Investigation of the quasi-ternary system LaMnO3–LaCoO3–“LaCuO3”. II: The series LaMn0.25−xCo0.75−xCu2xO3−δ and LaMn0.75−xCo0.25−xCu2xO3−δ
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- Tietz, F., Arul Raj, I., Fu, Q.X. et al. J Mater Sci (2009) 44: 4883. doi:10.1007/s10853-009-3746-7
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This paper investigates the crystal structure, thermal expansion, and electrical conductivity of two series of perovskites (LaMn0.25−xCo0.75−xCu2xO3−δ and LaMn0.75−xCo0.25−xCu2xO3−δ with x = 0, 0.025, 0.05, 0.1, 0.15, 0.2, and 0.25) in the quasi-ternary system LaMnO3–LaCoO3–“LaCuO3”. The Mn/Co ratio was found to have a stronger influence on these properties than the Cu content. In comparison to the Co-rich series (LaMn0.25−xCo0.75−xCu2xO3−δ), the Mn-rich series (LaMn0.75−xCo0.25−xCu2xO3−δ) showed a much higher Cu solubility. All compositions in this series were single-phase materials after calcination at 1100 °C. The Co-rich series showed higher thermal expansion coefficients (αmax = 19.6 × 10−6 K−1) and electrical conductivity (σmax = 730 S/cm at 800 °C) than the Mn-rich series (αmax = 10.6 × 10−6 K−1, σmax = 94 S/cm at 800 °C). Irregularities in the thermal expansion curves indicated phase transitions at 150–350 °C for the Mn-rich series, while partial melting occurred at 980–1000 °C for the Co-rich series with x > 0.15.
Perovskite-type oxides, especially LaMeO3−δ with Me = Cr, Mn, Fe, Co, Ni, Cu, have been investigated by many research groups because of their interesting electrical, magnetic, and oxygen transport properties which makes them suitable for application as functional materials in solid oxide fuel cells or as giant magnetoresistant materials. In order to tailor the properties for a certain application, elemental substitution at A-sites or B-sites of the perovskite lattice is often used as this forms complex perovskite-type oxides. For instance, complex oxides in the system LaMnO3–LaCoO3 have been widely investigated in terms of their crystal structure, electrical and ionic conductivity, catalytic, magnetic and thermal expansion behavior [1–10]. Crystallographic studies have also been conducted on LaCoO3–LaCuO3 and LaMnO3–LaCuO3 systems [11–13]. As far as we know, however, systematic studies on the complex system of LaMnO3–LaCoO3–LaCuO3 have not yet been performed.
In our previous study, the sintering behavior and properties of the oxides in the series La(Mn0.5Co0.5)1−xCuxO3−δ with x = 0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1 were investigated and the results were reported as the first part of investigations into this complex system . Some remarkable results were obtained along this series of perovskite compositions: LaMn0.3Co0.3Cu0.4O3−δ crystallized as a single phase with an orthorhombic perovskite structure. Among the synthesized compositions, this compound showed the highest electrical conductivity in air at 800 °C (155 S cm−1) and the highest thermal expansion coefficient (α30–800 °C = 15.4 × 10−6 K−1). The “LaCuO3−δ” composition crystallized as a mixture of La2CuO4 and CuO at 900 °C and as a single phase with a monoclinic La2Cu2O5-type structure at 1100 °C, although previous investigations have shown that other phases are preferably formed [15, 16].
For this study, perovskites with different Mn/Co ratio were prepared and systematically characterized, i.e. seven oxides in the series LaMn0.25−xCo0.75−xCu2xO3−δ and another seven oxides in the series LaMn0.75−xCo0.25−xCu2xO3−δ with x = 0, 0.025, 0.05, 0.1, 0.15, 0.2, and 0.25 for both series. The X-ray crystallographic data, the thermal expansion coefficient data, and the electrical conductivity data were obtained on sintered samples fabricated from these oxide powders and compared as a function of the copper content.
Fourteen powders with different compositions within the series LaMn0.25−xCo0.75−xCu2xO3−δ and LaMn0.75−xCo0.25−xCu2xO3−δ (both with x = 0, 0.025, 0.05, 0.1, 0.15, 0.2, and 0.25) were synthesized by the Pechini method  using nitrate solutions of La, Mn, Co, and Cu in the corresponding metallic ratios. A detailed description of the synthesis process is given in Ref. . After obtaining the raw powder and calcination at 600 °C for 3 h, the powders were subjected to chemical analysis using inductively coupled plasma with optical emission spectroscopy (ICP-OES, TJA-IRIS-INTREPID spectrometer) to confirm the nominal stoichiometry. In addition, two samples of each prepared powder were heat-treated in air at 900 and 1100 °C for 6 h. The crystal phase composition of these samples was determined by X-ray diffraction analysis using a Siemens D5000 diffractometer with CuKα radiation. Lattice parameters and contents of secondary phases were determined by Rietveld refinement.
