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The perovskite (1−x)YMnO3−xBiFeO3 binary system is very promising because of its multiferroic end members. Nanocrystalline phases have been recently obtained by mechanosynthesis across the system, and the perovskite structural evolution has been described. Two continuous solid solutions with orthorhombic Pnma and rhombohedral R3c symmetries were found, which coexist within a broad compositional interval of 0.5 ≤ x ≤ 0.9. This might be a polar‐nonpolar morphotropic phase boundary region, at which strong phase‐change magnetoelectric responses can be expected. A major issue is phase decomposition at moderate temperatures that highly complicates ceramic processing. This is required for carrying out a sound electrical characterization and also for their use in devices. We present here the application of Spark Plasma Sintering to the ceramic processing of YMnO3‐BiFeO3 phases. This advanced technique, when combined with nanocrystalline powders, allowed densifying phases at reduced processing temperatures and times, so that perovskite decomposition was avoided. Electrical measurements were accomplished, and the response was shown to be mostly dominated by conduction. Nonetheless, the intrinsic dielectric permittivity was obtained, and a distinctive enhancement in the phase coexistence region was revealed. Besides, Rayleigh‐type behavior characteristic of ferroelectrics was also demonstrated for all rhombohedral phases. Magnetic characterization was performed in this region, and antiferromagnetism was shown. [ABSTRACT FROM AUTHOR]

Abstract: The ferromagnetic perovskite oxide BiMnO3 is a highly topical material, and the solid solutions it forms with antiferromagnetic/ferroelectric BiFeO3 and with ferroelectric PbTiO3 result in distinctive polar/nonpolar morphotropic phase boundaries (MPBs). The exploitation of such a type of MPBs could be a novel approach to engineer novel multiferroics with phase‐change magnetoelectric responses, in addition to ferroelectrics with enhanced electromechanical performance. Here, the interplay among crystal structure, point defects, and multiferroic properties of the BiMnO3–BiFeO3–PbTiO3 ternary system at its line of MPBs between polymorphs of tetragonal P4mm (polar) and orthorhombic Pnma (antipolar) symmetries is reported. A strong dependence of the phase coexistence on thermal history is found: phase percentage significantly changes whether the material is quenched or slowly cooled from high temperature. The origin of this phenomenon is investigated with temperature‐dependent structural and physical property characterizations. A major role of the complex defect chemistry, where a Bi/Pb‐deficiency allows Mn and Fe ions to have a mixed‐valence state, in the delicate balance between polymorphs is proposed, and its influence in the magnetic and electric ferroic orders is defined. [ABSTRACT FROM AUTHOR]

The perovskite structure is one of the most wonderful to exist in nature. It obeys to a quite simple chemical formula, ABX 3 , in which A and B are metallic cations and X, an anion, usually oxygen. The anion packing is rather compact and leaves interstices for large A and small B cations. The A cation can be mono, di or trivalent, whereas B can be a di, tri, tetra, penta or hexavalent cation. This gives an extraordinary possibility of different combinations and partial or total substitutions, resulting in an incredible large number of compounds. Their physical and chemical properties strongly depend on the nature and oxidation states of cations, on the anionic and cationic stoichiometry, on the crystalline structure and elaboration techniques, etc. In this work, we review the different and most usual crystalline representations of perovskites, from high (cubic) to low (triclinic) symmetries, some well-known preparation methods, insisting for instance, in quite novel and original techniques such as the mechanosynthesis processing. Physical properties are reviewed, emphasizing the electrical (proton, ionic or mixed conductors) and catalytic properties of Mn- and Co-based perovskites; a thorough view on the ferroelectric properties is presented, including piezoelectricity, thermistors or pyroelectric characteristics, just to mention some of them; relaxors, microwave and optical features are also discussed, to end up with magnetism, superconductivity and multiferroïsme. Some materials discussed herein have already accomplished their way but others have promising horizons in both fundamental and applied research. To our knowledge, no much work exists to relate the crystalline nature of the different perovskite-type compounds with their properties and synthesis procedures, in particular with the most recent and newest processes such as the mechanosynthesis approach. Although this is not intended to be a full review of all existing perovskite materials, this report offers a good compilation of the main compounds, their structure and microstructure, processing and relationships between these features. [ABSTRACT FROM AUTHOR]

