Cation Size Disorder Research Articles

Chemical durability of multicomponent lanthanide zirconate solid solutions

Multicomponent zirconate solid solutions A2Zr2O7 (A= (Sm,Yb), (Sm,Yb,Gd), (Sm,Yb,Gd,Er), and (Sm,Yb,Gd,Er,Dy)) were synthesized through spark plasma sintering and their chemical durability was investigated via semi-dynamic leaching tests in pH = 1 nitric acid at 90 °C. Incongruent dissolution occurred for all samples with the preferential elemental release of zirconium in all A2Zr2O7 except for high entropy (Sm,Yb,Gd,Er,Dy)Zr2O7 which displays a congruent release of the A-site elements and Zr. A strong positive correlation was found between the lanthanide elemental release rates and the A-site cation size disorder, whereas a negative correlation was observed between the elemental release rates and mixing entropy.

Preparation and thermophysical properties of a novel dual-phase and single-phase rare-earth-zirconate high-entropy ceramics

A novel non-equimolar (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 composed of coexisting defect fluorite and pyrochlore phases and (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 with defect fluorite structure high entropy ceramics were firstly prepared by high temperature solid state method. The average grain growth rate of high entropy (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 and (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 ceramics is much lower than that of Nd2Zr2O7 during annealing at 1300 °C for 100 h, which may be ascribed to the sluggish diffusion effect of high entropy ceramics. Additionally, both of (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 and (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 possess higher thermal expansion coefficients and lower thermal conductivities than Nd2Zr2O7. The thermal expansion coefficients of (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7 and (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 at 1000 °C are 12.9 × 10−6 and 12.6 × 10−6 K−1, respectively. In addition, the thermal conductivity of dual-phase (Nd0.58Gd0.05Y0.05Er0.05Yb0.27)2Zr2O7 at room temperature up to 1000 °C is significantly lower than that of single-phase (Nd0.2Gd0.2Y0.2Er0.2Yb0.2)2Zr2O7, such as, 1.215 and 0.957 W·m−1·K−1 at 1000 °C, respectively, which may be due to cation size disorder and lattice distortion.

Size disorder: A descriptor for predicting the single- or dual-phase formation in multi-component rare earth zirconates

Eighteen different 5-component equimolar rare earth zirconates were successfully synthesized by solid state reaction method. Among them, 5 of them exhibit a high-entropy fluorite structure, 8 have a high-entropy pyrochlore structure, and the remaining 5 have dual-phase structures of both pyrochlore and fluorite. For high-entropy rare earth zirconates with single-phase structure, the lattice parameter is almost linearly correlated with the average cation radius ratio. In combination with a large number of previous reports, a summary is made to study the synergistic effects of the average cation radius ratio, configuration entropy and size disorder on the single-phase formation ability of multi-component rare earth zirconates. The conclusion verifies that the size disorder of about 5% is the boundary for the multi-component rare earth zirconates to form a single-phase or dual-phase region, while the average cation radius ratio of 1.467 is the demarcation between a pyrochlore and a defective fluorite structure in the single-phase zone, and the configuration entropy had less effect. Excessive size disorder leads to the enrichment or barrenness of certain components, which is the main reason for the formation of dual phase ceramics. • 18 Different equimolar 5-component rare earth zirconates have been prepared. • The size disorder ( δ *) can predict the single-phase formation ability of zirconates. • δ * = 5% is the boundary between the single- and dual-phase rare earth zirconates. • The single-phase forming ability of rare-earth zirconates is independent of entropy.

