Phenomenological description of doped manganites. Electron bandwidth, crystal local structure and Curie temperature
Introduction
Colossal magnetoresistance (CMR) and magnetocaloric effects (MCE) observed in the perovskite-like rare earth manganese oxides (manganites) generated a considerable interest to their properties controlled by the substitution in A [1]- and B [2], [3], [4], [5], [6]-positions in the ABO3 cell. Parent LnMnO3 oxides are antiferromagnetic insulator with trivalent manganese ions [1]. The partial substitution of Ln3+ by the divalent alkaline-earth ions causes an essential change in the properties of the materials. La1-zDzMnO3 (D – alkaline earth elements, Pb) manganites became ferromagnetic conductors at 0.2 ≤ z ≤ 0.4 and show CMR and MCE effects at Curie temperature, TC [1]. The ferromagnetism and the conductivity of the perovskites are explained in frames of the extended double exchange (DE) Zener model. According to the model, the noted features are caused by the transfer of mobile (itinerant) eg electrons inside the Mn4+ (3d3: ) matrix, and because the eg spins are oriented along the spins of Mn 3d orbitals [1], [7]. The electronic bandwidth, W, is used to estimate the strength of DE interaction. The W depends on the Mn-O distance and the Mn–O–Mn bond angle and on the overlapping of Mn 3d and O 2p orbitals. The dependence is usually described by the empirical expression W ~ cos(Θ/2)/d3.5, where Θ = π – Mn-O-Mn is an averaged tilt bond angle and d = Mn-O is an averaged bond length in the MnO6 octahedra, the averaging is performed over the three crystallographically independent values associated with the apical and basal oxygens [8], [9], [10]. Remarkably, according to Radaelli et al. [9], the dependence of W on Mn-O-Mn and Mn-O , obtained with the neutron diffraction measurement, was proportional to TC: W ∞TC.
The partial substitution of Mn by other transition metals (M) causes a decrease in the eg electron density which leads to weakening of the DE interaction [1], [11]. Also, the substitution favours antiferromagnetism, AFM, because the coupling in M–O–Mn bonds usually has antiferromagnetic nature [1]. Electron configuration (electron factor, Ef) and ionic radii (local crystal structure – structural factor, Sf) of (other) transition metal ions differ from those of Mn ions. This complicates the interpretation of properties of B-site substituted perovskites. Namely, during the study of the La0.7Ca0.3Mn0.96(Al1-xInx)0.04O3 oxides [12], the variation in metal-insulator transition temperature, TMI, was attributed to the enhanced Mn–O–M bending, or, in other words, to the change in the local crystal structure. A similar conclusion was made by Huang with co-workers [13] when observing a decrease in TC with x in the study of La0.7Ca0.3Mn1-xScxO3: authors attributed the TC change to the local strain effect induced by the essentially larger ionic radius of Sc3+ than those of Mn3+. Ghosh et al. [14] also concluded that for La0.7Ca0.3Mn0.95M0.05O3 (M = Cr, Fe, Co, Ni, Cu, Mn and Zn) perovskites, the magnetotransport properties are controlled by the local strain effect which depends on the dopant ion size. However, Zhao et al. [15] attributed the TC change to the different electron configuration of M-ions in the La0.7Sr0.3Mn0.9M0.1O3 (M = Al, Cr, Mn, Fe, Co, Ni, Cu, and Ga) oxides. According to Dubroka et al. [16], the change in the properties of La1−ySryMn1−xMxO3 (M = Cr, Co, Cu, Zn, Sc or Ga) depends mainly on the ionic radii of dopants and partially on their electron configuration. At the same time, Ulyanov et al. [17], [18], in the study of the La0.7Ca0.3Mn1-xScxO3 (x = 0.0–0.07) [17] and La0.7Ca0.3Mn0.95M0.05O3 (M = Al, Ga, Fe, Mn, and In) [18] manganites, stated that the decrease in TC is related to both electron and structural factors, and the change in the Ef-parameter suppresses the TC more strongly than that of the Sf-parameter.
Thus, in spite of numerous publications devoted to the study of properties of the A- and B-site substituted perovskites, some important features of the manganites are not well investigated, analysed or explained. There is no explanation for the dependence of TC on the doping degree, z, in the A-site substituted Ln1-zDzMnO3 manganites. There is also no clear explanation for the TC changes caused by differences in the electron configuration and the local crystal structure of Mn ions, as well as those of the substituent transition metal ions in B-site substituted perovskites. Here, we present a study of La0.7Ca0.3Mn0.95M0.05O3 (M = Mn, Al, Al/In, and In) perovskites to shed light on the above peculiarities. The choice of dopants allows to distinguish between the effect of the change in local electron and structural configurations on the TC. Namely, Al3+ ions (Ne configuration) and In3+ ions (d10 configuration) with completed outer shells do not participate in the ferromagnetic ordering and could be considered as ions having similar effects on the change in electron structure of undoped La0.7Ca0.3MnO3 oxide and different effects on the local crystal structure due to different ionic sizes of the dopants (see also [12], [17], [18], [19], [20] on this matter).
Section snippets
Experimental
La0.7Ca0.3Mn0.95M0.05O3 (M = Mn, Al, 0.5Al+0.5In, and In) were prepared by the conventional solid state method using the La2O3, Mn2O3, Al2O3 and In2O3 oxides and 1150 °C sintering temperature, as in ref. [21]. X-ray Cu Ka diffraction (XRD) analysis was performed with the diffractometer D/MAX-2500V/PC (Rigaku, Japan) to deduce the crystal structure and the lattice parameters of the perovskites. An inductive magnetometer by an APD Cryogenics device was used to measure the magnetic susceptibility.
Results and discussions
Fig. 1 presents the XRD patterns of the sintered manganites. The samples show the orthorhombic Pnma structure. The corresponding crystallographic data are presented in Table 1. According to the data, the lattice parameters of La0.7Ca0.3Mn0.95M0.05O3 increase with the dopant ionic radii, rd (= 0.535, 0.645 and 0.8 Å for the Al3+, Mn3+, and In3+ ions, respectively [22]). Temperature dependencies of AC susceptibility, χ(T), are presented in Fig. 2. The TC decreases sharply due to the substitution
Conclusions
The La0.7Ca0.3Mn0.95M0.05O3 (M = Mn, Al, Al/In, and In) oxides were analysed in terms of doping and its influence on the fundamental properties of the manganites. Doping causes a decrease in the TC, which is mostly related to the difference in the electron configurations of the Mn3+ and dopant M ions, and is partially caused by a change in the local crystal structure due to a difference in the M 3+ and Mn3+ ion radii. To describe the observed dependence of TC on doping in both the A- and
Acknowledgements
The reported study was funded by Russian Foundation for Basic Research (RFBR) according to the research projects No. 18-08-01071A and No. 16-03-00888a.
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