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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 effects of non-magnetic Ti substitution on the structural, electrical and magnetic properties of LaBaMnTiO (0≤ x≤0.1) are investigated and compared to those existing in LaBaMnCrO (magnetic Cr). The structural refinement by the Rietveld method revealed that Ti-doped samples crystallize in the cubic lattice with space group $\mathrm{Pm}\bar{3}\mathrm{m}$, while samples with Cr crystallize in the hexagonal setting of the rhombohedral $\mathrm{R}\bar{3}\mathrm{C}$ space group for identical contents of dopant. The most relevant structural features are an increase of the lattice parameters, of the cell volume and of the inter-ionic distances with increasing Ti doping level. Both series of samples show a decrease of the paramagnetic-ferromagnetic transition temperature when the amount of chromium or titanium increases. Transport measurements show that when increasing the metal doping, the resistivity increases whereas the metallic behavior of the parent compound LaBaMnO is destroyed. For a substitution higher than 5 at.% of Ti and 10 at.% of Cr, the samples exhibit a semiconducting behavior in the whole range of temperature, for which the electronic transport can be explained by variable range hopping and/or small polaron hopping models. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic hysteresis displacement, thermal inversion of the magnetization, hysteresis loops jumps and crossing branches of hysteresis loops at low magnetic fields are reviewed. Most of these phenomena have been observed in magnetic oxide systems, particularly in perovskite-type manganites and chromites. The paper takes into account structural considerations and different geometrical parameters, such as volume or thin layers. [Copyright &y& Elsevier]

Abstract: The magnetic phase transition and the magnetic entropy change (−ΔSM ) in the La0.67Ba0.33Mn0.9Cr0,1O3 manganite were investigated by measuring the magnetization as a function of temperature. The maximum magnetic entropy change (−ΔSM ) and the relative cooling power (RCP) are found to be, respectively, 4.20Jkg−1 K−1 and 238Jkg−1 for a 5-T field change, making of this material a promising candidate for magnetic refrigeration near room temperature. To investigate the nature of the paramagnetic (PM) to ferromagnetic (FM) phase transition, found to be of second-order, we performed a critical exponents study by dc-magnetization M(H,T) measurements around the Curie temperature T C, in a temperature range including the critical region |ɛ|=|T − T C|/T C ⩽0.05. From the derived values of the critical exponents (β =0.380, γ =1.345), we conclude that La0.67Ba0.33Mn0.9Cr0.1O3 belongs to the three-dimensional Heisenberg class with short-range interaction. Scaling relations are obeyed, indicating renormalized interactions around T C. By investigating the field dependence of RCP and ΔSM , it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott method. [Copyright &y& Elsevier]

Abstract: We have investigated the structural, magnetic and electrical transport properties of a series of ABO3-type perovskite compounds, La0.67Ba0.33Mn1−x Cr x O 3 (0≤ x ≤0.15), which strongly depend on the doping level x. The slight difference between the ionic radii of Cr3+ and Mn3+ causes no change in the structure when x ≤0.1, remaining rhombohedral (space group R-3C), while for x =0.15 the structure becomes cubic (space group Pm-3m). Energy dispersive X-ray analysis (EDAX) confirms the expected stoichiometry of all samples. Upon Cr doping on the Mn site, the lattice parameters, the unit cell volume and the Bh name="sbnd" />B bond angle are reduced. All samples present a single magnetic transition from ferromagnetic to paramagnetic phase, showing a decrease of the Curie temperature T c and the magnetization M when x increases (x ≤0.15). However, Cr doping makes the saturation magnetization at 5K to decrease, which indicates that the Cr3+ moments tend to be antiparallel to the Mn3+ moments at low temperature. The Cr-doped manganites exhibit a large variation in resistivity values. The increase of Cr doping (x ≤0.15) leads to an increase of the electrical resistivity. Below 10at.% of Cr3+, the electrical resistivity shows a metallic behavior, which is well fitted by the relation ρ = ρ 0 + ρ 2T2 + ρ 4.5T4.5, indicating the importance of the grain/domain boundary, the electron–electron scattering effects and, to a lesser extent, the electron–(magnon, phonon) scattering effects in the mechanism of conduction. On the other hand, the 15at.% of Cr3+ doping makes the material to exhibit a semiconductor behavior, for which the electronic transport can be explained by a variable range hopping (VRH) and small polaron hopping (SPH) models. Results are consistent with a reduction of the number of available hopping sites for the Mn e g (↑) electron due to the substitution of Mn3+ by Cr3+, which suppresses the double exchange (DE) interactions. [Copyright &y& Elsevier]

