Magnetic properties in the system BaCo1−xMnxO3 and SrCo1−xMnxO3 (0 ≦ x ≦ 1)

The compounds in the systems of BaCo_1?xMn_xO₃ (0 ≦ x ≦ 1) and SrCo_1?xMn_xO₃ (0 ≦ x ≦ 1) were prepared at an oxygen pressure of 1400 bars. The former had a two-layer hexagonal structure and that of the latter was cubic perovskite type. From the variation of the unit-cell parameters and of the magnetic properties, it is found that the Co⁴⁺ ions change from the low-spin to the high-spin state. In the system of SrCo_1?xMn_xO₃, the change of magnetic property from ferromagnet to antiferromagnet is related to the spin state of Co⁴⁺ ions located at the octahedral sites.     

The isotropic temperature factors of Sr(Co1−xMnx)O3 (x = 0, 0.1, 0.5, 0.8, and 1.0)

The cubic perovskite Sr(Co_1?xMn_x)O₃ has a maximum value of a-axis at x = 0.3 and a change of spin state of Co⁴⁺ ion from low to high. To elucidate these properties, the isotropic temperature factor (B) of strontium, cobalt, manganese, and oxygen atoms for x = 0, 0.1, 0.5, 0.8, and 0.1 have been derived from powder X-ray diffraction measurements. The isotropic temperature factor of oxygen for x = 0, 0.1, and 1.0 is small and that for x = 0.5 and 0.8 is large. This fact suggests that the oxygen ion deviates from the center of the CoOMn bond in the solid solutions with $\text{x ≧ 0.3}$. Larger CoO₆ octahedra and smaller MnO₆ octahedra, which are connected by corner sharing of oxygens of the octahedron, are distributed statistically.     

Magnetoresistance in the ferromagnetic metallic perovskite SrFe1−xCoxO3

The investigation of the magnetic and transport properties of the oxygen deficient perovskites SrFe_1−xCo_xO_3−δ shows that these compounds exhibit both ferromagnetism and metallicity in a wide compositional range (0≤x≤0.70). Negative magnetoresistance is evidenced for the first time in these oxides, in contrast to SrCoO_3−δ. These properties are explained by superexchange interactions between cobalt and iron according to the scheme Fe³⁺OCo⁴⁺↔Fe⁴⁺OCo³⁺. This model is strongly supported by ⁵⁷Fe Mössbauer measurements which show the existence of two sites at room temperature, high spin localized Fe⁴⁺ and delocalized Fe^3+α sites, whereas magnetic disordering suggesting spin fluctuations is observed at 5 K as soon as cobalt is introduced into the SrFeO₃ structure.     

Behavior of Mn3+ ions in four-layered hexagonal (Sr1−x−yLaxBay)MnO3

Stoichiometric four-layered hexagonal (4H) (Sr_1?x?yBa_xLa_y)MnO₃ was synthesized using a standard ceramic technique. Rietveld analysis at room temperature indicated that the Mn–O(1) distance increased and the Mn–O(2) distance decreased with the increase in x. The samples were n-type semiconductors and exhibited hopping conductivity in a small-polaron model below 533 K. The Mn³⁺ ion acted as a donor and the electron transfer became active through the Mn³⁺–O–Mn⁴⁺ path. The samples were antiferromagnetic and the Néel temperature (T_N) was constant regardless of y when x was fixed to 0.3, whereas T_N shifted to a high temperature when y was fixed to 0.02. The face-sharing Mn³⁺–O(2)–Mn⁴⁺ interaction strengthened as the Mn–O(2) distance decreased, and T_N shifted to a high temperature as a result.     

X-ray photoelectron spectroscopy study of (Ca1−xLax)MnO2.97 (0.1 ≦ x ≦ 0.4)

The X-ray photoelectron spectroscopy (XPS) of perovskite-type (Ca_1−xLa_x)MnO_2.97 (0.1 ≦ x ≦ 0.4) was measured at room temperature. From the absolute values of the binding energy difference (ΔBE) of Ca2pO1s, La3dO1s, and Mn2pO1s, both the chemical bonding of MnO and CaO become more covalent, and that of LaO becomes more ionic with increasing x. The electron transfer of the MnOMn path is dominant, and the electrical properties are strongly influenced by the decrease of the electron transfer of the MnO(Ca,La)OMn path.     

