Perovskite-type Solid Solutions Research Articles

Redox behavior and transport properties of La 0.5− xSr 0.5− xFe 0.4Ti 0.6O 3− δ (0< x

The range of perovskite-type solid solution formation in the system La 0.5− x Sr 0.5− x Ti 0.6Fe 0.4O 3− δ in oxidizing conditions is determined from X-ray diffraction and Mössbauer spectroscopy data to correspond to approximately 0–10% of the A-site concentration, similar to other numerous perovskite systems. Reduction and subsequent reoxidation of the oxides leads to a narrowing of this range and the segregation of the hematite phase at x=0.05. Increasing A-site deficiency results in the formation of oxygen vacancies and decreasing stability in reducing environments. The total conductivity of La 0.5− x Sr 0.5− x Ti 0.6Fe 0.4O 3− δ ( x=0.02–0.10) ceramics is essentially independent of composition in air and increases with increasing x in reducing atmospheres, due to increasing concentrations of n-type charge carriers and the formation of metallic iron. Partial decomposition of the perovskite phase in hydrogen, resulting in metal Fe formation, was found to be reversible when the A-site deficiency is small ( x<0.05). Mössbauer spectroscopic data showed that, contrary to other perovskite-type titanates–ferrites, the concentration of Fe 4+ cations in the perovskite lattice of oxidized La 0.5− x Sr 0.5− x Ti 0.6Fe 0.4O 3− δ is negligible.

Preparation and gas-sensitive properties of LaFe 1−yCo yO 3 semiconducting materials

In this paper, the ultrafine powders of the perovskite-type solid solution LaFe 1− y Co y O 3 (0≤ y≤0.40) were prepared by chemical coprecipitation. The preparation conditions, phase constituents, conductance and gas-sensing properties were investigated. The results demonstrate that LaFe 1− y Co y O 3 is a p-type semiconducting material and it has high sensitivity and good selectivity to C 2H 5OH when y=0.20.

Transport Properties and Thermal Expansion of Sr0.97Ti1−xFexO3−δ (x=0.2–0.8)

Increasing iron concentration in perovskite-type solid solutions Sr0.97Ti1−xFexO3−δ(x=0.2–0.8) was found to decrease the unit cell volume and to increase the partial ionic and electronic conductivities as well as the thermal expansion. The oxygen permeation through Sr0.97(Ti,Fe)O3−δ membranes is limited by both bulk ionic conduction and surface exchange rates. Prolonged stabilization under oxygen chemical potential gradients suggests formation of ordered microdomains in the perovskite lattice. The activation energy for the ionic conductivity, evaluated from oxygen permeation and Faradaic efficiency data, is nearly independent of iron content, varying in the range 97–104 kJ/mol. Thermal expansion coefficients of Sr0.97Ti1−xFex O3−δ (x=0.4–0.8) vary from (11.7–13.8)×10−6 K−1 at temperatures of 300–720 K to (16.6–27.0)×10−6 K−1 at higher temperatures. In comparison with the Sr(Ti, Co)O3−δ system, strontium titanates ferrites possess somewhat poorer transport properties, but lower thermal expansion.

Synthesis of compositionally homogeneous Pb(Zr xTi 1− x) 1− y(Mg 1/3Ta 2/3) yO 3 by wet–dry combination method and its dielectric properties

Ternary perovskite-type solid solutions of Pb(Zr x Ti 1− x ) 1− y (Mg 1/3Ta 2/3) y O 3 (PZTMT) were synthesized by a wet–dry combination method in which a coprecipitation process was applied to B-site cations (Zr 4+, Ti 4+, Mg 2+ and Ta 5+). The compositional homogeneity in calcined powders and dielectric properties of sintered ceramics were investigated. The powder X-ray diffraction analysis revealed that PZTMT powders prepared by the wet–dry combination method had no compositional fluctuation unlike that prepared by a conventional dry method. The sintered PZTMT prepared by the wet–dry combination method exhibited much higher dielectric constant than that prepared by the conventional dry method. The improvement of the compositional homogeneity and the density led to the improvement of the dielectric properties.