Influence of A-Site cation size-disorder on the magnetocaloric response of La0.6CaxSr0.4-xMnO3 nanoparticles

Building on previous work finding a significant magnetocaloric effect (MCE) in manganite nanoparticles having the formula La0.60Cax Sr0.40-xMnO3 (0<x<0.4), which show promise for application in magnetic refrigeration systems. These were synthesized via a Pechini sol-gel method, as verified by powder x-ray diffraction and laser ablation-inductively coupled plasma-mass spectrometry. The temperature-dependence of the magnetic entropy change between 200K and 320K in a magnetic field of μH=3T was evaluated by means of Maxwell’s relations and isothermal MH curves. The reported values of ΔSmax increase from 1.5 J kg-1K-1 to 5.6 J kg-1K-1 as x increases from 0 to 0.4. Ms; crystallite size and b and c lattice parameters increase with x. The results show a clear positive correlation between calcium concentrations and the value of -ΔS, suggesting that a larger atom at the calcium site negatively impacts the magnetocaloric effect, thereby decreasing magnetic refrigeration viability.

Combustion synthesis and characterization of Ln1−xMxCr0.9Ni0.1O3 (Ln = La and/or Nd; M = Sr and/or Ca; x ≤ 0.25) perovskites for SOFCs anodes

A series of chromite perovskites with the general formula Ln1−xMxCr0.9Ni0.1O3 (Ln = La and/or Nd; M = Sr and/or Ca; x ≤ 0.25) has been prepared by three combustion synthesis routes using a different combustible substance each time: glycine, urea and sucrose. In order to isolate the effect of divalent dopant concentration from the A cation steric effects, the whole group has a fixed mean A cation radius, < rA> ≈ 1.22Å, and cation size disorder, σ2(rA) ≈ 0.0001Å2, but variable doping x. Their crystal structure, microstructure, electrical properties and expansion coefficients have been investigated on the basis of their possible use as anode materials for intermediate temperature solid oxide fuel cells (SOFC). Cell parameters, grain sizes, expansion coefficients and conductivities all are found to be dependent on x and the combustible substance used. The most interesting relationship is the negative dependence of the conductivity with x under H2 atmosphere: conductivity decreases with doping which is the opposite to the expected behavior for a p-type doped perovskites and has not been reported before.

Effect of synthesis conditions on electrical and catalytical properties of perovskites with high value of A-site cation size mismatch

The perovskite La0.15Sm0.35Sr0.08Ba0.42FeO3−δ has been prepared by the glycine nitrate route, varying the calcination temperature, fuel/oxidizer ratio and cooling rate, in order to study the sample preparation influence on the properties in the context of their application as a electrode material for SOFCs. The obtained materials have been characterized by X-ray diffraction, scanning electron microscopy, electrical and BET surface area measurements, and also the reaction between oxygen and CO, which can occur in SOFCs during the conversion of chemical energy into electrical energy. As overall results, all the samples present a phase segregation showing two perovskites with rhombohedral structure. SEM images show a well-necked morphology of the powders which are composed of nanosized particles and agglomerations of grains. The BET specific surface area of the samples decreases as calcination temperature increases, as well as for the quenched sample. The measured electronic conductivity values (<50 S/cm) are characteristic for samples with these high values of σ2(rA) (A cation size disorder). The catalytic activity tests for the CO oxidation reaction showed a T50% (“light-off temperature”, defined as the temperature at which 50% conversion of CO is achieved) value about 440°C–450 °C, CO conversion reaching 100% at approximately 600 °C for all the prepared perovskites. Then, for the La0.15Sm0.35Sr0.08Ba0.42FeO3−δ perovskite, CO conversion temperature is lower than usual SOFCs operating temperature. This points out to the technological interest of these materials in the framework of reducing the operating temperature of SOFCs.

Spin Nematicity and Large Low‐Field Positive Magnetoresistance in a Half‐Doped Manganite: An Approach Exploiting Cation Size Disorder

In principle, material disorder such as random dopants disfavors the long range electronic liquid crystal (ELC) order in quantum matters. It is reported that a minor A site cation size disorder (σ2 = 0.001 Å2) in half‐doped manganites (La1‐xYx)0.5(Ca1‐ySry)0.5MnO3 interestingly leads to spin nematicity which is accompanied by a large positive magnetoresistance (up to 40%) at fields (H ≤ 1 kOe at 60 K) much lower than that (Hcr ≈ 35 kOe) critical for melting the weakened antiferromagnetic charge‐orbital ordering (AFM‐COO). The spin nematicity, emerging with a suppressed orthorhombicity, is ascribed to the destabilization of the d3x2‐r2/d3y2‐r2 orbital stripe alternation and the consequent magnetic symmetry breaking. The non‐trivial low field magneto­resistance is found to be determined by the spin polarized hopping along the nematic filaments and across the perpendicular ferromagnetic zigzag Mn–O chains in the AFM‐COO host. While the present low‐field magneto­resistance holds great potential for future spintronic devices, the approach for spin nematic order based on cation size disorder opens up a new avenue for questing ELC and exotic properties in strongly correlated materials.