The powders calcined at 600 °C were uniaxially pressed to bars (40 × 5 × 4 mm3) and sintered at either 1300 °C (for x ≤ 0.15) or 1100 °C (for x ≥ 0.2) for 6 h in air. Densities of sintered samples were measured by the Archimedean method. The thermal expansion between 30 and 800 °C was determined using a Netzsch DIL 402C dilatometer. The total electrical conductivity of the sintered samples was measured by a 4-probe DC technique at temperatures between 100 and 900 °C in air using silver wires and silver paste as contacts.
Results and discussion
Elemental analysis of the powders
Nominal composition, analytical composition, and crystalline phases observed after calcination at 900 °C and 1100 °C for 6 h
Crystalline phases (numbers in parentheses indicate the phase content in wt%)
Prh + La2O3 + Co3O4
Prh + La2O3 (1) + Co3O4 (1)
Prh + La2O3 + Co3O4
Prh + La2O3 (1) + Co3O4 (3)
Prh + La2O3 + L2C + Co3O4
Prh + L2C (3)
Prh + L2C + La2O3 + Co3O4
Prh + L2C (1)
Prh + L2C
Prh + L2C (3)
Prh + L2C
Prh + L2C2 (2) + L2C (7)
Prh + L2C + CuO
Prh + L2C2 (22) + L2C (2)
Por + L2C
Por + L2C
For the LaMn0.75−xCo0.25−xCu2xO3−δ series, both the apparent and corrected conductivity show similar profiles, i.e. a conductivity maximum is observed at x = 0.10 at the lower temperature range (<400 °C) or at x = 0.15 at the higher temperature range (>400 °C). From Figs. 10 and 11, it appears that lower activation energies correspond to higher electrical conductivity. In addition, compared to the Mn-rich series (LaMn0.75−xCo0.25−xCu2xO3−δ), the Co-rich series (LaMn0.25−xCo0.75−xCu2xO3−δ) shows significantly higher electrical conductivity, which is consistent with the fact that LaCoO3-based perovskites have a much higher electrical conductivity than LaMnO3-based perovskites [1, 10, 22]. The higher electrical conductivity of the cobaltites is due to the narrower band gap and the shifted Fermi level in the electronic band structure . Although both series contain Mn and Co on the B-site of the perovskite structure, the electrical conductivity of LaMn0.75Co0.25O3−δ and LaMn0.25Co0.75O3−δ reflects the electronic band structure of LaMnO3 and LaCoO3, respectively . The addition of Cu in both series leads to a higher conductivity and lower activation energy which clearly indicates the occupation of electronic states within the band gap of valence and conduction band, either as single energy levels (low Cu concentrations) or as small narrow bands (high Cu concentrations), similar to the A-site substitution with Sr in these perovskites . In the case of the Co-rich series, the band gap becomes so small that the thermal energy at elevated temperatures is sufficient to activate electrons occupying energy levels in the conduction band. As shown in Fig. 8a, the maximum conductivity of the samples with x = 0.20 and x = 0.25 was observed at 500 and 400 °C corresponding to a thermal energy of RT = 6.43 and 5.56 kJ/mol (i.e. 0.067 and 0.058 eV), respectively (R = 8.3144 J K−1 mol−1). These values correspond very well with the measured activation energies at lower temperatures of these two samples (0.081 and 0.060 eV, cf. Fig. 10) and indicate a thermally activated electronic conduction in Cu-rich perovskite materials similar to Ni-based peovskites (LaNiO3, LaNi1−xFexO3) [26, 28–30].
Two series of perovskite oxides (LaMn0.25−xCo0.75−xCu2xO3−δ and LaMn0.75−xCo0.25−xCu2xO3−δ with x = 0, 0.025, 0.05, 0.1, 0.15, 0.2, and 0.25) in the quasi-ternary system LaMnO3–LaCoO3–“LaCuO3” have been investigated in terms of crystal structure, thermal expansion and electrical conductivity. In comparison to the Co-rich series (LaMn0.25−xCo0.75−xCu2xO3−δ), the Mn-rich series (LaMn0.75−xCo0.25−xCu2xO3−δ) can tolerate higher concentrations of Cu in the lattice without forming Cu-containing secondary phases. After calcination at 1100 °C, all compositions in the Mn-rich series crystallized as single-phase orthorhombic perovskites showing no significant variation of the lattice parameters. The Co-rich series showed a much higher thermal expansion coefficient and electrical conductivity than the Mn-rich series. Irregularities in the thermal expansion curves indicated phase transitions at 150–350 °C for the Mn-rich series, while partial melting occurred at 980–1000 °C for the Co-rich series with higher Cu contents. For both series, the influence of the Cu content on the thermal expansion coefficient is not as pronounced as on the electrical conductivity. The electrical conductivity of the Co-rich series increased with the Cu content at temperatures below 600 °C. At higher temperatures, metallic conduction was observed for compositions with x > 0.15. For the Mn-rich series, however, a conductivity maximum was observed at x = 0.10-0.15, and all compositions showed semiconducting behavior.
The authors thank P. Lersch (deceased) and M. Ziegner for XRD measurements, A. Hilgers and M.-T. Gerhards for dilatometric measurements, and H. Lippert and N. Merki (FZJ-ZCH) for chemical analyses.