Structural Imperfection and Electrical Conductivity in the Perovskite-Type Solid-Solution System, Ba(Zn1/3Ta2/3)1-xMxO3-.DELTA. (M=Y, In, Ga).

Diffraction-line broadening was observed in the X-ray diffraction (XRD) profiles for samples sintered in a relatively short period in the perovskite-type solid-solution systems, Ba(Zn1/3Ta2/3)1-xMxO3-δ(M=Y, In, Ga), and it decreased with prolonged sintering period. The line broadening was ascribed to a fluctuation of lattice spacing (Δd/d) that originated from structural imperfections due to ionic displacement, based on a β cosθ vs sinθ plot. Furthermore, as a characteristic tendency, when a larger dopant ion substituted for the (Zn1/3Ta2/3) site in the solid-solution systems, a larger initial value of Δd/d was observed which also decreased upon prolonged sintering. Electrical conductivity was strongly dependent upon Δd/d. The relationship between electrical conductivity and Δd/d was discussed from the viewpoint of disordering of the oxide ion.

Critical behavior in the perovskite-like system Y(Ni,Mn)O3

The electrical and magnetic properties of the perovskite-type solid solution YNixMn1−xO3 have been studied at different temperatures and magnetic fields. Electrical conductivity measurements have shown a semiconductor behavior throughout the solid solution. The room temperature conductivity increases with the Ni concentration until x=1/3, and then decreases for higher Ni contents. The effective moment in the paramagnetic state shows a clear break at x=xcrit=1/3, presenting a constant value at x≤xcrit and decreasing at higher nickel concentration. In the ordered state, other magnetic parameters show similar discontinuities versus the nickel content, such as a sudden increase of the magnetization at high field or a maximum in the coercive strength of the ferromagnetic-like M(H) loop. By charge-conservation arguments we can readily identify such critical value as an optimum conversion of Mn3+ into Mn4+, leading to the charge-balance formula Y3+[Ni1/32+(Mn3+0.5Mn4+0.5)2/3]O32−, that is, equal amounts of Mn3+ and Mn4+ in the sample. Magnetic data shows that, at x≤xcrit, the solid solution behaves as a spin-glass and/or antiferromagnetic system, while at x>xcrit, full ferromagnetic characteristics are observed.

Ionic conductivity of La(Sr)Ga(Mg,M)O 3− δ (M=Ti, Cr, Fe, Co, Ni): effects of transition metal dopants

Oxygen-ion conductivity of the perovskite-type solid solutions (La,Sr)Ga 1−zM 2O 3− δ (M=Ti, Cr, Fe, Co; z=0–0.20), LaGa 1− y− z Mg y M z O 3− δ (M=Cr, Fe, Co; y=0.10–0.20, z=0.35–0.60) and LaGa 1− z Ni z O 3− δ ( z=0.20–0.50) was studied using the techniques of oxygen permeation, Faradaic efficiency, ion-blocking electrode and the e.m.f. of oxygen concentration cells. Oxygen-ion transference numbers vary from 2×10 −6 to 0.98 throughout the series and p-type electronic conductivity increases with increasing transition metal content. Substitution of Ga with higher valence cations (Ti, Cr) decreases ionic conductivity whereas small amounts of Fe or Co (∼5%) increase ionic conductivity. For higher transition metal contents, lower levels of oxygen-ion conductivity and an increase in the activation energy, E A, for ionic transport, from 60 (5%-doped) to 230 kJ/mol (>40%-doped) are observed. In heavily doped phases, E A tends to decrease with temperature and, above 1170 K, values are similar to the undoped phase suggesting that an order–disorder transition takes place. Factors affecting the observed ionic conductivity trends are discussed.

New oxyfluoride lead-free ferroelectric relaxors in the BaTiO 3–BaZrO 3–CaLiF 3 system

New dielectric oxyfluoride ceramics have been obtained in the ternary system BaTiO 3–BaZrO 3–CaLiF 3. X-ray powder diffraction studies determined the limits of perovskite-type solid solutions. Dielectric measurements were performed over the frequency and temperature ranges 10 2–2×10 5 Hz and 77–450 K respectively. This ternary system shows both classical ferroelectric and relaxor ferroelectric behaviours depending on the composition. These results are related to ionic substitution and chemical bonding. These compositions are of interest for applications (dielectrics for capacitors, actuators) since they are environment-friendly.

