Cationic Nonstoichiometry Research Articles

Effect of cationic nonstoichiometry on thermoelectric properties of layered calcium cobaltite obtained by field assisted sintering technology (FAST)

This work addresses the problem of obtaining of Ca3Co4O9+δ-based ceramics with enhanced thermoelectric performance. Phase-inhomogeneous layered calcium cobaltite ceramics with cationic nonstoichiometry were prepared by solid-state reactions method and a field assisted sintering technology (FAST). Comprehensive experimental characterizations were conducted on the prepared bulk samples, focusing on their phase composition, as well as thermal (including thermal expansion, thermal diffusivity, and thermal conductivity), electrical (encompassing electrical conductivity and the Seebeck coefficient), and functional properties (such as power factor and figure-of-merit). The FAST technique allowed to obtain ceramics with low porosity and high electrical conductivity, which increased as the Ca:Co ratio within the samples decreased, while sample phase inhomogeneity considerably increased the Seebeck coefficient. The best thermoelectric performance was demonstrated for cationic nonstoichiometric Ca3Co4.4O9+δ, which power factor and figure-of-merit values at 825 °C reached 427 μW⋅m−1⋅K−2 and 0.146, respectively.

Cationic nonstoichiometry, structural and magnetic properties of hexagonal Tm2-xMnxO3 manganites

We studied the mechanism of formation of cation-nonstoichiometric compositions of the Tm2-xMnxO3 (x = 0.96, 0.98, 1, 1.02, 1.04, 1.06) solid solution with a hexagonal crystal structure. The entire region of homogeneity of compositions with an excess/deficiency of Mn and Tm was covered. Their structural and magnetic properties were studied by X-ray diffraction and magnetometry. The analysis of the concentration dependence of the structural parameters allows us to conclude that the formation of compositions with an excess of Tm occurs through the formation of vacancies in the Mn sublattice. The mechanism of formation of solid solutions with an excess of Mn includes the reaction of manganese disproportionation: 2Mn3+=Mn4++Mn2+. The Mn2+ ions formed as a result of this reaction occupy thulium sites, while manganese sites are occupied by Mn3+ and Mn4+ ions. Magnetic properties have been studied as functions of temperature in the temperature range 5 – 300 K. It was established that all the samples undergo a transition from the high-temperature PM state to AF one at T = TN1. The temperature TN1 both at x < 1 and x > 1 is reduced and takes a value in the range of 81–84 K. With decreasing temperature (T < TN1) in samples with x = 0.96, 1.04, 1.06 an additional first-order transition to a new AF state is observed in the vicinity of T∼45 K. The Curie-Weiss temperature ΘCW monotonically decreases for compositions with x < 1. At the same time with x > 1 a nonlinear dependence of ΘCW (x) is observed.

Cationic Nonstoichiometry, Crystal Structure, and Magnetic Properties of Eu1-xMn1+xO3

Cationic Nonstoichiometry, Crystal Structure, and Magnetic Properties of Eu1-xMn1+xO3

The impact of Mn nonstoichiometry on the oxygen mass transport properties of La0.8Sr0.2Mn y O3±δ thin films

Oxygen mass transport in perovskite oxides is relevant for a variety of energy and information technologies. In oxide thin films, cation nonstoichiometry is often found but its impact on the oxygen transport properties is not well understood. Here, we used oxygen isotope exchange depth profile technique coupled with secondary ion mass spectrometry to study oxygen mass transport and the defect compensation mechanism of Mn-deficient La0.8Sr0.2Mn y O3±δ epitaxial thin films. Oxygen diffusivity and surface exchange coefficients were observed to be consistent with literature measurements and to be independent on the degree of Mn deficiency in the layers. Defect chemistry modeling, together with a collection of different experimental techniques, suggests that the Mn-deficiency is mainly compensated by the formation of antisite defects. The results highlight the importance of antisite defects in perovskite thin films for mitigating cationic nonstoichiometry effects on oxygen mass transport properties.

Chemistry, Local Molybdenum Clustering, and Electrochemistry in the Li2+xMo1-xO3 Solid Solutions.