Observation par r.p.e. de l'effet d'un traitement thermique en atmosphère réductrice sur l'état d'oxydation du manganèse dans les solutions solides (La2O3)1−x(CeO2)x

Manganese-doped (～200 ppm) single-crystal (La₂O₃)_1?x(CeO₂)_x samples, with x = 0.20, 0.25, and 0.30 were investigated by ESR before and after annealing at 500°C for 5 hr in a hydrogen atmosphere. Spectra obtained before annealing showed that the valence state of manganese depended upon the amount of CeO₂ in the solid solutions. After annealing the valence changes Mn⁴⁺ → Mn³⁺ and Mn³⁺ → Mn²⁺ were evident.     

Synthesis and characterization of La0.8Sr1.2Co0.5M0.5O4−δ (M=Fe, Mn)

The M⁴⁺-containing K₂NiF₄-type phases La_0.8Sr_1.2Co_0.5Fe_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O₄ have been synthesized by a sol–gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M⁴⁺ to M³⁺ and of Co³⁺ to Co²⁺. The vacancies are confined to the equatorial planes of the K₂NiF₄-type structure. A partial reduction of Mn³⁺ to Mn²⁺ also occurs to achieve the oxygen stoichiometry in La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6. La_0.8Sr_1.2Co_0.5Fe_0.5O_3.65 contains Co²⁺ and Fe³⁺ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La_0.8Sr_1.2Co_0.5Fe_0.5O₄, La_0.8Sr_1.2Co_0.5Mn_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6 induce spin glass properties in these phases.     

Synthesis and characterization of La0.8Sr1.2Co0.5M0.5O4−δ (M=Fe, Mn)

The M⁴⁺-containing K₂NiF₄-type phases La_0.8Sr_1.2Co_0.5Fe_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O₄ have been synthesized by a sol-gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M⁴⁺ to M³⁺ and of Co³⁺ to Co²⁺. The vacancies are confined to the equatorial planes of the K₂NiF₄-type structure. A partial reduction of Mn³⁺ to Mn²⁺ also occurs to achieve the oxygen stoichiometry in La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6. La_0.8Sr_1.2Co_0.5Fe_0.5O_3.65 contains Co²⁺ and Fe³⁺ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La_0.8Sr_1.2Co_0.5Fe_0.5O₄, La_0.8Sr_1.2Co_0.5Mn_0.5O₄ and La_0.8Sr_1.2Co_0.5Mn_0.5O_3.6 induce spin glass properties in these phases.     

Synthesis and characterization of La1+xSr2−xCoMnO7−δ (x=0,0.2; δ=0,1)

The n=2 Ruddlesden-Popper phases LaSr₂CoMnO₇ and La_1.2Sr_1.8CoMnO₇ have been synthesized by a sol-gel method. The O6-type phases LaSr₂CoMnO₆ and La_1.2Sr_1.8CoMnO₆ were produced by reduction of the O7 phases under a hydrogen atmosphere. The materials crystallize in the tetragonal I4/mmm space group with no evidence of long-range cation order in the neutron and electron diffraction data. Oxygen vacancies in the reduced materials are located primarily at the common apex of the double perovskite layers giving rise to square pyramidal coordination around cobalt and manganese ions. The oxidation states Co³⁺/Mn⁴⁺ and Co²⁺/Mn³⁺ predominate in the as-prepared and reduced materials, respectively. The materials are spin glasses at low temperature and the dominant magnetic interactions change from ferro- to antiferromagnetic following reduction.     

Study of Pr1−xMn1+xO3 perovskites

The structural and magnetic properties of the Pr_1?xMn_1+xO₃ perovskites were studied. The increase of x (i.e., $\text{Pr}\text{Mn}\text{< 1}$) leads to the decrease of the orthorhombic deformation and of the Néel temperature and, simultaneously, to an increase of the ferromagnetic contribution. The latter effect is explained from the suggested distribution of the cations (Pr³⁺_1?xMn²⁺_x)_A(Mn³⁺_1?xMn⁴⁺_x)O^2?₃ by the double exchange of Mn³⁺Mn⁴⁺ pairs at the B—sublattice.     

