for Ln=La and Ce while the oxides adopt C-type structure for other lanthanides and Y. The temperature programmed oxygen evolution occurs at two distinct stages. The conductivity–temperature plot shows discontinuities corresponding to the onset of oxygen evolution. Though the powder XRD shows the C-type structure for LnPrO_3+y (Ln=Nd, Sm, Gd, Dy,

Acknowledgements

M.R and M.G.K wish to thank the EPSRC for research fellowships. The authors also thank Mr. R. Reynolds for electrical conductivity and Ms. E. Williams for TPD measurements.

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Slow-cooling results in monophasic samples. In contrast quenching samples [8,10,11] potentially results in multiphasic products with variable oxygen stoichiometries. In-situ powder X-ray diffraction experiments can follow the oxygen uptake and the associated increase in symmetry from bixbyite to fluorite structures.
The reduction and oxidation of the solid solution Y_2−xPr_xO_3+δ (0.0≤x<2.0 and δ≤1.0) is investigated with emphasis on potential solid state electrolyte applications in solid oxide fuel cells. The fully reduced solid solution Y_2−xPr_xO₃ (0.0≤x<2.0) crystallizes in the bixbyite structure $(I a \bar{3})$ . The oxidized solid solution Y_2−xPr_xO_3+δ (0.0≤x<1.4) forms bixbyite phases $(I a \bar{3})$ whereas Y_2−xPr_xO_3+δ (1.4≤x<2) compositions form fully disordered defect fluorite structures $(F m \bar{3} m)$ with variable oxide defect concentrations. The two cation positions are investigated in detail using synchrotron powder X-ray and time of flight neutron diffraction data. In the bixbyite structures the 8c cation site splits into the 16c cation site and the 24d cation position migrates toward the ideal fluorite coordination upon oxidation. Reductive in-situ diffraction experiments reveal the co-existence of the fluorite and bixbyite structure only in a narrow temperature range. During oxidation of the bixbyite phase a new 16c oxide anion site is populated. The impact of the 16c oxide site population on the cation sublattice is being discussed.
Order/Disorder and in Situ Oxide Defect Control in the Bixbyite Phase YPrO<inf>3+δ</inf>(0≤δ<0.5)
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Structure, oxygen stoichiometry and electrical conductivity of LnPrO3+y (Ln=Y and Lanthanide) oxides

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Solid State Ion.

Solid State Ion.

J. Eur. Ceram. Soc.

J. Eur. Ceram.Soc.

J. Eur. Ceram.Soc.

Solid State Ion.

Stud. Surf. Sci. Catal.

Solid State Ion.

Mater. Sci. Eng.

Structure, oxygen stoichiometry and electrical conductivity of LnPrO_3+y (Ln=Y and Lanthanide) oxides