Correlation between defect chemistry and expansion during reduction of doped LaCrO 3 interconnects for SOFCs

Reduction of doped lanthanum chromite results in a linear expansion of the sample depending on dopant concentration, temperature, and oxygen content within the sample. In order to elucidate the defect structure of the perovskite-type oxide solid solution La 1− x Ca x CrO 3− δ ( x=0.10 and 0.20), the nonstoichiometry δ was measured as a function of oxygen partial pressure from 900 to 1100°C by means of a thermogravimetric method. The thermal expansion in reducing environments was measured as a function of oxygen partial pressure at 1000°C and then related to the change in oxygen stoichiometry. A linear correlation between thermal expansion and oxygen nonstoichiometry was found.

Oxygen nonstoichiometry and defect equilibrium in the perovskite-type oxides La 1− xSr xMnO 3+ d

Oxygen nonstoichiometry of perovskite-type oxide solid solution La 1− x Sr x MnO 3+ d ( x=0–0.5) is summarized in a plot of oxygen content, 3+ d, vs. log P(O 2) at 873–1273 K. For x≤0.4, two plateaus are observed in the plot, one around the stoichiometric point as to the lattice site, i.e. (3+ d)=3.00, and the other at low temperatures and high P(O 2) at which (3+ d) was saturated. At the latter plateau, the mean Mn valence was +3.3–3.4 independent of x and the saturated oxygen content decreased linearly with increasing x. A defect model is proposed to rationalize the (3+ d) vs. log P(O 2) relationship as well as the reported electronic properties between the two plateaus, which is based on the following assumptions. (i) Because the trivalent cation vacancies cause a large electronic imbalance and local lattice distortion, they do not stay close to each other and close to the Sr La′ site. To express the spacing among these defects, we introduce the concept of a vacancy excluding space around each cation vacancy and Sr La′. (ii) With the formation of cation vacancies, a nonbonding oxygen 2p level is formed by the oxide ions around the vacancies. The formed level is narrow and the mobility of electrons in this level is low. We assume that the conductive e g(↑) level and the low mobility t 2g(↓) level of Mn are very close to each other and that these levels are lower than the nonbonding oxygen 2p level. This nonbonding oxygen 2p level serves as the hole-trap. The (3+ d) vs. log P(O 2) relationships between the two plateaus can be explained by assuming that the volume of the vacancy excluding space around a trivalent cation vacancy and that around Sr La′ are nine and three unit cells, respectively, of pseudo-cubic perovskite-type ABO 3. Applying the proposed electronic structure, nonstoichiometry of the oxygen-deficient composition could be explained by the random distribution of oxide-ion vacancies. However, analysis based on the defect equilibrium could not identify the cation vacancy site, whether it is a La site or a Mn site or both a La and Mn site, because the results are almost identical whatever the cation vacancy site. Considering the reported neutron diffraction analysis results, we report a detailed analysis placing the cation vacancies predominantly on the La site.

EPR spectra and electrical conductivity of perovskite-like BaBi 1− xLn xO 3− δ (Ln = La, Pr)

Perovskite-type solid solutions of BaBi 1− x Pr x O 3− δ ( x = 0–1.0) and BaBi 1− x La x O 3− δ ( x = 0–0.4) were prepared by the standard ceramic synthesis technique. Substituting bismuth with the rare-earth cations was found to result in transition of monoclinic crystal lattice to orthorhombic at x ≥ 0.2 and in increasing perovskite unit cell volume. At moderate dopant concentrations, the electronic conduction in BaBi l− x Ln x O 3− δ occurs via transport of electron holes between Bi 5+ and Bi 3+ cations, decreasing with x and oxygen partial pressure. Electrical conductivity of the BaBi 1− x Pr x O 3− δ solid solutions at x = 0.4–0.8 was ascertained to be essentially independent of composition due to increasing contribution of praseodymium cations to the electronic transport. Relationships between electrical conductivity, electron paramagnetic resonance (EPR) and infrared absorption (IR) spectra of BaBi 1− x Ln x O 3− δ were discussed. Thermal expansion coefficients of the BaBi 1− x Ln x O 3− δ ceramics calculated from dilatometric data vary in the range of (11.0–13.0) × 10 −6 K −1 at 300–1050 K.