Oxidative nonstoichiometry in perovskites,an experimental survey; the defect structure of an oxidized lanthanum manganite by powder neutron diffraction

Oxide perovskites showing oxidative nonstoichiometry (ABO_3+x) have been investigated. The structure of LaMn³⁺_0.76Mn⁴⁺_0.24O_3.12 has been investigated by powder neutron diffraction and a composition (La_0.94±0.02□_0.06±0.02)(Mn³⁺_0.745Mn⁴⁺_0.235□_0.02)O₃ with partial elimination of La₂O₃ and vacancies on both the A and B metal sites determined. A much smaller degree of nonstoichiometry has been found for LaVO_3+x(x ? 0.05), and LaCrO₃, and EuTiO₃ did not show nonstoichiometry under the conditions used. A single phase region from Ba_0.8La_0.2Ti⁴⁺_0.8Ti³⁺_0.2O_3.0 to Ba_0.8La_0.2Ti⁴⁺O_3.1 has been confirmed for lanthanum-doped BaTiO₃, but the solubility of La³⁺ in SrTiO₃ is very small; consideration of the ionic radii indicates that the dopant ion of higher oxidation state must be significantly smaller than the normal ion to stabilize a wide nonstoichiometric region with B site vacancies. The extensive nonstoichiometry shown by LaMnO_3+x, in contrast to the other lanthanum-transition-metal perovskites LaBO₃, may result from the much larger reduction in ionic radius from Mn³⁺ to Mn⁴⁺ than is found for other transition-metal ions.     

Magnetotransport properties of Mo substituted La0.7Ca0.3Mn1−xMoxO3 perovskites

We studied the effects of Mo substitution on the structural, transport, and magnetic properties of the La_0.7Ca_0.3Mn_1−xMo_xO₃ (x ≤ 0.1) samples. Powder X-ray diffraction analysis reveals that the samples studied crystallize in the orthorhombic structure with space group Pbnm. Both particle size and morphology change significantly as the Mo content x varies. The metal-insulator transition temperature (T_MI) and Curie temperature (T_C) decrease monotonically as x increases. Magnetization data reveal that long-range FM ordering persists in all samples and the saturation moment decreases linearly as x increases. The smaller depression rate of dT_C/dx observed is mainly ascribed to the increased amount of Mn²⁺ ions with Mo doping, which opens the FM coupling between Mn²⁺–O–Mn³⁺ in the samples.     

Inducement of ferromagnetic–metallic phase and magnetoresistance behavior in charged ordered monovalent-doped Pr0.75Na0.25MnO3 manganite by Ni substitution

In this communication, the study on the effect of Ni²⁺ substitution on structural, magnetic and electrical transport properties were performed in Pr_0.75Na_0.25Mn_1-xNi_xO₃ (x = 0–0.10) ceramics synthesized using conventional solid-state method. X-ray diffraction patterns showed that all samples were present in single phase and crystallized in orthorhombic structure with Pnma space group. Rietveld refinement analysis revealed unit cell volume slight increase with increase Ni concentration, thereby indicating partial substitution of Ni²⁺ at Mn³⁺. The presence majority of Ni²⁺ states in the compound were confirmed by X-ray photoelectron spectrum. Tolerance factor calculation suggested that Ni substitution exerted no strong effect on structural distortion. For un-doped sample (x = 0), AC susceptibility (χ′) against temperature (T) curve showed paramagnetic (PM)–antiferromagnetic(AFM) behavior at Neel temperature (T_N) of approximately 170 K. Furthermore, resistivity (ρ) against temperature (T) curve showed an insulating behavior for the whole measured temperature range. The χ′ against T curve of x = 0 sample showed broad peak at approximately 218 K which was attributed to the onset of charge ordered (CO) state. No such broad peak was observed in Ni-substituted samples which indicated the weakening of CO state. Moreover, χ′ measurements exhibited successful inducement of PM–FM transition with Curie temperature (T_C), decreasing from 132 K (x = 0.02) to 92 K (x = 0.08). Electrical resistivity measurement on samples (x = 0.02–0.08) displayed inducement of metal–insulator transition, where transition temperature (T_MI) decreased and resistivity increased, with x before re-entrant insulating behavior at x = 0.10. Notably, upturn resistivity was observed below 40 K for x = 0.06 and 0.08 samples. The suppression of CO state and inducement of ferromagnetic-metallic (FMM) state beginning from x = 0.02 sample was attributed to the reduced degree of Jahn–Teller distortion and Coulomb interaction among Mn ions, as well as the presence of ferromagnetic superexchange (FM SE) interaction among Ni²⁺–O–Mn⁴⁺ which improved the alignment charge carrier spins and induced the double-exchange (DE) interaction among Mn³⁺–O–Mn⁴⁺. The decrease in T_C and T_MI with increased x may be due to the enhanced AFM SE interactions of Mn³⁺–O–Mn³⁺, Mn⁴⁺–O–Mn⁴⁺ and Ni²⁺–O–Ni²⁺ which decreased the FM SE interaction of Ni²⁺–O–Mn⁴⁺. Consequently, the effective DE interaction was decreased. In addition, the decreased metallic behavior and re-entrant insulating behavior for x = 0.10 sample was due to the strong AFM interaction between Ni²⁺ ions which consequently contributed to the suppression of FM SE and DE interactions. The observed upturn resistivity below 40 K for x = 0.06 and 0.08 samples was attributed to the Kondo-like effect which resulted from the interaction between itinerant conduction electron spin and localized spin impurity.     

