Thermodynamic quantities and defect equilibrium in La2−xSrxNiO4+δ

In order to elucidate the relation between thermodynamic quantities, the defect structure, and the defect equilibrium in La_2−xSr_xNiO_4+δ, statistical thermodynamic calculation is carried out and calculated results are compared to those obtained from experimental data. Partial molar enthalpy of oxygen and partial molar entropy of oxygen are obtained from δ-P(O₂)-T relation by using Gibbs-Helmholtz equation. Statistical thermodynamic model is derived from defect equilibrium models proposed before by authors, localized electron model and delocalized electron model which could well explain the variation of oxygen content of La_2−xSr_xNiO_4+δ. Although assumed defect species and their equilibrium are different, the results of thermodynamic calculation by localized electron model and delocalized electron model show minor difference. Calculated results by the both models agree with the thermodynamic quantities obtained from oxygen nonstoichiometry of La_2−xSr_xNiO_4+δ.     

Thermodynamic properties of complex oxides in the La-Ni-O system

Complex oxides La₂NiO_4+δ, La₃Ni₂O_7−δ, La₄Ni₃O_10−δ and LaNiO_3−δ, the members of Ruddlesden-Popper series La_n+1Ni_nO_3n+1, were prepared using citrate precursors. The stability range of LaNiO_3−δ in air as well as the oxygen nonstoichiometry of La₃Ni₂O_7−δ and La₄Ni₃O_10−δ as a function of temperature and oxygen partial pressure was determined by means of thermogravimetric technique. Decomposition temperatures of La₃Ni₂O_7−δ, La₄Ni₃O_10−δ and LaNiO_3−δ in air were determined by conductivity measurement method. The boundary of stability for La₄Ni₃O_10−δ was determined by EMF measurements of galvanic cell with oxygen conducting solid electrolyte. The isothermal (1400 K) projection of La-Ni-O system phase diagram to the plane “log(PO₂)-relative mole fraction of metal components” was suggested.     

Effect of nonstoichiometry on Raman scattering of VO2 films

We report on Raman scattering of VO2 films prepared by radio frequency magnetron sputtering under different conditions. Our investigations revealed that the dominated Raman peaks shift towards high frequency for both V-rich and O-rich VO2 films, compared with the stoichiometry VO2 films. The experimental evidence is presented and the cause for nonstoichiometry dependence of Raman spectra of VO2 films is discussed.     

Characterization of nonstoichiometric nickel manganite spinels NixMn3−x□3δ/4O4+δ by temperature programmed reduction

Cation deficient spinels Ni_xMn_3−x□_3δ/4O_4+δ (0≤x≤1) have been prepared by thermal decomposition of mixed oxalates Ni_x/3Mn_(3−x)/3(C₂O₄)·nH₂O in air at 623 K. They have been characterised by temperature programmed reduction (TPR) under H₂, the reaction being followed by gravimetric and powder X-ray diffraction measurements. It has been shown that TPR proceeds in several steps. The first steps correspond to the loss of nonstoichiometric oxygen leading to the formation of a stoichiometric oxide. During the following stages the manganese cations are reduced, causing the spinel structure to be destroyed, and the formation of solid solution of NiO in a cubic MnO. Subsequently, Ni²⁺ cations undergo a reduction to metallic nickel, and, finally, a mixture of nonstoichiometric MnO_1−δ and metallic nickel is formed. These oxides contain a high level of vacancies which vary with the nickel content with a maximum of δ≈1 near x=0.6. This nonstoichiometry is ascribed both to the presence of Ni³⁺ and excess of Mn⁴⁺.     

A semi-empirical equation for oxygen nonstoichiometry of perovskite-type ceramics

Explicit equations correlating oxygen nonstoichiometry to oxygen partial pressure and temperature are important for applications of perovskite-type ceramics as membranes, adsorbents and catalysts in various chemical reaction and separation processes. A semi-empirical equation for oxygen nonstoichiometry on perovskite-type ceramics is reported in this paper. Though derived from the results of a point defect model on a perovskite-type ceramic material, La_0.1Sr_0.9Co_0.5Fe_0.5O_3−δ, this equation describes very well the experimentally measured oxygen nonstoichiometry data for two perovskite-type ceramics measured in this work and three perovskite-type ceramics reported in the literature. The major advantage of this semi-empirical equation lies in its simplicity, explicitness and accuracy. This equation is coupled with oxygen permeation equation to predict oxygen permeation current density through two perovskite-type ceramic membranes. The predicted data agree very well with the results reported in the literature using a complex defect reaction model.     

Anionic high-temperature conduction in single crystals of nonstoichiometric phases R1 ? y M y F3 ? y (R = La-Lu, M = Ca, Sr, Ba) with the tysonite (LaF3) structure

Electrophysical properties of single crystals of nonstoichiometric phases R_{1 − y} M _y F_{3 − y} , where R = La-Lu, M = Ca, Sr, or Ba, with the tysonite (LaF₃) structure, which are present in a metastable state after being grown and cooled, are measured in the temperature interval extending from 300 to 1073 K. It is discovered that, during a sufficiently long high-temperature investigation, solid solutions R_{1 − y} Ca _y F_{3 − y} , where R = Tb, Dy, or Ho, undergo irreversible variations in the phase composition in the temperature region 723 to 823 K. This level of temperatures, which correspond to partial decomposition of phases R_{1 − y} Ca _y F_{3 − y} with the rare-earth elements of the end of the period, lies above the temperatures to which the fluoride solid electrolytes are usually heated when used in solid-state electrochemical devices. The temperature and concentration dependences of the phases’ electroconduction are explained in the framework of the vacancy mechanism of anionic transport. Original Russian Text ? N.I. Sorokin, B.P. Sobolev, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 4, pp. 420–431. The paper is dedicated to the memory of Prof. M.W. Breiter, formerly of the Vienna Technical University, Austria.     

Lattice Dynamics in LaTa2-2xNb2xVO9-δ (x=0–0.1) Solid Solutions

The oxygen nonstoichiometry (x) of LaTa_2-2xNb_2xVO₉- (x = 0–0.1) solid solutions was studied using Xray phase analysis, vibrational spectroscopy, and radiospectroscopy. A correlation was found between (x) and the unit cell volume V(x) of the solid solutions. It was shown that the infrared spectra of LaTa₂VO₉- change in passing from = 0 to 0. The structural position of the oxygen vacancy in LaTa_2-2xNb_2xVO₉- is discussed.     

Corrigendum to “EuBaFe2O5+w: Valence mixing and charge ordering are two separate cooperative phenomena” [J. Solid State Chem. 180 (2007) 148-157]

An error in the expression for the Fe²⁺, Fe³⁺ valence-mixing probability in RBaFe₂O_5+w is acknowledged and correct formula is derived. The new formula slightly improves the least-squares fit to the experimental concentrations of the Mössbauer component Fe^2.5+ as a function of the oxygen-nonstoichiometry parameter w for R=Eu.     

Structural Features of Solid Phases and Topology of Phase Diagrams

The paper considers model phase diagrams of binary and ternary systems involving transformations (transitions) of phase equilibria. The relationship between the type of structural solidstate transformation and the type of phase diagram is shown. Topological series of phase diagrams of systems with continuous and limited types of solid solutions are considered, including phase diagrams with polymorphic transitions between intermediate solid phases of variable composition.