Electrochemical Synthesis of Ferromagnetic LaMnO3 and Metallic NdNiO3

doi:10.1006/jssc.1995.1044

Journal of Solid State Chemistry

Volume 114, Issue 1, January 1995, Pages 294-296

https://doi.org/10.1006/jssc.1995.1044 Get rights and content

Abstract

While NdNiO_3-δ prepared ordinarily is highly oxygen-deficient and exhibits a metal-insulator transition at low temperature, it has been possible to prepare nearly metallic NdNiO_3-δ (δ = 0.05) by electrochemical oxidation. On progressive electrochemical oxidation, orthorhombic, insulating LaMnO₃ transforms to a rhombohedral and then to a cubic structure. The last two phases are ferromagnetic and exhibit metal-insulator transitions.

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Oxygen diffusion in perovskites, such as LaNiO3 and SrCoO2.5, with high covalency and highly conducting character, usually follow the peroxide (O−) pathway (i.e., formed by releasing a hydrogen ion from the adsorbed OH–, followed by oxidation to O−) [21]. A direct intercalation of O2– is suggested for perovskites, such as LaMnO3−δ, with less conducting character and covalency [17,22]. Perovskite halides have been evaluated as energy conversion materials.
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Cation-deficient lanthanum manganite oxides: Experimental and theoretical studies
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The value of unit formula of 58.60 Å3 volume also well agrees with Ref. [25]. It has been shown that this proportion of Mn4+ in the parent LaMnO3 itself can be varied by chemical or electrochemical means [29,53,54]. For example, by heating the sample in a He atmosphere at 973 K, we obtain LaMnO3 with about 18% Mn4+ and by heating in air at different temperatures ranging from 973 to 1273 K this proportion decreases from 32 to 12%.
Cation-deficient lanthanum manganite oxides with 0.8 ≤ La/Mn ≤ 1.25 were synthesized using a multi-step decomposition of gel precursors and investigated from experimental and theoretical point of view. The XRD (X-Ray Diffraction) analysis of the mixed LaMnO₃ oxide crystal concludes to a hexagonal structure, space group R-3c, excluding the presence of pure oxides such as La₂O₃, Mn₂O₃, or MnO₂ whatever the ratio La/Mn is. Oxides with nominal formulae La_1−xMnO_3+y and LaMn_1−x′O_3+y contain more than one defect structure involving valence defect (holes h), anionic vacancies as well as cationic vacancies in A and B sublattices of the perovskite structure. With the increase of La or Mn non-stoichiometry, the oxygen content y decreases more with La-deficient compositions than with Mn-deficient ones. The La/Mn ratio influences strongly the relationship between [h], [V_O], [V_La^‴] and [V_Mn^‴].
The DFT-GGA (Density functional Theory, Generalized-Gradient Approximation) simulation of these compounds using VASP (Vienna Ab-initio Simulation Package) concludes that the electronic structure for the optimized stoichiometric La₆Mn₆O₁₈ is not optimal, relative to that expected considering Mn(III) ions with four alpha electrons each. The non-stoichiometry is the easiest way of reducing the Jahn–Teller instability by depopulating the half-filled e_g orbitals. A partial oxidation is then stabilizing. Creation of defects, either an O insertion or a cationic vacancy, allows finding an ideal count. In the case of pure cationic defects (missing a La(III) or a Mn(III) cation), the amount of vacancies is one missing cation upon twelve. The compact nature of lanthanum manganite oxide does not allow oxygen insertion within the bulk structure and oxidation can only be achieved at the surface without O penetration. The formation of antisites is endothermic. The creation of mixed vicinal vacancies, one oxygen and one cationic vacancy (La or Mn), is exothermic for a concentration of defects of one defect per twelve cations.
Highly cation-deficient manganese perovskite, La<inf>1-x</inf>Mn <inf>1-y</inf>O<inf>3</inf> with x=y
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Oxidative (δ>0) nonstoichiometry in the perovskite ‘LaMnO_3+δ’ has been known to be manifested not with O interstitials but rather with cation vacancies of equal amounts at the two cation sites, La and Mn, i.e. La_1−xMn_1−yO₃ with x=y. Here, we report the fabrication of samples with record-high cation-vacancy concentrations (x>0.12 or δ>0.4) by means of a variety of high-pressure oxygenation techniques. Linear (negative) dependence of the cell volume on x was observed within the whole x range investigated, down to 56.9 Å³ (per formula unit) for a sample oxygenated at 5 GPa and 1100 °C using Ag₂O₂ as an excess oxygen source. With increasing degree of cation deficiency in La_1−xMn_1−xO₃, the ferromagnetic transition temperature was found to follow a bell shape with respect to x exhibiting a maximum of ∼250 K about x≈0.1. For moderately oxygenated samples large magnetoresistance effect was evidenced.
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The rare-earth nickelates are among few perovskite oxides showing very sharp temperature-induced metal/insulator transition. The synthesis and study of oxygen-deficient NdNiO_3−δ and SmNiO_3−δ are described in this paper. The results obtained indicate that the oxygen nonstoichiometry greatly influences transport properties of these compounds. The sharpness of metal/insulator transition strongly rises with decrease of oxygen deficiency without any noticeable change of transition temperature. The boundaries of oxygen and cation homogeneity ranges of Nd_1−xNiO_3−δ were determined for the first time. The thermogravimetric study was used to explain certain difficulties in oxidation of as-synthesized oxygen-deficient NdNiO_3−δ samples.
Synthesis and characterization of the ceramic system La<inf>x</inf>Nd<inf>1-x</inf>NiO<inf>3-δ</inf> (0≤x≤0.5)
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Lanthanum and neodymium nickelate (La_xNd_1−xNiO_3−δ) with x equal to 1.0, 0.9, 0.7 and 0.5 were prepared with molar ratios of 1:1.5 and 1:3. The resins were heated at 300°C for 2 h and then calcinated at 800°C for 12 h. The resulting powders were analyzed by thermogravimetry, infrared spectroscopy and X-ray powder diffraction. Chemical analysis of the powders showed that the oxygen vacancy number (δ) becomes higher with the substitution of lanthanum by neodymium for both molar ratios. X-ray patterns showed the formation of perovskite in all samples analyzed. Nevertheless, a small amount of a secondary phase was observed when neodymium content increases in the oxides. This phase was attributed to NiO.
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2001, Journal of Solid State Chemistry
Mn K-edge X-ray absorption experiments have been carried out on the self-doped LaMnO_3+δ system and compared to a (La_1−xCa_x)MnO₃ reference series. For both systems, the absorption edge and the unit cell volume vary linearly with the formal Mn(IV) content, indicating a direct correlation between the hole count in Mn 3d states and the concentration of substituents or cationic vacancies in the perovskite structure. However for the LaMnO_3+δ system, a deviation from linearity occurs in the volume change at high δ values. This discontinuity is ascribed to a limit of solubility for the cationic vacancy insertion, evidenced for 30% of Mn(IV). Larger hole contents can still be measured in samples synthesized at low temperature and showing a small grain size, but these excess holes are then localized on Mn surface sites. Based on resistivity and thermopower measurements, the differences in hole doping through aliovalent substitution or vacancy insertion are discussed, highlighting the existence of a large ferromagnetic insulating composition range in the LaMnO_3+δ system for 0.1≤δ≤0.15.

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Brief CommunicationElectrochemical Synthesis of Ferromagnetic LaMnO3 and Metallic NdNiO3

Abstract

Brief Communication
Electrochemical Synthesis of Ferromagnetic LaMnO₃ and Metallic NdNiO₃