Abstract: RECo0.50Mn0.50O3 perovskites (RE: La, Y, Er) have been prepared by an auto-combustion method and calcined at 700°C, 850°C and 950°C. YCo0.50Mn0.50O3 forms at temperatures as low as 700°C, much lower than those required in solid-state ceramic synthesis. All materials show adequate specific surface when calcined at 700°C. ErCo0.50Mn0.50O3, which has the largest surface area, shows the lowest activity due to the presence of segregated phases, while YCo0.50Mn0.50O3, which reaches the perovskite structure at lower temperatures, exhibits the highest catalytic activity. The catalytic activity, evaluated from the total combustion of methane, can be related to the presence of the perovskite structure, an adequate surface area and the reducibility of the material. At the calcination temperatures of 850°C and 950°C, at which the surface area and crystalline degrees are almost the same for the La-, Y- and Er-systems, no differences in the catalytic activity were detected. The catalytic activity, higher for materials calcined at lower temperatures, is progressively lost due to sintering processes occurring at higher calcinations temperatures, as confirmed by SEM observations. [Copyright &y& Elsevier]

Abstract: The magnetic phase transition and magnetic entropy change (−ΔS M ) in the La0.40Nd0.30Sr0.30Mn0.70Cr0.30O3 perovskite were investigated by measuring the magnetization as a function of temperature. The ferromagnetic transition temperature T C and the critical exponents β and γ, determined by analyzing the Arrott plots, are found to be T C =191K, β =0.433, γ =1.053 and δ =3.486. These values for the critical exponents are close to the mean-field values. In order to estimate the spontaneous magnetization M s (T) at a given temperature, we use a process based on the analysis, in the mean-field theory, of the magnetic entropy change (−ΔS M ) versus the magnetization data. An excellent agreement is found between the spontaneous magnetization determined from the entropy change ((−ΔS M ) vs. M 2) and the classical extrapolation from the Arrott curves (μ 0 H/M vs. M 2), thus confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in this system and in other compounds as well. [Copyright &y& Elsevier]

Abstract: The effect of Ti substitution on the magnetocaloric effect of the La0.70Sr0.30MnO3 perovskite was investigated. The magnetic entropy change (−ΔS M ) was deduced by two methods: a Maxwell relation and the Landau theory. The magnetocaloric data displays a large value of the magnetic entropy change (−ΔS M ) near the Curie temperature (T C =210K), which increases when increasing the applied magnetic field. A good agreement is found between the experimental (−ΔS M ) and the one estimated by Landau theory. The relative cooling power values vary from 49 to 288Jkg−1 upon variation of the applied magnetic field at 1 and 5T. Under 5T, the relative cooling power value for La0.70Sr0.30Mn0.90Ti0.10O3 is about 70% of the conventional refrigerant Gd material. As a result, the herein reported compound can be considered as a promising material in magnetic refrigeration technology. [Copyright &y& Elsevier]

Abstract: We have successfully prepared a single-phase Ce-doped sample (La0.56Ce0.14)Sr0.30MnO3 by a solid-state route, without CeO2 oxide segregation. Compared to the undoped sample La0.70Sr0.30MnO3, the structure remains hexagonal with space group without a large variation of the cell parameters and volume. The thermal variation of the magnetization shows that the Ce-doped sample exhibits a ferromagnetic to paramagnetic transition at 357K and an anti-ferromagnetic coupling below 100K between the ferromagnetic Mn moments and the Ce moments and thus form a ferrimagnetic ground state. Comparing our results to those of the literature, we can suggest that the cerium ions are in a trivalent state (Ce3+) and the anomalous behaviour attributed to a Kondo-like effect. [Copyright &y& Elsevier]