Effect of the Cation Size Disorder at the A-Site on the Structural Properties of SrAFeTiO&amp;lt;SUB&amp;gt;6&amp;lt;/SUB&amp;gt; Double Perovskites (A = La, Pr or Nd)

In this paper, the cation size disorder effect of the A-site on the structural properties of the SrAFe- TiO6 (A = La, Pr or Nd) was investigated. The compounds were synthesized—as the best of our knowledge—for the first time by conventional and precursor method to get crystalline materials. The results obtained from the experimental measurements carried out on new double perovskite materials were presented. The data of X-ray diffraction (XRD), Fourier Transform Infra Red FTIR were measured at room temperature. From the X-ray diffraction, and by means of standard Rietiveld method, all the samples have the same structure (orthorhombic) with Pnma space group. The difference in the tolerance factor is clearly noticed and refers to the cation size disorder at the A-sites. The Fourier Transform Infra Red FTIR measurement has been done; the results of it confirm the double perovskite structure and the difference between the samples were noticed. The tolerance factors for the samples altered from SrLaFeTiO6 up to SrNdFeTiO6 and this difference return to ionic radius and cation size effect.

Structure and properties of perovskites for SOFC cathodes as a function of the A-site cation size disorder

The structure, electronic conductivity and electrochemical properties of the oxide perovskites with the Ln0.5M0.5FeO3−δ composition (Ln=La, Sm; M=Ba, Sr) are studied in the context of their possible use as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The main objective of the work is to analyse the influence of the A-site cation size disorder (σ2(rA)) on the properties of these compounds. The A-site cation size disorder of these samples varies from 0.0286 to 0.089Å2, whilst the doping (x) and the average ionic radius of the A-site cations (<rA>) are fixed to 0.5 and ~1.30Å, respectively.The samples are prepared using the glycine-nitrate method. X-ray powder diffraction (XRD) shows that all of the compounds have rhombohedral symmetry (space group: R-3c). The structure is highly dependent on σ2(rA); the lattice parameters and unit cell volume decrease with decreasing σ2(rA). All of the samples have a porous microstructure with fine grain sizes (100–250nm).The electrical conductivity (σ) measurements on the bulk samples shows a strong dependence on the A-site cation size disorder. Electrochemical impedance spectroscopy (EIS) measurements of the cathodes on the yttria-stabilised zirconia (YSZ) electrolyte-based symmetrical cells indicate that the lowest polarisation resistance value is observed for the sample with the lowest σ2(rA). The effect of introducing a samarium doped ceria (SDC) interlayer between the best performing cathode and the electrolyte is also studied.

Effect of A-site cation size on magnetic and charge-ordering properties of Ln0.5Sr0.5Mn0.9Cu0.1O3 (Ln=La, Pr, Nd, or Ho)

The structural, magnetic, and electrical properties of Ln0.5Sr0.5Mn0.9Cu0.1O3 (Ln = La, Pr, Nd, or Ho) perovskite manganites have been investigated to explore the influence of A-site cation radius (〈rA〉) and the A-site cation size-disorder (σ2) on the various interdependent phenomena such as ferromagnetism (FM), phase separation (PS), and charge ordering (CO). The temperature dependence magnetization (M–T) curve of La-based sample shows four distinct points at ∼269 K, 255 K, 200 K, and 148 K corresponding to strong FM, cluster glass (CG), weak FM, and charged ordered antiferromagnetic (COAFM) transitions, respectively. Our investigation shows that Neel temperatures (TN) increases, whereas Curie (TC) and irreversibility temperatures (Tirr) decrease with decreasing 〈rA〉, i.e., with increasing σ2. Furthermore, the value of the magnetization decreases and resistivity increases with decreasing 〈rA〉. All samples exhibit insulating behavior in the temperature range 77–300 K and above 110 K the electronic conduction mechanism has been described within the framework of the variable range hopping (VRH) model.