A broad range of cationic nonstoichiometry has been demonstrated for the Li-rich layered rock-salt-type oxide Li2MoO3, which has generally been considered as a phase with a well-defined chemical composition. Li2+xMo1-xO3 (-0.037 ≤ x ≤ 0.124) solid solutions were synthesized via hydrogen reduction of Li2MoO4 in the temperature range of 650-1100 °C, with x decreasing with the increase of the reduction temperature. The solid solutions adopt a monoclinically distorted O3-type layered average structure and demonstrate a robust local ordering of the Li cations and Mo3 triangular clusters within the mixed Li/Mo cationic layers. The local structure was scrutinized in detail by electron diffraction and aberration-corrected scanning transmission electron microcopy (STEM), resulting in an ordering model comprising a uniform distribution of the Mo3 clusters compatible with local electroneutrality and chemical composition. The geometry of the triangular clusters with their oxygen environment (Mo3O13 groups) has been directly visualized using differential phase contrast STEM imaging. The established local structure was used as input for density functional theory (DFT)-based calculations; they support the proposed atomic arrangement and provide a plausible explanation for the staircase galvanostatic charge profiles upon electrochemical Li+ extraction from Li2+xMo1-xO3 in Li cells. According to DFT, all electrochemical capacity in Li2+xMo1-xO3 solely originates from the cationic Mo redox process, which proceeds via oxidation of the Mo3 triangular clusters into bent Mo3 chains where the electronic capacity of the clusters depends on the initial chemical composition and Mo oxidation state defining the width of the first charge low-voltage plateau. Further oxidation at the high-voltage plateau proceeds through decomposition of the Mo3 chains into Mo2 dimers and further into individual Mo6+ cations.

The cationic and oxygen nonstoichiometry of sodium-gadolinium molybdates

The content of all matrix elements, including oxygen, was determined by X-ray spectral microanalysis in series of single crystals of sodium-gadolinium molybdates (NGM) grown from melt by Czochralski technique. All grown NGM crystals, including those grown from stoichiometric charge, are non-stoichiometric and contain a molybdenum deficiency of about 3% in average. The NGM congruently melting composition has been determined. For crystals grown from melt of equimolar (stoichiometric) composition, the content of cationic vacancies in the (Gd + Na) sublattice is close to zero, while the composition of crystal differs from the initial melt. As the Gd excess increases, the content of cationic vacancies in the (Gd + Na) sublattice increases up to 10%. It has been found that cation-deficient NGM crystals are also anion-deficient. The concentration of oxygen vacancies varies from 0.5% to 5% of the amount of anion sites. Possible mechanisms responsible for the formation of cationic and anionic vacancies in NGM crystals are discussed.

Thermodynamic and Structural Properties of CuCrO2 and CuCr2O4: Experimental Investigation and Phase Equilibria Modeling of the Cu–Cr–O System

This work considers the equilibria between the stable phases of the Cr–Cu–O ternary chemical system at atmospheric pressure and in a large temperature range that includes the high-temperature liquid phase. Based on a thorough evaluation of literature data, we performed complementary experimental investigations of solid-phase properties (especially the mixed oxides) by implementation of spark plasma sintering and microanalysis, high-temperature X-ray diffraction combined with Rietveld analysis, differential thermal analysis, and thermogravimetry. We show that neither the spinel CuCr2O4 nor the delafossite CuCrO2 phases exhibit significant cationic nonstoichiometry. We provide an assessment of the thermodynamic functions of the spinel up to 1200 K, taking into account the α/β phase transition. We propose, for the first time, a consistent thermodynamic description of the Cr–Cu–O system based on the Calphad method, allowing the computation of reactions and phase equilibria, including the high-temperature liquid phase described by the modified quasichemical model.

INFLUENCE OF DEFECTS FORMATION WAY ON PROPERTIES OF PROTON CONDUCTORS BASED ON LANTHANUM SCANDATE

The structure and properties of proton conductors based on LaScO3 synthesized by acceptor doping and the creation of a deficiency in La cation were studied. It has been shown that the number of intercalated protons in the oxide lattice increases with an increase in the degree of cationic doping and is higher when doped with strontium than calcium. It has been noted that in lanthanum scandate samples with cationic non-stoichiometry, the number of intercalated protons decreases with increasing lanthanum deficiency, which is probably due to the possibility of binding oxygen vacancies VO•• to (LaLa''' - VO••)' complexes and blocking their participation in the process of water dissolution. The activation energies of the conductivity of the volume and grain boundaries of lanthanum scandate synthesized by various methods are determined. It was shown that the conductivity and activation energy of conductivity of the volume of grains of samples doped with strontium are higher than when doped with calcium. It is noted that the activation energy of conductivity of samples with cationic non-stoichiometry is practically independent of the lanthanum deficiency.