Structural variations and cation distributions in Mn3−xCoxO4 (0 ≤ x ≤ 3) dense ceramics using neutron diffraction data

Single phase ceramics of cobalt manganese oxide spinels Mn_3?xCo_xO₄ were structurally characterized by neutron powder diffraction over the whole solid solution range. For x < 1.75, ceramics obtained at room temperature by conventional sintering techniques are tetragonal, while for x ≥ 1.75 ceramics sintered by Spark Plasma Sintering are of cubic symmetry. The unit cells, metal–metal and metal–oxygen average bonds decrease regularly with increasing cobalt content. Rietveld refinements using neutron data show that cobalt is first preferentially substituted on the tetrahedral site for x < 1, then on the octahedral site for increasing x values. Structural methods (bond valence sum computations and calculations based on Poix's work in oxide spinels) applied to our ceramics using element repartitions and [M–O] distances determined after neutron data refinements allowed us to specify the cation distributions in all phases. Mn²⁺ and/or Co²⁺ occupy the tetrahedral site while Mn³⁺, Co²⁺, Co^III (cobalt in low-spin state) and Mn⁴⁺ occupy the octahedral site. The electronic conduction mechanisms in our highly densified ceramics of pure cobalt and manganese oxide spinels are explained by the hopping of polarons between adjacent Mn³⁺/Mn⁴⁺ and Co²⁺/Co^III on the octahedral sites.     

Perovskite-like La1−xKxMnO3 and related compounds: Solid state chemistry and the catalysis of the reduction of NO by CO and H2

The new compounds La_1?xM_xMnO₃ (0.05 ? x ? 0.4 for M = K; x = 0.2 for M = Na, Rb) have been prepared. La_1?xK_xMnO₃ (0.05 ? x ? 0.4), LaMnO_3.01, LaMnO_3.15, La_0.8Na_0.2MnO₃, and La_0.8Rb_0.2MnO₃ have been used as catalysts in the reduction of NO. La_0.8K_0.2MnO₃ has also been used in the catalytic decomposition of NO. The activity of these catalysts is related to the presence of a Mn³⁺/Mn⁴⁺ mixed valence and to the relative ease of forming oxygen vacancies in the solid. The presence of cation vacancies in LaMnO_3.15 and the substitution of La³⁺ by alkali ions in LaMnO₃ increases the catalytic activity. The reduction of NO involves both molecular and dissociative adsorption of NO.     