Characterization of Ln 0.5M 0.5FeO 3–δ (Ln = La, Nd, Sm; M = Ba, Sr) perovskites as SOFC cathodes

Ln 0.5M 0.5FeO 3–δ (Ln = La, Nd, Sm; M = Ba, Sr) perovskites have been synthesised by the glycine-nitrate route in order to obtain compounds with good morphological and electrical characteristics to be used as SOFC cathodes. A systematic study of the influence of the average ionic radius of the A cation (<r A>), on the structural, morphological and electrical properties of these perovskites has been performed. The obtained materials have been characterised by ICP-AES analysis, X-ray powder and neutron powder diffraction, scanning electron microscopy and four-probe conductivity measurements. In this study <r A> has been changed in the range 1.34 ≥ <r A > ≥ 1.25 Å, while keeping constant the doping level, x = 0.5 (in A position) and A cation size disorder, σ 2 (r A) = 0.0161 Å 2. Overall, the sample with the higher average ionic radius showed the highest values of conductivity. Chemical compatibility of the proposed cathodes with yttria-stabilised zirconia electrolyte (YSZ) has also been analysed. The data indicate that additional phase formations exist in all systems at temperatures above ~ 1000 °C.

Effect of the A Cation Size Disorder on the Properties of an Iron Perovskite Series for Their Use as Cathodes for SOFCs

AbstractIn order to develop a cathode that can be used in intermediate temperature solid oxide fuel cells, a family of iron perovskites with the general formula Ln0.5M0.5FeO3–δ (A = Ln1–xMx; Ln = La, Nd and/or Pr; M = Sr, Ca or/and Ba) with fixed divalent dopant concentration x to 0.5, mean A cation radius 〈rA〉 to 1.25 Å but with different A cation size disorder σ2(rA) has been prepared by the conventional solid state reaction. These compounds have been characterised by power X‐ray diffraction (XRD), high‐resolution scanning electron microscopy (SEM), electrical conductivity, I–V measurements and AC impedance spectroscopy. All the samples have a rhombohedral crystal structure (space group: R‐3c). SEM images show that the average grain size increases with increasing σ2(rA) in the bulk samples. The DC four‐probe measurements indicate that electrical conductivity is strongly dependent on the σ2(rA). Electrochemical impedance spectra at 700 and 800 °C revealed that better electrochemical cathode performances are for samples with smaller values of σ2(rA).

Structural and magnetic ordering inLa0.5Ca0.5−xBaxMnO3 (0 < x ≤ 0.5) manganite

We report a structural, magnetic and transport study inLa0.5Ca0.5−xBaxMnO3 (0<x≤0.5) using neutron diffraction, magnetization and resistivity measurements.The samples crystallize in an orthorhombic structure withPnma space group and no structural transition is observed with Ba doping.Substituting Ca with Ba leads to an increase in average A-site ionic radii,⟨rA⟩ and cationsize disorder (σ2). Consequently, the nature of magnetic ordering undergoes changes as a function ofx. At lowtemperature for x≤0.1, the CE-type antiferromagnetic spin structure is stabilized, similar to the parent compound. Atx = 0.15, the magnetic phase is a mixture of CE-type and A-type spin structure. Forx≥0.2, the ferromagnetic phase is stabilized. The signature of orbital orderingin addition to spin and charge ordering is observed in all samples withx≤0.15.TC,TCO andTN remain nearlyconstant for x≤0.2. The transport studies reveal that for samplesx≤0.2, adiabatic hopping of small polarons governs the transport behaviour aboveTCO (∼160 K). A parallel combination of small polaron hopping and variablerange hopping mechanisms is found to describe resistivity data belowTCO. Atx = 0.2, an insulator to metal transition is observed as temperature is lowered, and atx = 0.5 the sample becomes metallic below 280 K. In the metallic regime, resistivity data followsthe Bloch–Grüneisen model together with contributions from high frequency optical phononsand magnetic scattering. The experimentally established phase diagram from the presentstudy is compared with theoretical studies in the intermediate electron–phonon interactionregime.