Abstract: Substituted LaMn1−y Co y O3 perovskite-type oxides (0.0≤ y ≤1.0), prepared by the citrate method, have been investigated as catalysts in the total combustion of acetylacetate. The characterization results indicate variation in specific surface area, crystal structure, infrared spectra and reducibility. The crystal phase transformation was found to occur for y Co values above 0.5. The role of Mn is to stabilize the cobaltite structure, which does not tolerate more than 30% substitution of Co for Mn. The catalytic activity in the total combustion of acetylacetate can be related with changes in the crystal structure and specific surface area. The higher intrinsic activity of the cobaltite doped with 10wt% Mn, y Co =0.9, can be explained by the changes in crystal and electronic properties. Magnetic properties (susceptibility, ZFC/FC and M-loops) show strong ferromagnetic interactions in the range (0.0< y Co ≤0.5) while antiferromagnetism is progressively settled at higher cobalt concentrations. [Copyright &y& Elsevier]

Abstract: The transport properties of Nd-doped perovskite materials (La0.7−x Nd x )Sr0.3Mn0.7Cr0.3O3 (x≤0.30) were investigated using impedance spectroscopy techniques over a wide range of temperatures and frequencies. AC conductance analyses indicate that the conduction mechanism is strongly dependent on temperature and frequency. The DC conductance plots can be described using the small polaron hopping (SPH) model, with an apparent reduction of the polaron activation energy below the Curie temperature T C. Complex impedance plots exhibit semicircular arcs described by an electrical equivalent circuit. Off-centered semicircular impedance plots show that the Nd-doped compounds obey to a non-Debye relaxation process. The conductivity of grains and grain-boundaries has been estimated. The activation energies calculated from the conductance and from time relaxation analyses are comparable. This indicates that the same type of charge carriers is responsible for both the electrical conduction and relaxation phenomena. [Copyright &y& Elsevier]

Abstract: We present the structural and magnetic properties of polycrystalline samples of the solid solution (La0.70−x Y x )Ba0.30Mn1−x Fe x O3 (x =0.00, 0.10, 0.20 and 0.30). Samples have been prepared by a conventional solid-state reaction method in air. Rietveld refinements of the X-ray powder diffraction data show a structural transition from rhombohedral () to orthorhombic (Pbnm) symmetry when x ≥0.10. The ZFC, FC and M(H) measurements lead to conclude that the samples with x ≥0.10 behave like spin-glass systems. The substitution of Mn3+ ions by Fe3+ ions triggers antiferromagnetic interactions between the Fe3+ and Mn4+ spins. The values of the magnetization (M(H)) decrease when increasing the Fe content. On the basis of a simple model of Mn and Fe spins pointing in opposite directions, we found the experimental data close to the calculated values, confirming the antiferromagnetic alignment between Fe and Mn moments. [Copyright &y& Elsevier]

Abstract: The structural, magnetic and electrical properties of (La0.70−x Nd x )Sr0.30Mn0.70Cr0.30O3 perovskites (0≤ x ≤0.30) prepared by the usual ceramic procedure were investigated. Structural Rietveld refinement revealed that these compounds crystallize in a rhombohedral perovskite structure when x =0, 0.10 and 0.20, while for x =0.30 the structure becomes orthorhombic (Pbnm). It was found that the substitution of La by Nd reduces the Curie temperature (T C). The FC, ZFC, M(H) and AC susceptibility measurements show typical canted-antiferromagnetism for the Nd-doped samples, in which a ferromagnetic component coexists with predominant antiferromagnetic interactions. The values of the magnetization (M(H)) decrease very slightly when increasing the Nd content, compared to the undoped sample (M S values at 5T and 2K are, respectively, 47.9, 47.3 and 47.5emu/g for x =0.10, 0.20 and 0.30, compared to 48.2emu/g for x =0), indicating that the Nd3+ contribution is negligible compared to the total moment of the ferromagnetic (Mn/Cr) network. The resistivity increases by several orders of magnitude with Nd-doping and the semi-conducting behaviour persists in the whole temperature range. The interaction between Mn4+–O–Cr3+and Cr3+–O–Cr3+ is responsible for the semi-conducting state. [Copyright &y& Elsevier]