A-site defects in LaSrMnO3 perovskite-based catalyst promoting NOx storage and reduction for lean-burn exhausts

Herein, we report the high De-NOx performance of the A-site defective perovskite-based Pd/La0.5Sr0.3MnO3 catalyst. The formation of the defective perovskite structure can be proved by both the increased Mn4+/Mn3+ ratio and serious lattice contraction due to cationic nonstoichiometry. It promotes the Sr doping into perovskite lattice and reduces the formation of the SrCO3 phase. Our results demonstrate that below 300 °C the A-site defective perovskite can be more efficiently regenerated than the SrCO3 phase as NOx storage sites due to the latter's stronger basicity, and also exhibits the higher NO oxidation ability than the A-site stoichiometric and excessive catalysts. Both factors promote the low-temperature De-NOx activity of the Pd/La0.5Sr0.3MnO3 catalyst through improving its NOx trapping efficiency. Nevertheless, above 300 °C, the NOx reduction becomes the determinant of the De-NOx activity of the perovskite-based catalysts. A-site defects can weaken the interactions between perovskite and Pd, inducing activation of Pd sites by in-situ transformation from PdO to metallic Pd in the alternative lean-burn/fuel-rich atmospheric alternations, which boosts the De-NOx activity of the Pd/La0.5Sr0.3MnO3 catalyst. The Pd/La0.5Sr0.3MnO3 catalyst exhibits the high sulfur tolerance as well. These findings provide insight into optimizing the structural properties and catalytic activities of the perovskite-based catalysts via tuning formulation, and have potential to be applied for various related catalyst systems.

Insights on the Stability and Cationic Nonstoichiometry of CuFeO2 Delafossite.

CuFeO2, the structure prototype of the delafossite family, has received renewed interest in recent years. Thermodynamic modeling and several experimental Cu-Fe-O system investigations did not focus specifically on the possible nonstoichiometry of this compound, which is, nevertheless, a very important optimization factor for its physicochemical properties. In this work, through a complete set of analytical and thermostructural techniques from 50 to 1100 °C, a fine reinvestigation of some specific regions of the Cu-Fe-O phase diagram under air was carried out to clarify discrepancies concerning the delafossite CuFeO2 stability region as well as the eutectic composition and temperature for the reaction L = CuFeO2 + Cu2O. Differential thermal analysis and Tammann's triangle method were used to measure the liquidus temperature at 1050 ± 2 °C with a eutectic composition at Fe/(Cu + Fe) = 0.105 mol %. The quantification of all of the present phases during heating and cooling using Rietveld refinement of the high-temperature X-ray diffraction patterns coupled with thermogravimetric and differential thermal analyses revealed the mechanism of formation of delafossite CuFeO2 from stable CuO and spinel phases at 1022 ± 2 °C and its incongruent decomposition into liquid and spinel phases at 1070 ± 2 °C. For the first time, a cationic off-stoichiometry of cuprous ferrite CuFe1- yO2-δ was unambiguous, as evidenced by two independent sets of experiments: (1) Electron probe microanalysis evidenced homogeneous micronic CuFe1- yO2-δ areas with a maximum y value of 0.12 [i.e., Fe/(Cu + Fe) = 0.47] on Cu/Fe gradient generated by diffusion from a perfect spark plasma sintering pristine interface. Micro-Raman provided structural proof of the existence of the delafossite structure in these areas. (2) Standard Cu additions from the stoichiometric compound CuFeO2 coupled with high-temperature X-ray diffraction corroborated the possibility of obtaining a pure Cu-excess delafossite phase with y = 0.12. No evidence of an Fe-rich delafossite was found, and complementary analysis under a neutral atmosphere shows narrow lattice parameter variation with an increase of Cu in the delafossite structure. The consistent new data set is summarized in an updated experimental Cu-Fe-O phase diagram. These results provide an improved understanding of the stability region and possible nonstoichiometry value of the CuFe1- yO2-δ delafossite in the Cu-Fe-O phase diagram, enabling its optimization for specific applications.