Synthesis and photoluminescent properties of Y1 − x La x PO4:Eu3+ nanophosphor

In this paper, Y_{1 ? x} La _x PO₄:Eu³⁺ (x = 0.5, 0.7, and 0.3) nanophosphors were synthesized by a rather simple method. The products present different morphologies. For Y_{1 ? x} La _x PO₄:Eu³⁺, they have similar phase composition of a mixture of monoclinic LaPO₄ and tetragonal YPO₄. Furthermore, the luminescence behavior of Eu³⁺ has been investigated in this type of matrices. In Y_{1 ? x} La _x PO₄:Eu³⁺, the ⁵D₀-⁷F₁ magnetic dipole transition is dominant, indicating that the Eu³⁺ site is inversion symmetry. The difference in the Eu-O charge transfer (CT) band with La³⁺ ion concentration suggests the difference in the ionicity of the Eu-O bond. Among those products, the red to orange intensity ratio (R/O) of ⁵D₀-⁷F₂ to ⁵D₀-⁷F₁ value of Eu³⁺ is different, furthermore, for La³⁺ x = 0.3, the R/O value of Eu³⁺ is the biggest on the contrary, indicating that the inversion symmetry Eu³⁺ is lowest.     

Behavior of Co4+ ion in K2NiF4-type (Ca1+xSm1−x)CoO4

Orthorhombic K₂NiF₄-type (Ca_1+xSm_1−x)CoO₄ (0.00 ≤ x ≤0.15) with space group Bmab has been synthesized by the polymerized complex route. The cell parameters (a and b) decrease, while the cell parameter (c) increases with increasing Co⁴⁺ ion content. The global instability index (GII) indicates that the crystal stability of (Ca_1+xSm_1−x)CoO₄ is not influenced by the Co⁴⁺ ion content. (Ca_1+xSm_1−x)CoO₄ is a p-type semiconductor and exhibits hopping conductivity in the small-polaron model at low temperatures. The magnetic measurement indicates that (Ca_1+xSm_1−x)CoO₄ shows paramagnetic behavior above 5 K, and that the spin state of both the Co³⁺ and Co⁴⁺ ions is low. The Co⁴⁺ ion acts as an acceptor, and the electron transfer becomes active through the Co³⁺–O–Co⁴⁺ path as the Co⁴⁺ ions increase.     

Influence of the structure on electric and magnetic properties of La0.8Na0.2Mn1−xCoxO3 perovskites

The influence of the cobalt substitution for manganese ions in the mixed valence perovskites La_0.8Na_0.2Mn_1−xCo_xO₃ (0?x?0.2) was investigated by X-ray, electric transport and magnetic measurements. The study carried out on sintered polycrystalline samples revealed the rhombohedral () structure and the insulator-metal transition connected with a ferromagnetic arrangement in the whole concentration range. Increasing concentration of cobalt ions leads to a gradual decrease of PM-FM and I-M transition temperatures. An influence of the cobalt ions on the observed behavior is attributed to charge compensation Mn³⁺→Mn⁴⁺ leading to the formation of stable couples Mn⁴⁺-Co²⁺. Therefore the double-exchange interactions Mn³⁺-O²⁻-Mn⁴⁺ partly vanish and they are replaced by positive superexchange interactions Mn⁴⁺-O²⁻-Co²⁺, but of a semiconducting character.     

Electron transport and magnetic properties of La1−xSrxCo1−yTiyO3 (x = 0a or x = yb)

The two systems (a) and (b) for different values of x were synthesized. Their electron transport and magnetic properties show a change in behavior above a critical value of x. Unlike the system La_1?xSr_xCoO₃, itinerant electron ferromagnetism is not observed. This is explained on the basis of the absence of an itinerant band of Co⁴⁺ whose generation is restricted on account of substitution of Ti⁴⁺. Electron transport in these two systems is compared with that of LaCoO₃ or La_1?xSr_xCoO₃ and is discussed in view of the presence of different valence states of cobalt and change in crystal field splitting. Spin-state equilibria in these two systems are similar to that in LaCoO₃.     

Sur de nouveaux ferrites oxyfluorés d'yttrium ou de gadolinium à structure grenat

The oxyfluoride garnets of formula Y₃Fe_5?xM_xO_12?xF_x and Gd₃Fe_5?xM_xO_12?xF_x (M = 3d transition element) result from partial substitution of O^2? by F^? in Y₃Fe₅O₁₂ and Gd₃Fe₅O₁₂ oxides. The cationic charge compensation is obtained by replacing the Fe³⁺ ions by divalent ions as Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺ ions. The site occupied by some of these ions (Mn²⁺, Ni²⁺, Zn²⁺) is determined by magnetic or Mössbauer measurements.