Elsevier

Materials Chemistry and Physics

Volume 182, 1 October 2016, Pages 459-465
Materials Chemistry and Physics

Physical properties of Sr2FeIrO6 and Sr1.2La0.8FeIrO6 double perovskites obtained by a new synthesis route

https://doi.org/10.1016/j.matchemphys.2016.07.057Get rights and content

  • Sr2−xLaxFeIrO6 (x = 0.0 and 0.8) samples were synthesized using a new heating route.

  • La3+ to Sr2+ substitution induces the structural transition from I2/m → P21/n.

  • The high temperature magnetic transition previously reported for x = 0.8 is probably due to impurity.

  • Fe3+–Ir5+ to Fe3+–Ir4+ valence configuration changes govern the physical properties.

Abstract

Previous works on Sr2-xLaxFeIrO6 double perovskite (DP) series reported a possible ferromagnetic transition at T ∼ 700 K for the x = 0.8 concentration, for which was observed the presence of spurious Fe2O3 phase. In order to prevent the formation of this impurity phase and check if this high temperature magnetic transition is intrinsic of the material, different synthesis routes became necessary. In this work, polycrystalline samples of Sr2-xLaxFeIrO6 (x = 0.0 and 0.8) have been synthesized by solid state reaction using a new heating treatment. The sample's properties were investigated by synchrotron x-ray powder diffraction (SXRD), transmission electron microscopy (TEM), magnetic susceptibility, specific heat and electrical resistivity, and compared with the previously reported results. The SXRD data revealed a structural transition induced by La to Sr substitution (I2/mP21/n). Moreover, it was not detected the presence of Fe2O3 on the samples obtained by the new route, which might be related to the absence of high temperature magnetic ordering. The magnetometry results indicated the emergence of Ir4+ with La doping, being corroborated by specific heat measurements which suggest Fe3+/Ir5+ and Fe3+/Ir4+ configurations for x = 0.0 and 0.8 compounds, respectively. Temperature dependent electrical resistivity measurements showed that Sr2+ to La3+ substitution leads to a decrease of electrical resistivity, possibly associated with the increase in the number of Ir valence electrons.

Introduction

Double perovskite (DP) compounds have attracted great attention due to their interesting physical properties and potential for technological applications in magneto-electronic devices [1], [2]. These materials have the general formula A2BBO6, where A is a rare-earth/alkaline-earth cation, while B and B′ are two different transition-metal ions. Usually, the most interesting physical phenomena observed for these oxides are associated with the BB′ interactions and to the strong correlation between structural, electronic and magnetic properties [3], [4], [5].

In this context, Fe-based DPs became more noticed since the discovery of half-metallic behavior and room temperature ferrimagnetism (FIM) on Sr2FeMoO6 and Sr2FeReO6 [1]. Contrastingly, initially Ir-based DPs have received less attention, despite its extended 5d orbital delocalization and different possible valence states. But recently the interest in iridates emerged due to Ir exotic magnetic properties. For instance, recent results observed for Sr2IrO4 indicates a quantum topological state of matter [6]. Also, studies on Sr2YIrO6 and related compounds suggest magnetic ordering of Ir5+ ions, which are anticipated to have zero net magnetic moment singlet ground state in the strong spin-orbit coupling [7], [8], [9].

Due to the intrinsic ionic and magnetic inhomogeneity commonly observed on transition-metal oxides, Ir-based DPs may also serve to application purposes. La1.5 Ca0.5 CoIrO6 was recently reported as a disordered system for which the presence of multiple magnetic phases leads to a re-entrant spin glass (RSG) behavior [10]. This RSG character, induced by the structural disorder, is believed to be closely connected to the spontaneous exchange bias effect observed for this compound. To further explore the interesting properties above mentioned and verify the currently proposed ideas, the investigation of other Ir-based DPs is of fundamental importance.

In a previous work we have reported that the A2−x LaxFeIrO6 (A = Ca,Sr) systems evolve from an antiferromagnetic (AFM) ground state for the end members (x = 0.0 and 2.0) to a FIM order for intermediate doping (x ∼ 1) [11]. It was also observed some controversial results. Firstly, the x = 0.8 concentration presented evidence of a high temperature ferromagnetic transition (∼700 K). But it was also detected the presence of Fe2 O3 impurity phase (∼1%). Moreover, our laboratory x-ray diffraction (XRD) pattern for the x = 0.0 concentration was best described as belonging to P21/n monoclinic space group at room temperature, while Qasim et al. recently reported this same compound to belongs to the monoclinic I2/m space group [12], and earlier Battle et al. reported it to crystallizes in triclinic I1¯ space group [13]. Thus, the structural and magnetic properties of Sr2-xLaxFeIrO6 series of compounds are not completely established and further investigation is necessary to shed some light on the understanding of the system.

In this work, the x = 0.0 and 0.8 concentrations of Sr2-xLaxFeIrO6 series were synthesized by the solid-state reaction technique using a different heating treatment, in order to prevent the presence of Fe2 O3 spurious phase. The structural properties of these new materials, here called route-b compounds, were investigated using synchrotron x-ray powder diffraction (SXRD and transmission electron microscopy (TEM) data, and compared to those of our previous work (route-a compounds). The magnetic evolution of the system was studied by means of temperature-dependent magnetization. The structural and magnetic analysis for both compounds were supported by specific heat and electrical resistivity measurements, in order to explore the interplay between structural, electronic and magnetic properties in this series.

Section snippets

Experimental details

Polycrystalline samples of Sr2FeIrO6 (x = 0.0) and Sr1.2La0.8FeIrO6 (x = 0.8) were synthesized by solid-state reaction in a conventional tubular furnace and air atmosphere. For the now on called route-b compounds, stoichiometric amounts of SrCO3, La2 O3, Fe2 O3 and metallic Ir were mixed and heated at 800 °C for 12 h. After this first step, samples were re-grinded and heated progressively at 975 °C (2 days), 1050 °C (5 days) and 1100 °C (7 days). Finally, the materials were grinded, pressed

Results and analysis

Fig. 1 shows the SXRD patterns and corresponding Rietveld analysis for x = 0.0 and 0.8 compounds growth by the route-b method. In contrast to our earlier laboratory XRD results [11], the more accurate and higher resolution SXRD pattern for x = 0.0 was better fitted with the I2/m space group, in agreement with Qasim's et al. report of SXRD and neutron diffraction studies [12]. Some refined parameters are listed in Table 1. The XRD results for the samples growth by the route-a method are also

Conclusion

Two representative concentrations (x = 0.0 and 0.8) of the Sr2-xLaxFeIrO6 series were synthesized using a new heating treatment, for which the furnace temperature and baseline were increased. The physical properties of both compounds were compared to those reported for samples obtained by different routes. Rietveld refinement of SXRD data for the x = 0.0 sample indicated this compound crystallizes in I2/m monoclinic space group at room temperature, in agreement with Qasim et al. work [12].

Acknowledgments

This work was supported by the Brazilian funding agencies CNPq (Grants No. 470.613/2012-2; 304649/2013-9; 442230/2014-1), FAPESP (Grants Nos. 2006/60440-0; 2007/50968-0; 2012/04870-7) and FAPERJ (Grant No. 111.382/2013). LNLS is acknowledged for concession of beamtime.

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