Piezoelectric and dielectric properties of solid solution of PbZrO 3–PbTiO 3–Pb(Mg 1/3Nb 2/3)O 3 system prepared by wet–dry combination method

Ternary perovskite-type solid solutions of Pb(Zr x Ti 1− x ) 1− y (Mg 1/3Nb 2/3) y O 3 (PZTMN) were synthesized by wet–dry combination method in which a wet-process was applied to B-site cations. These solid solutions had no compositional fluctuation and high density. Their dielectric constant and electromechanical planar coefficient ( k p) were higher than those of PZTMN prepared by the conventional dry method. Moreover, their compositional dependence of k p exhibited a peak at the morphotropic phase boundary, while PZTMN prepared by the dry method did not.

Theory of Morphotropic Phase Boundary in Solid Solution Systems of Perovskite-Type Oxide Ferroelectrics: p-e Hysteresis Loop

The p-e hysteresis loop near the morphotropic phase boundary in the perovskite-type oxide solid solution system is discussed on the basis of a Landau-type free energy function. It was found that the coercive field in the p-e hysteresis loop decreases upon approaching the morphotropic phase boundary and vanishes at the morphotropic phase boundary, because of the instability in the direction perpendicular to the polarization.

Cation ordering in the perovskite-type Sr2−xLaxCo1−yTa1+yO6

Abstract Perovskite-related solid solutions, Sr 2− x La x Cu 1− y Ta 1+ y O z , were synthesized by solid-state reaction at 1200°C in air. The crystalline phases were identified by powder X-ray diffraction. The perovskite-type solid solutions with different ordering types were observed including simple cubic perovskite-type (random), the Ba(Sr 1/3 Ta 2/3 )O 3 -type (1:2 ordering) and the (NH 4 ) 3 FeF 6 -type (1:1 ordering). The 1:2 ordering phase formed near the Sr 2 (Co 0.67 Ta 1.33 )O 6 composition. In the composition range, 0≤ y ≤0.2, the random and the 1:1 ordered phases formed. In the higher La 3+ content region, the 1:1 ordering phase was observed. The valency state of Co decreased with increasing La 3+ content and the perovskite cell expanded resulting from the increase of the average ionic radii on (Co,Ta) site. The 1:1 ordering of the ions on the octahedral site was found to be affected by the size difference of the ions more than the charge difference in this system.

Distortion and cation ordering in LaSr(Ni1−xCux)TaO6

The perovskite-type solid solution LaSr(Ni 1− x Cu x )TaO 6 was synthesized and characterized by powder X-ray diffraction and iodometric titration. The stoichiometric solid solutions were synthesized at 1050–1100°C in air. The compounds for 0≤ x ≤0.6 have the (NH 4 ) 3 FeF 6 -type structure with ordering between (Ni 2+ , Cu 2+ ) and Ta 5+ . Tetragonal distorted (NH 4 ) 3 FeF 6 -type phases due to the Jahn-Teller effect formed in the compositional range, x ≥0.7 and the monoclinic distortion was observed for x =0.9 and 1.0. This result suggests that the tetragonal distortion occurs when the amount of Cu 2+ becomes more than 35% of the octahedral site in this system.

A Theory of Morphotropic Phase Boundary in Solid-Solution Systems of Perovskite-Type Oxide Ferroelectrics

The origin of the appearance of the morphotropic phase boundary in perovskite-type oxide solid solution systems is clarified on the basis of a Landau-type free-energy function expressed in terms of polarization and strain components. The dielectric susceptibilities and piezoelectric constants are shown to increase in the vicinity of the morphotropic phase boundary.

Cation Ordering in the Oxygen Deficient Perovskite Sr2-xLaxMg1-yTa1+yOz.