Structural and magnetic characterisation of the perovskite oxides La0.7Ca0.3−x NaxMnO3

Abstract X-ray powder diffraction (XRD) and magnetic measurements were performed in order to investigate the effect of Na+ ion substitution for Ca2+ ions on the crystallographic structure, the character of magnetic ordering, and the effect of transition temperature in La0.7Ca0.3−x NaxMnO3 manganites series (0 ⩽ × ⩽ 0.2). All samples crystallise in an orthorhombic structure with the Pnma space group. We have found a strong dependence of structural and magnetic properties on the cation-size disorder parameter σ 2. The temperature dependence of magnetization of all samples obeys the Bloch T 3/2 law. The values of the spin wave constant at low temperature B increase with the increase of x and the Curie temperature decreases. It is concluded that the substitution of Ca by Na+ ions causes a decrease in total exchange integral Aof the samples.

Effects of quenched disorder on Griffiths phase and EPR line width in La0.67–2xNd2xCa0.33–xSrx MnO3 manganites

AbstractIt is widely accepted that there is Griffiths phase (GP) present in the paramagnetic phase in doped manganites. We studied the effect of quenched disorder on the region of GP in connection with the temperature dependence of the line width (ΔH) of the electron paramagnetic resonance (EPR) in La0.67–2x Nd2x Ca0.33–x Srx MnO3 (x = 0, 0.1, 0.15, 0.2 and 0.25). With this approach it is found that ΔH (T), the minimum in the line width (ΔHmin) and Tmin are reproduced. With increasing values of the quenched disorder the values of ΔHmin are found to increase. A numerical relation that correlates the temperature range in which GP exists and the cation size disorder is obtained and found to be in agreement with some reported data. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Phase separation and stability inSm0.50Sr0.50MnO3: effects of cation dopants

The Sm0.50Sr0.50MnO3 system has been studied with respect to identifying the various electronicand magnetic phases and the effects of introducing different cation dopantson these phases and their respective stabilities. Magnetic, resistive andthermoelectric studies have been performed as a function of temperature inSm0.50Sr0.50MnO3,(Sm0.50Nd0.50)0.50Sr0.50MnO3 andSm0.50(Sr0.50Ca0.50)0.50MnO3. The parentcompound (Sm0.50Sr0.50MnO3) lies close to the ferromagnetic–antiferromagnetic (FM–AFM) phase boundary and theconsequent effects of the dopants are explained in terms of the changes in the one electronbandwidth and the cation size variation and disorder. Below 130 K the ground state forSm0.50Sr0.50MnO3 composition is seen to be a mixture of ferromagnetic and A-type antiferromagnetic phases.Partial substitution of a larger size cation (Nd) in place of Sm raises the ferromagnetictransition temperature while at the same time stabilizing the AFM phase. This latter trendis inferred from the differences in the hysteresis behaviour of the two systems and isexplained in terms of the decrease of the size mismatch factor between cations. However,partial substitution of the smaller size cation (Ca) in place of Sr almost eliminates theferromagnetic phase, leaving the system in a charge ordered state, which appears to beaccompanied by the CE type of charge–orbital ordering, exhibiting extremely large valuesof resistivity.