Abstract: The critical properties of the manganese perovskite La0.67Ba0.33Mn0.98Ti0.02O3 around the paramagnetic-ferromagnetic phase transition were investigated through various techniques such as modified Arrott plot, Kouvel–Fisher method and critical isotherm analysis based on the data of static magnetic measurements recorded around the Curie temperature T c. The magnetic data analyzed in the critical region using the above methods yield the critical exponents β =0.551±0.008 with T c =310.47K±0.10 (from the temperature dependence of the spontaneous magnetization below T C) and γ =1.020±0.024 with T C =310.11K±0.14 (from the temperature dependence of the inverse initial susceptibility above T C) and δ =2.826, determined separately from the isothermal magnetization at T C. These critical exponents fulfill the Widom scaling relation δ =1+ γ/β, implying that the obtained values of β and γ are reliable. Based on these critical exponents, the magnetization–field–temperature (M–H–T) data around T c collapse into two curves obeying the single scaling equation . The values deduced for the critical exponents in La0.67Ba0.33Mn0.98Ti0.02O3 are close to the theoretical prediction of the mean-field model rather than the universal theory of 3D-Heisenberg, 3D-Ising and tricritical mean-field models. Moreover we have investigated the validity and usefulness of the theoretical modeling in our compound La0.67Ba0.33Mn0.98Ti0.02O3 based on the mean-field analysis of the magnetic entropy change (‑∆S M) versus the magnetization data. Results obtained through this approach are compared to those obtained from extrapolation of the Arrott curves. An excellent agreement was obtained between this approach with the one obtained from the extrapolation of the Arrott curves. [Copyright &y& Elsevier]

The preparation and crystalline features of perovskite solid solution Y(Co[sub x]Mn[sub 1-x])O[sub 3], x = 0.6 and 0.7, belonging to the Co-rich region of the pseudobinary system YMnO[sub 3]-YCoO[sub 3] have been studied. Dilatometric experiments performed under different atmospheres have been carried out for determining the best conditions to obtain single-phase samples. The influence of the thermal cycle and of the sintering density on the appearance of secondary phases has also been established. Firing in O[sub 2] atmosphere favors the formation of the pure perovskite. The cooling rate plays an important role for inhibiting the formation of secondary phases. All of the parameters depend ultimately on the final density of the polycrystalline bodies. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue on diverse topics including ferroelectric, dielectric, pyroelectric, piezoelectric, magnetic, catalytic, photovoltaic and electronic conduction properties of perovskites, structure and property interplay of perovskites and organometallic perovskites.

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: Thermal and reduction–oxidation stability of substituted LaMn1−y Co y O3 perovskite-type oxides (0.0≤ y Co ≤1.0) prepared by the citrate route have been studied by means of surface area, X-ray diffraction, FTIR spectroscopy and magnetic properties. The perovskite orthorhombic structure is found for y Co ≤0.5, with the exception of y Co =0.1, which corresponds better to rhombohedral LaMnO3.15. For y Co >0.5 the diffraction profiles are quite similar to the cobaltite’s rhombohedral structure. Magnetic iso-field studies (ZFC-FC) reveal that, for y Co ≤0.50, the system presents an antiferromagnetic canted-like ordering of the Mn/Co sublattice, in which the presence of divalent Co ion creates Mn3+–Mn4+ pairs that interact ferromagnetically through the oxygen orbital. This interpretation is confirmed by the magnetization loops, in which the magnetic moment increases when substituting Mn for Co. Therefore, the general trend is: for y Co ≤0.5, the Co ions are inserted in the manganite structure and for y Co >0.5, the Mn ions are inserted in cobaltite structure. The enhancement of the ferromagnetic properties and the thermal stability against reduction for y Co =0.5 is attributed to optimized Co2+–Mn4+ interactions. [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]

The structural properties of the YMexMn1−xO3 (Me=Cu, Ni, Co) pseudobinary oxides have been studied by X-ray diffraction and electrical measurements. The powders were prepared by solid state reaction between the corresponding oxides. The incorporation in solid solution of small divalent cations, Cu2+, Ni2+, and Co2+, substituting for Mn in the hexagonal YMnO3 compound, leads to a phase transition in which a perovskite-type structure is formed. The amount of substituting cation necessary for such a transition depends on the cation nature and, to a small extent, on the ionic radius. The phase transition depends strongly on the progressive substitution of the Jahn–Teller Mn3+ cation and therefore of the cooperative Jahn–Teller interaction weakness. The steric influence plays a secondary role, as is shown by the very small variation of the tolerance factor, t, as a function of the cation content. The solid solutions with perovskite-type structure show semiconducting behavior. The conductivity mechanism is of a thermally activated small polaron hopping. [Copyright &y& Elsevier]

The effect on the geometric, electronic, and catalytic properties of LaMnO 3 oxides of replacement of 50% Mn with Cr, Fe, Co, and Ni in combustion reactions of dimethyl ether has been studied. The similarity in the ionic radius does not produce larger differences in the geometric properties of the compounds, whereas the differences in the stability of their cation valences produce significant changes in their electronic properties. The high stability of Cr 3+ and Fe 3+ species does not increase the Mn 4+ /Mn 3+ redox pair ratio in LaMn 0.5 Cr 0.5 O 3 and LaMn 0.5 Fe 0.5 O 3 . Meanwhile, the high stability of Co 2+ and Ni 2+ species favors the oxidation of Mn 3+ to Mn 4+ and the mobility of anionic oxygen in the LaMn 0.5 Co 0.5 O 3 and LaMn 0.5 Ni 0.5 O 3 catalysts, which improves their catalytic performance. [ABSTRACT FROM AUTHOR]

