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Ionics DOI 10.1007/s11581-017-2220-9

ORIGINAL PAPER

Effect of Fe doping on structural, magnetic, and electrical properties of non-magnetic and magnetic rare earth-based perovskite chromites La0.5Nd0.5Cr1-xFexO3 (0 ≤ x ≤ 1) Surby Gupta 1 & Arun Mahajan 1 & Suram Singh 1 & Devinder Singh 1

Received: 13 May 2017 / Revised: 14 June 2017 / Accepted: 29 June 2017 # Springer-Verlag GmbH Germany 2017

Abstract The structural, magnetic, and electrical properties of new series La0.5Nd0.5Cr1-xFexO3 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) have been reported. All the phases crystallize in the orthorhombic symmetry with Pbnm space group. The unit cell volume of La0.5Nd0.5Cr1-xFexO3 increases monotonically with increasing Fe doping. Magnetization studies showed that all our investigated phases (except x = 0.8 and 1.0) exhibit a paramagnetic–anti-ferromagnetic transition at low temperature. The variation in Neel temperature (TN) with increasing x can be explained in terms of magnetic interactions due to Fe doping. Irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization is clearly seen close to TN. All the phases show that insulating behavior and the transport properties are dominated by the polaron hopping mechanism with increase in polaron hopping energy with Fe content. Keywords Ceramic method . Rietveld refinements . Magnetic properties . Transport properties

Introduction Structural and magnetic properties of mixed metal oxides are very complex, especially when the oxide contains more than one magnetic species, or when the crystal structure permits some degree of atomic disorder [1, 2]. The perovskite oxides =

with combination of 3d and 4d or 5d elements like AB1−x B== x O3 (where A is an alkaline earth or rare earth element, and B//B// are * Devinder Singh [email protected] 1

Department of Chemistry, University of Jammu, Jammu 180 006, India

transition metal elements) are easy to synthesize as ordered compounds because of the large difference in the atomic radii. Synthesis of 3d–3d ordered compounds is however difficult because of the very similar ionic sizes. The rare earth orthochromites ReCrO3 (Re = rare earth element) exhibit anti-ferromagnetic behavior with magnetic ordering transition temperature in the range 112–282 K depending upon the type of rare earth element [3, 4]. Both the undiluted perovskites LaCrO 3 and NdCrO 3 are orthorhombic having antiferromagnetic structure with Neel temperatures 256 and 225 K, respectively, along with a weak ferromagnetic component due to spin canting [5, 6]. Substitution of Fe on Cr site gives rise to interesting magnetic properties [7–10]. Azad et al. [11] have shown that LaFe0.5Cr0.5O3 has an orthorhombic structure, GdFeO3 type (space group Pbnm), with random positioning of the Fe and Cr cations in the B sublattice, the same structure featured by the simple perovskites LaCrO3 and LaFeO3. According to the Kanamori–Goodenough (KG) rule, if Fe and Cr were in an ordered arrangement at the B-site, Fe3+(d5)–O–Cr3+(d3) would show ferromagnetic (FM) behavior due to the super-exchange interaction [12, 13]. Magnetic properties of LaFe1-xCrxO3 (0 ≤ x ≤ 1) are strongly dependent on the composition [5]. With increase of Cr content, magnetization first increases, reaches a maximum at x = 0.5, and then starts to decrease. Magnetization reversal has been reported in NdCr1-xFexO3 (x = 0.05 to 0.2) compounds due to competition between weak ferromagnetic component of Cr3+ and paramagnetic moments of Nd3+ and Fe3+ under the influence of negative internal magnetic field [6]. Magnetic properties of ReFe1xCrxO3 (Re = La, Y, or Nd) systems are closely related to Cr doping level for Fe, and the dominant magnetic interactions in them are anti-ferromagnetic [14–16]. It has been reported that the phases NdCr1-xFexO3 show magnetization reversal, while same phenomenon has not been shown by LaCr1-xFexO3 phases, so it would be interesting to

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study magnetic properties of La0.5Nd0.5Cr1-xFexO3 system containing both non-magnetic La and magnetic Nd ions at A-site. Moreover, to the best of our knowledge, the electrical properties of these systems have not been reported so far. In view of this, new materials of composition La0.5Nd0.5Cr1xFexO3 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) have been prepared by solid-state reaction method and their structural, magnetic, and electrical properties have been investigated.

Experimental Polycrystalline samples of La0.5Nd0.5Cr1-xFexO3 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by conventional heat treatment of solid-state reaction method using La2O3 (99.9%, Aldrich products), Nd2O3 (99.9%, Loba Chemicals products), Fe2O3 (99.9%, Aldrich products), and Cr2O3 (99.9%, Aldrich products) as starting materials. La2O3 and Nd2O3 were dried by heating in a furnace at 1000 °C for 12 h so as to drive out moisture and carbon dioxide that have been easily absorbed from air by the oxides. The reactant powders were weighed corresponding to the stoichiometry of the desired phases and then mixed in cyclohehane using an agate mortar and pestle to produce the intended compositions. The mixed powders were heated in the furnace at 800 °C in air for 10 h. The powders were then taken out from the furnace, thoroughly ground, pressed into pellets, and heated at 1300 °C in air for 60 h with a number of grinding and pelleting cycles to ensure the

homogeneity of the samples. Finally, the samples were cooled down slowly to room temperature in the furnace. The phase analysis was carried out by powder X-ray diffraction (XRD) with a PANalytical X’Pert PRO MRD, Netherlands, using Ni-filtered CuKα radiation. For Rietveld refinements, the data were collected in the 2θ range 20–100o with a step size of 0.017o and step duration of 21 s. The surface morphology and microstructures of the samples were studied by scanning electron microscope FE-SEM Quanta 200 FEG. The elemental analysis of the samples was done by energy dispersive X-ray analysis (EDX) using INCA attachment with the SEM instrument. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements were performed using Faraday technique in the temperature range 80–300 K at 0.3 T magnetic field. The electrical resistivity of the sintered pellets of the phases was recorded by four probe method in the liquid nitrogen temperature range. Thin copper wires were attached to the surface of pellet for the purpose of electrodes with silver epoxy.