Charge Transfer and Defect Structure in BaCeO3

Electrical conductivity (at 460–990°С) and ion and proton transference numbers (at 550–950°С) of nominally undoped BaCeO3 have been studied as dependent on temperature, pO2 (2.1 × 10–4 to 10–15 Pa), and pН2O (40–2340 Pa). For determining the defect model, small additives of aliovalent dopants Nd3+ (up to 1 at %) and Ta5+ (up to 0.5 at %) were used. The effect of cationic nonstoichiometry of barium cerate on the electrical conductivity and reaction with the gas phase has been considered. Charge transfer in ВаСеО3 is explained using the model of defect formation in the reaction of ВаСеО3 with the gas phase.

A-site nonstoichiometry and B-site doping with selected M3 + cations in La2-xCu1-y-zNiyMzO4-δ layered oxides

In this work studies on a modification of Cu-based, layered oxides with K2NiF4-type structure are presented concerning formation of cationic nonstoichiometry in the A-sublattice of the parent La2CuO4, as well as an introduction of selected M3+ cations (Sc, Ga, In) at the B-site into La1.9Cu1-xMxO4 and Ni-containing La1.9Cu0.45Ni0.45M0.1O4. It is shown that synthesis of phase-pure La2-xCuO4 can be done with values of x≤0.1, and in the A-site nonstoichiometric materials further doping with M3+ is also achievable. Formation of solid solutions for M3+-doped samples follows the expected trend, with the widest range observed for the smallest Sc3+ cation, z≤0.1. Apart from a different structural behavior of the doped oxides at high temperatures, it is documented that the discussed substitution affects strongly the oxygen content (e.g. high concentration of the oxygen vacancies is present in La1.9CuO4-δ), as well as electrical conductivity. This is documented by the high-temperature X-ray diffraction, thermogravimetric, iodometic titration and electrical measurements. Since the compounds possess good transport properties, are stable at elevated temperatures, exhibit moderate thermal expansion and are Co-, Sr- and Ba-free, the proposed doping strategy seems interesting concerning development of the oxygen-transporting membranes based on Cu-containing oxides. For example, 1mm thick La1.9Cu0.9Sc0.1O4-δ membrane was found to deliver 0.37mLcm−2min−1 oxygen flux at 950°C, a very high value for the K2NiF4-type materials.

Nanostructural inhomogeneity of EuBa2Cu3O6 + δ superconductors

Samples of high-temperature superconducting oxide EuBa2Cu3O6 + δ (Eu-123) with total cationic composition Eu: Ba: Cu = 1: 2: 3 are investigated by means of local X-ray microanalysis and high-resolution transmission electron microscopy. The cationic nonstoichiometry of Eu-123 oxide is revealed. The particles of the studied samples are inhomogeneous in structure on the nanoscale, with two types of inhomogeneities: one with typical sizes of one to several nanometers, and one with typical sizes of 10 to 20 nm, respectively.

Electrical Conduction and Dielectric Properties of the Rb‐doped CaCu3Ti4O12

Ca1−xRbxCu3Ti4O12 (x = 0, 0.03, and 0.05) ceramics were synthesized by the sol‐gel method. Their microstructure and electrical properties were investigated. In the Rb‐doped samples, the Cu‐rich and Ti‐poor grain‐boundary layers are formed, and electrical properties are also changed by doping: With the increase in doping concentration, the grain resistivity and the grain‐boundary Schottky potential barrier are changed, the grain‐boundary resistivity is enhanced, and the low‐frequency dielectric constants and loss are reduced. These results were discussed in terms of the internal barrier layer capacitor (IBLC) mechanism, particularly focusing on the electrical properties in grains and the cationic nonstoichiometry at grain boundaries.

Cationic nonstoichiometry of oxides of the Ba-Cu-O and Y-Ba-Cu-O systems

The structure and cationic composition of oxides of the Ba-Cu-O system with cubic BaCuO2 and tetragonal BaCu2O2 structure and YBa2Cu3O6 + δ oxide of the Y-Ba-Cu-O system are studied. The oxides are found to be nonstoichiometric in cationic composition when the composition differs noticeably from the corresponding formula relation with conservation of the crystalline structure’s type.