Polycrystalline samples La 0.7− x Nd x Sr 0.3 Mn 0.7 Ti 0.3 O 3 ( x =0.10; 0.20 and 0.30) were prepared by a high-temperature solid-state reaction technique. The X-ray diffraction shows that all the samples crystallize in the orthorhombic structure, Pbnm space group, with presence of a minor unreacted Nd 2 O 3 . The electrical response was studied by impedance complex spectroscopy over a broad frequency range (40–100 MHz) at room temperature. The values of ac conductivity for all samples were fitted by the Jonscher law σ ( ω ) = σ dc + A ω s . For x =0.10 and 0.20, hopping occurs between neighboring sites, whereas for x =0.30 the hopping process occurs through longer distance. Complex impedance plots exhibit semicircular arcs described by an electrical equivalent circuit, which indicates that the Nd-doped compounds obey a non-Debye relaxation process. [ABSTRACT FROM AUTHOR]

Abstract: Critical behavior in the La0.6Sr0.4Mn0.8Fe0.1Cr0.1O3 ceramics was studied using magnetization methods. Results show that the paramagnetic–ferromagnetic transition is of second order. Based on the critical behavior analysis using the Banerjee criterion and the Kouvel–Fisher method, we find the critical exponents: β=0.395±0.010, γ=1.402±0.010, and δ=5.208±0.007, for which the magnetic interaction is satisfied within the three-dimensional Heisenberg model. Results indicate the presence of short-range interactions. The magnetic entropy change (−ΔS M) reached maximum values of 1.75, 1.45, 1.15, 0.8 and 0.43JKg−1 K−1 under a magnetic field variation of 5, 4, 3, 2 and 1T, respectively. Nevertheless, these (−ΔS M) values are much low for any potential application at this moment. The nature of this phenomenon is discussed in relation to the characteristics of the magnetic phase transition and critical exponents. [Copyright &y& Elsevier]

Abstract: The electrical properties and the mechanism of conduction of the simultaneously substituted La0.7−x Y x Ba0.3Mn1−x Fe x O3 perovskite (0≤x≤0.30) have been studied. The insertion of Y3+ and Fe3+ ions in the parent compound La0.7Ba0.3MnO3 leads to an increase of the resistivity. The undoped sample (x=0) shows a metallic behavior, which can be fitted by the relation ρ(T)=ρ 0+ρ 2T2+ρ 4.5T4.5, indicating the importance of electron–magnon scattering effects in this material. All the other samples (x≥0.10) are semiconductors throughout the studied temperature range (80–290K). Several models have been used to fit their temperature-dependent resistivity: thermal activation, adiabatic nearest-neighbor hopping of small polarons (Holstein theory) and variable range hopping (VRH) models. The fits show that the electronic transport in semiconducting La0.7−x Y x Ba0.3Mn1−x Fe x O3 is well described and dominated by the VRH mechanism, for which the hopping distance (a) grows with increasing Fe3+ doping, thus increasing the average hopping energy W. [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: The structural, magnetic and electrical properties of Cr and Fe simultaneously substituted in the perovskite La0.6Sr0.4Mn1−2x Cr x Fe x O3 have been studied. The presence of Cr and Fe had no significant effect on the structural properties. Curie temperature and saturation magnetization decrease with increase in Cr and Fe contents. For x=0.20 and 0.25, a steep drop of zero field-cooled (ZFC) magnetization at low temperature signifies the formation of cluster- or spin-glass state. A weak hysteresis at low fields seems to be an indication of phase separation. All the resulting magnetization curves can be explained by a superposition of both ferromagnetic and antiferromagnetic components. All the samples are semiconducting throughout the temperature range studied. Resistivity can be described by the adiabatic small polaron hopping and the variable range hopping model. It was found that the transport mechanism is dominated by the VRH model with an increase of Mott localization energy, which explains the increase of resistivity. [ABSTRACT FROM AUTHOR]

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]

Abstract: The (Mn1−x Cd x )Cr2S4 phases (0≤ x ≤0.6) have been synthesized from the corresponding elements at 1123K. These samples were characterized by powder X-ray diffraction (XRD) and magnetic susceptibility. The (Mn1−x Cd x )Cr2S4 compounds crystallize in the space group Fd-3m with cell parameters a =10.101(6)Å, 10.139(3)Å, 10.165(2)Å, and 10.192(1)Å for x =0, 0.2, 0.4 and 0.6, respectively. An overall ferrimagnetic behavior is observed for all samples. The ferromagnetic component increases rapidly when manganese is substituted by non-magnetic cadmium, as shown by ZFC/FC measurements. At the same time, the value of the magnetization M 50 at 50kOe, deduced from M(H) loops, also increases with increasing cadmium content because the antiferromagnetic alignment between chromium and manganese spins is progressively lost, leading toward well aligned moments pointing into the same direction. These results are explained by a rearrangement of the chromium spins when Mn located at the tetrahedral sites, is substituted by Cd. [Copyright &y& Elsevier]