Perovskite-type solid solutions, Sr2-xLaxMg1-yTa1+yOz, were synthesized and cation ordering between Mg2+ and Ta5+ in the solid solutions was investigated. In the Sr2-xLaxMgTaOz system, the (NH4)3FeF6-type (1:1 order) phase was formed in the compositional range, x≥0.4. However, few degree of the cation ordering between Mg2+ and Ta5+ was also observed in the solid solutions, 0≤x≤0.3. These results suggest that the cation ordering between Mg2+ and Ta5+ in Sr2-xLaxMgTaOz system is affected by the average charge of A-site cation or, in other words, by the oxygen content. The 1:2-type ordering was observed in the Sr2-xLaxMg1-yTa1+yOz solid solutions, x=0 for y≥0.3, and x≤0.1 for y=0.1, 0.2. In particular, the disappearance of the 1:2 ordering due to small amount of La3+ substitution was observed in the stoichiometric Sr2Mg0.67Ta1.33O6-LaSrMgTaO6 solid solution. However, in the case of the composition containing oxygen vacancy, the 1:2 ordered phase was formed at the composition Mg/Ta=0.8/1.2 and 0.9/1.1, although they have 20% and 35% higher Mg2+ content than in the case of Mg/Ta=1/2, respectively. These results suggest that the 1:2 ordered structure is stabilized by the presence of oxygen vacancy.

Synthesis and Lattice Distortion of the Perovskite-Type Oxides Sr2(Sr1-xMx)TaOz(M=Ca, Nd).

The perovskite-type solid solutions Sr2(Sr1-xMx)TaOz (M=Ca, Nd) were synthesized. Lattice distortion and cation ordering in the octahedral site of the perovskite type structure were investigated using Rietveld analysis of powder X-ray diffraction data. Cubic perovskite-type phases with cation ordering in octahedral site were obtained for Sr2(Sr1-xCax)TaO5.5 (x=0-1), and Sr2(Sr0.9Nd0.1)TaO5.55. Two types of monoclinic perovskite-type phases were obtained for Sr2(Sr1-xNdx)TaOz. The unit cell of the solid solutions for x=0.3-0.7 was close to a tetragonal cell. The distortion of the solid solutions was due to tilting octahedra and a difference in the direction of tilt was observed between the compounds for x=0.3-0.7 and x=0.9-1.0. Degree of monoclinic distortion increased with increasing Nd3+ content. Cation ordering in the octahedral sites was observed in all the compositions investigated for Sr2(Sr1-xNdx)TaOz. Increase of Nd3+ content possibly causes increase of oxygen content z. The present results suggest that increase of oxygen content is one of the reasons to cause the lattice distortion for Sr2(Sr1-xMx)TaOz (M=Ca, Nd).

Oxygen ionic transport in A-site-deficient perovskites La(Pb)FeO 3

Formation of perovskite-type solid solutions were found in the oxide system La 1−xPb xFeO 3−δ (x = 0.1–0.3). Synthesis and sintering of the ceramics at temperatures above 1300 K led to a cation deficiency in the lanthanum sublattice, due to lead oxide evaporation. Electrical conductivity and oxygen permeation fluxes were ascertained to increase with increasing x. Oxygen permeability values of the La 1−xPb xFeO 3−δ cation-deficient ceramics were considerably less than those of La 1−xSr xFeO 3−δ. Permeation fluxes through La 1−xPb xFeO 3−δ were demonstrated to be limited by the bulk ionic conductivity. Thermal expansion coefficients of the oxides were (10.0–10.4) × 10 −6 K −1.

Lithium Ion Mobility and Activation Energy for Lithium Ion Conduction in A-Site Deficient Perovskites La&lt;sub&gt;1/3-&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Li&lt;sub&gt;3&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;TaO&lt;sub&gt;3&lt;/sub&gt;

The lithium ion conductivity of perovskite-type solid-solutions La1/3-xLi3xTaO3 was measured by the ac impedance method and the activation energy for conduction was determined. The lithium ion mobility was calculated in order to discuss the conductivity behavior. While the ordering at the A-sites as well as the tetragonal distortion lowered the mobility for tetragonal perovskites (x≤0.06), the mobility decreased with decreasing the lattice parameter for cubic perovskites (0.075≤x≤1/6). The logarithm of the mobility was found to decrease linearly with increasing the activation energy. It was concluded that the lithium ion mobility was the major factor in influencing lithium ion conduction in this solid-solution system.