Isolating the effect of doping in the structure and conductivity of (Ln 1− xM x)FeO 3− δ perovskites

A series of iron perovskites with the general formula AFeO 3− δ (A = Ln 1− x M x ; Ln = La, Nd and/or Pr; M = Sr and/or Ca) has been prepared by conventional solid state reaction. In order to isolate the effect of divalent dopant concentration from the A cation steric effects, the whole group has a fixed mean A cation radius < r A> ≈ 1.22 Å and cation size disorder σ 2( r A) ≈ 0.003 Å 2 but variable doping x. The structure changes with x from orthorhombic (0.2 ≤ x ≤ 0.4) through rhombohedral (0.5 ≤ x ≤ 0.7) to a mixture of rhombohedral and cubic for x = 0.8. SEM images show that the average grain size increases with alkaline–earth content. All samples show a systematic dependence of conductivity upon doping, which can be directly related to the concentration of charge carriers. Conductivity fits indicate a p-type semiconducting, small-polaron hopping mechanism, in which the activation energies remain similar for all doping levels, x, throughout the series.

A-Site Cational Size Disorder Effects in La&lt;sub&gt;0.55&lt;/sub&gt;Ca&lt;sub&gt;0.45&lt;/sub&gt;MnO&lt;sub&gt;3&lt;/sub&gt;

The effects of A-site cation size disorder in ABO3 type ferromagnetic metallic La0.55Ca0.45MnO3 system have been studied by substituting La3+ and Ca2+ by other ions, while keeping the valency of Mn ions and the A-site cation mean radius &lt;rA&gt; constant in the substituted compounds with different A-site ionic radii variance σ2 = Σi (xiri 2 − &lt;rA&gt;2), where xi and ri are the atomic fraction and ionic radii of i-type ions at A-site, respectively. It is revealed that the A-site disorder induces the decreasing of the magnetization and the increasing of the resistivity. The ferromagnetic Curie point TC decreases with the increasing of the A-site ionic radii variance, too. Moreover, the ground state of the system transits from ferromagnetic metal to glass insulator upon increasing variance of the A-site ionic radii from 0.0003 for La0.55Ca0.45MnO3 to 0.009 for Gd0.55Sr0.45MnO3. It is argued that the suppression of the ferromagnetism is ascribed to the enhanced radial distortion of the MnO6 octahedra due to the increasing A-site disorder.

Influence of A-site cation size-disorder on structural, magnetic and magnetocaloric properties of La 0.7Ca 0.3− xK xMnO 3 compounds

The structural and magnetic properties of perovskite oxides La 0.7Ca 0.3− x K x MnO 3 (0 ≤ x ≤ 0.15) have been investigated to explore the influence of the A-site cation size-disorder ( σ 2). The materials were prepared by the solid-state method and then characterized by X-ray diffraction (XRD). The XRD data have been analyzed by Rietveld refinement technique. For K doping concentration x ≤ 0.075, the samples crystallize in the orthorhombic structure, while for x ≥ 0.1, the structure becomes rhombohedral. The variation of the magnetization M as a function of the applied magnetic field μ 0 H reveals the presence of a structural distortion leading to a reduction of the magnetization at low μ 0 H values. When increasing μ 0 H, the structural distortion decreases and for a high applied magnetic field, the M ( μ 0 H) curves saturate indicating the disappearance of the structural distortion. The influence of K doping concentration and the applied magnetic field on the magnetocaloric properties has been considered. A large magnetic-entropy change (|Δ S M| ∼ 5 J/kg K) is obtained in all samples at Curie temperatures between 270 and 280 K for an applied magnetic field of 3 T. These results show that these materials can be used as candidates for magnetic refrigerants near room temperature.

A comparative study on metamagnetic transitions in half-doped Pr 0.45La 0.05Ca 0.5MnO 3 and Pr 0.45Gd 0.05Ca 0.5MnO 3 manganites

Half-doped Pr 0.5Ca 0.5MnO 3 is an ABO 3-type, charge-ordered antiferromagnetic system with a negligible A-site cation size disorder ( σ 2). The A-site substitution by larger or smaller ions than Pr 3+/Ca 2+ in this system increases σ 2. We have studied and compared the σ 2 and cation size effects on metamagnetic transitions by substituting 5 atomic percentage of isovalent larger ion La 3+ or smaller ion Gd 3+ at A-site in this system. We find that the La substitution induces successive sharp step-like metamagnetic transitions at low temperature, in contrast to a smooth metamagnetic transition induced by Gd substitution.