Results and discussion The XRD patterns for La0.5Nd0.5Cr1-xFexO3 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) showed that all our synthesized samples were almost single phase with minor impurity of La2O3. All reflection lines were successfully indexed according to the orthorhombic perovskite structure. The data were refined by the Rietveld technique using the GSAS program [17]. Rietveld

Table 1 Structural parameters obtained from the Rietveld refinement of X-ray diffraction pattern for La0.5Nd0.5Cr1−xFexO3 samples. The atomic sites are as follows: La/Nd 4c[x, y, 0.25]; Fe/Cr 4b [0, 0.5, 0]; O(1) 4c[x, y, 0.25]; and O(2) 8d[x, y, z] in the space group Pbnm x

0.0

0.2

0.4

0.6

0.8

1.0

a (Ǻ) b (Ǻ) c (Ǻ)

5.4675(8) 5.4741(7) 7.7345(12)

5.4758(4) 5.4923(3) 7.7510(9)

5.5101(4) 5.4760(5) 7.7548(8)

5.5277(2) 5.4884(4) 7.7730(5)

5.5425(2) 5.4936(2) 7.7840(3)

5.5577(3) 5.4972(3) 7.7980(4)

231.49(1) −0.00173(21) 0.0755(17) 0.7157(13) 0.02947(21) 0.4955(17) 0.2735(13) 0.0326(13) 0.01687(8) 0.01233(3) 0.02601(6) 0.01795(8) 0.0798 0.0624 2.061

233.11(1) −0.00084(19) 0.0743(16) 0.7235(11) 0.03036(19) 0.4943(16) 0.2813(11) 0.0404(11) 0.01801(5) 0.01171(7) 0.02649(9) 0.01800(5) 0.0672 0.0518 1.640

233.99(1) −0.0032(4) 0.0727(28) 0.7216(18) 0.0280(4) 0.4927(28) 0.2794(18) 0.0385(18) 0.01920(5) 0.01249(4) 0.02057(5) 0.01453(5) 0.0894 0.0659 2.628

235.82(1) −0.00240(32) 0.0714(23) 0.7314(13) 0.02881(32) 0.4914(23) 0.2892(13) 0.0483(13) 0.01936(6) 0.01468(4) 0.02738(7) 0.01767(7) 0.0819 0.0571 3.218

237.01(1) −0.0049(4) 0.0466(28) 0.7432(13) 0.0263(4) 0.4666(28) 0.3010(13) 0.0501(13) 0.02258(8) 0.01179(6) 0.02633(7) 0.01560(7) 0.0821 0.0560 4.006

238.24(1) −0.0057(6) 0.0491(4) 0.7377(19) 0.0255(6) 0.4521(4) 0.2955(19) 0.0546(19) 0.02441(6) 0.01265(6) 0.02681(8) 0.02018(7) 0.0913 0.0642 3.681

V (Ǻ3) x

y

z Uiso (Ǻ2)

Rwp Rp χ2

La/Nd O(1) O(2) La/Nd O(1) O(2) O(2) La/Nd Fe/Cr O(1) O(2)

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refinements were carried out in the orthorhombic space group Pbnm for all the synthesized samples in which La/Nd atoms have been located at 4c (x, y, 0.25) position, Cr/Fe atoms at 4b (0, 0.5, 0), and oxygen atoms occupy two different sites, namely O1 at 4c (x, y, 0.25) and O2 at 8d (x, y, z). The background Rietveld refinement was fitted with a shifted Chebyschev polynomial function, and pseudo-Voigt function was employed to model the peak shapes in all cases. A sixth-order Chebyschev polynomial for the background, zero, LP factor, scale, pseudoVoigt profile function (U, V, W, and X), lattice parameters, atomic coordinates, and isothermal temperature factors Uiso were used in the refinement. Isotropic thermal displacement parameters, initially set at 0.025 Å2, were refined first for the metal atoms and then for the oxygen atoms with full occupancy. The occupation factors for the metals were fixed by taking sample stoichiometry into account, while those of oxygen atoms were refined. No evidence of oxygen nonstoichiometry could be obtained from the XRD structural refinements and the oxide ion sites were therefore fixed at full occupancy. The structural parameters obtained from the structural refinements are listed in Table 1. In this table, the residuals for the weighted pattern Rwp, the pattern Rp, and the goodness of fit χ2 are also reported. An excellent agreement was found between the experimental spectra and the calculated values. The observed and calculated diffraction profiles of the samples are shown in Fig. 1. Table 2 contains the selected Cr/Fe–O–Cr/Fe bond angles and individual Cr/Fe–O bond lengths as well as their averages. The average