Effect of metal vacancies on the energy parameters of s-, p-, and d-metal diborides

For a group of s-, p-, and d-metal diborides (MB2, where M = Mg, Al, Sc, Ti, V, Y, Zr, Nb, or Ta), variations in the cohesive energy and the formation energy in the presence of cation vacancies (MB2 → M0.75B2), as well as the formation energy of cation-site vacancies, are analyzed using the FLMTO-GGA full-potential approach. The cationic nonstoichiometry for MB2 phases is achievable only when they are prepared under nonequilibrium conditions.

A Relative of Sr4Fe6O13 with Double Perovskite Layers: Bi4Sr14Fe24O56, m = 2-Members of a Potential Series [(Sr,Bi)2Fe4O7-δ][(Sr,Bi)2Fe2O6]m

A new ferrite Bi4Sr14Fe24O56 (i.e., Bi4/3Sr14/3Fe8O18.66) has been synthesized. The high-resolution electron microscopy study of this oxide shows that its modulated structure consists of an original intergrowth of double perovskite layers [(Sr,Bi)2Fe2O6] with oxygen-deficient [(Sr,Bi)2Fe4O7-δ] layers. The latter are closely related to the [Sr2Fe4O7-δ] layers observed in Sr4Fe6O13, that is, built up of FeO5 bipyramids and distorted tetragonal pyramids. This new oxide appears as the member m = 2 of a potential series [(Sr,Bi)2Fe4O7-δ] [(Sr,Bi)2Fe2O6]m, which offers a great possibility of cationic and oxygen nonstoichiometry, in connection with the structure modulation.

New n = 2 members of the Li2Srn − 1MnO3n + 1 family, closely related to the Ruddlesden–Popper phases: structure and non-stoichiometry

New n = 2 members of the Li2Srn – 1MnO3n + 1 series closely related to the RP phases have been synthesized by solid state reaction for the first time: Li2SrNb2O7, Li2SrTa2O7, Li2EuTa2O7. The detailed structure determination of Li2SrNb2O7 performed by neutron diffraction and electron microscopy shows that it crystallizes in the space group Cmcm [a = 18.0071(5), b = 5.5979(3), c = 5.5920(3) A], i.e. with a symmetry different from that of LiAM2O7 and Li2AM2O7 previously synthesized by chemical or electrochemical intercalation. The double perovskite layers [SrNb2O6] are puckered due to a tilting of their [001] octahedral rows alternately in opposite directions while the tetrahedral [Li2O] layers are significantly distorted. The possibility of cationic non-stoichiometry in both Li and Sr sites is demonstrated by the synthesis of Li2Sr1 – xLn2x/3Nb2O7 (Ln = La, Nd) and Li2 – xSr1 – xLaxNb2O7 oxides. The stability of these phases is discussed in terms of matching of the octahedral and tetrahedral layers, taking into consideration the tilting of the octahedra, itself influenced by the size of the A site cations and the cationic non-stoichiometry. An extension to n = 3 members is made, with the synthesis of the oxides Li2Sr1.5Nb3O10 and Li2Ca1.5Nb3O10.

Coupled Ferroelectric-Ferroelastic Materials Including Tetravalent Elements

Lead has been substituted for cerium, thorium or uranium in Pb2.5 Nb5O15 and Pb2KNb5O15. Six phases with formula Pb2.5-2xMxNb5O15 and Pb2(1-x)MxKNb5O15 (M=Ce, Th, U) and tungsten bronze structure have been obtained. A cationic nonstoichiometry in the A and B sites appears for x>0. These phases have coupled ferroelastic-ferroelectric properties. The spontaneous strain and the Curie temperature decrease as lead is replaced by cerium, thorium or uranium.

Phase equilibria and thermodynamics of coexisting phases in rare-earth element-iron-oxygen systems. III. The europium-iron-oxygen system

X-ray and microstructural analysis methods were used to study the possibility of cationic nonstoichiometry in intermediate phases of the ferroperovskite-ferrogarnet type of the Eu 2O 3Fe 2O 3 system. It is shown that at 1350°C europium ferroperovskite has a narrow range of nonstoichiometry in the direction of excess iron oxide. The character of phase changes in the EuFeO system during changes in P O 2 has been studied; the equilibrium pressure of oxygen on ferrogarnet and magnetite has been calculated, and an equilibrium phase diagram of the type log P O 2 = f(composition) at 1473°K has been constructed. The relative stability of europium ferroperovskite and ferrogarnet has been evaluated.