Structural, electronic and magnetic properties of the series of double perovskites (Ca,Sr)2−xLaxFeIrO6

doi:10.1016/j.jssc.2014.01.007

Journal of Solid State Chemistry

Volume 212, April 2014, Pages 23-29

https://doi.org/10.1016/j.jssc.2014.01.007 Get rights and content

Highlights

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The double perovskite series ${(Ca, Sr)}_{2 - x} {La}_{x} {FeIrO}_{6}$ were synthesized.
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Changes in the Fe-Ir magnetic coupling due to La doping on both series.
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Evidence of high $T_{C}$ on ${Sr}_{1.2} {La}_{0.8} {FeIrO}_{6}$ .
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Indication of Mott insulator behavior on ${Sr}_{2} {FeIrO}_{6}$ .

Abstract

Polycrystalline samples of the series of double perovskites ${Sr}_{2 - x} {La}_{x} {FeIrO}_{6}$ were synthesized. Their structural, electronic and magnetic properties were investigated by X-ray powder diffraction, Mössbauer spectroscopy, magnetic susceptibility, heat capacity and electrical resistivity experiments. The compounds crystallize in a monoclinic structure and were fitted in space group $P 2_{1} / n$ , with a significant degree of Fe/Ir cationic disorder. As in ${Ca}_{2 - x} {La}_{x} {FeIrO}_{6}$ the Sr-based system seems to evolve from an antiferromagnetic ground state for the end members (x=0.0 and x=2.0) to a ferrimagnetic order in the intermediate regions ( $x ~ 1$ ). Since Mössbauer spectra indicate that Fe valence remains 3+ with doping, this tendency of change in the nature of the microscopic interaction could be attributed to Ir valence changes, induced by La³⁺ electrical doping. Upon comparing both Ca and Sr series, ${Sr}_{2 - x} {La}_{x} {FeIrO}_{6}$ is more structurally homogenous and presents higher magnetization and transition temperatures. Magnetic susceptibility measurements at high temperatures on ${Sr}_{1.2} {La}_{0.8} {FeIrO}_{6}$ indicate a very high ferrimagnetic Curie temperature $T_{C} ~ 700 K$ . For the Sr₂FeIrO₆ compound, electrical resistivity experiments under applied pressure suggest that this material might be a Mott insulator.

Graphical abstract

The Weiss constant as a function of La doping for the ${(Ca, Sr)}_{2 - x} {La}_{x} {FeIrO}_{6}$ series, indicating changes in Fe–Ir magnetic coupling on both families.

Introduction

Perovskite-type transition metal oxides with the general formula AB O₃ have been subject of intense scientific investigation since the discovery of magnetoresistance in manganese-based compounds [1], [2]. A variant of the perovskite structure, namely the double perovskite (DP) also presents interesting physical phenomena. These $A_{2} B' B ″ O_{6}$ materials can be seen as a duplication of the simple perovskite unit cell with cation A occupying the cube vertices while cations $B'$ and $B ″$ sit alternately at the center of the oxygen octahedron. Recently, tunneling-type magnetoresistance at room temperature in Sr₂FeMoO₆, Sr₂FeReO₆ and related compounds has attracted considerable interest due to its potentiality as magneto-electronic devices, such as magnetic tunnel junctions or low-field magnetoresistive sensors [3], [4], [5], [6], [7]. These materials are believed to be ferrimagnetic half-metals with highly spin polarized conduction electrons, which are understood as d-like electrons and strongly connected with the structural and magnetic properties. Actually, the level of delocalization of the cation $B ″$ electrons (e.g., Re and Mo), which generally present 4d or 5d character, is thought to be the key ingredient for the magneto-electronic properties of these families [4], [5], [6], [8], [9]. For instance, the large unquenched Re 5d orbital moments are crucially connected with the magneto-electric properties of the half-metallic Ba₂FeReO₆ [5].

Intense research in these compounds has been dedicated to study the effects due to the variation of the metallic/magnetic ions on the $B'$ and $B ″$ sites as well as charge carrier doping, where the divalent alkaline earth ions (e.g., Ca, Sr and Ba) on the A site are partially replaced by a trivalent rare earth ion such as La [10]. In this way one expects to understand the electronic structure and the magnetic exchange interaction in these DPs in detail. Despite the recent progress there is still an open debate about the particularities of the interplay among the structural, electronic and magnetic degrees of freedom in these materials. Since changing the number of conduction electrons, achieved by partially doping on the A site, is accompanied by significant steric effects as anti-site disorder, it is difficult to distinguish between electronic and structural influences [10], [11], [12], [13].

To further explore the open issues above and the real potential of DP compounds as magneto-electronic devices, investigation of other DPs with different 4d or 5d transition metal on the cation $B ″$ site is necessary. In this context, Ir, like Re and Mo, is a transition metal ion that can have strongly hybridized 5d electrons and exist in five different integer valence states (2+ to 6+). Therefore, it is interesting to study both the influence of structural changes (in bond length and bond angles) and the effect of charge carrier doping in other examples of DPs. In our previous reports [14], [15], we showed some preliminary results for the ${Ca}_{2 - x} {La}_{x} {FeIrO}_{6}$ and ${Sr}_{2 - x} {La}_{x} {FeIrO}_{6}$ series. Like other Ir-based DPs [16], these compounds were fitted well with space group $P 2_{1} / n$ with a significant Fe/Ir cationic disorder. However, different studies in Sr₂FeIrO₆ suggested two distinct symmetries, $I \bar{1}$ [17] and $I 2 / m$ [18]. Thus, there is an open debate about the structure of these compounds and new information concerning the Fe/Ir-based DP's is important to shed some light on the understanding of these systems.

In this work, we report studies of X-ray powder diffraction, magnetic susceptibility, magnetization as a function of applied magnetic field, Mössbauer spectroscopy, specific heat and electrical resistivity experiments in the series of compounds ${Sr}_{2 - x} {La}_{x} {FeIrO}_{6}$ . We discuss the Rietveld refinement of Sr₂FeIrO₆ and SrLaFeIrO₆ in three different space groups, $P 2_{1} / n$ , $I \bar{1}$ and $I 2 / m$ . We also report new results (magnetic, specific heat, and Mössbauer spectroscopy) for the analog compounds ${Ca}_{2 - x} {La}_{x} {FeIrO}_{6}$ and explore the connection among the structural, electronic and magnetic properties to bring some light on the role of cationic disorder in the microscopic magnetic interactions in these materials. Also, we investigate the very high Curie temperature $T_{C} ~ 700 K$ observed for intermediate regions ( $x ~ 1$ ) of the Sr-based series.

Section snippets

Experimental details

Polycrystalline samples of ${Sr}_{2 - x} {La}_{x} {FeIrO}_{6}$ (x=0.0, 0.5, 0.8, 1.0, 1.2, 1.6 and 2.0) were synthesized by solid-state reaction. Stoichiometric amounts of SrCO₃ and La₂O₃, Fe₂O₃ and Ir metallic powder were mixed and heated in air at 900 °C for 24 h in a conventional tubular furnace. After the first step the material was re-grounded and then pressed into pellets, which were subsequently heated at 1100 °C for 24 h in air. The ${Ca}_{2 - x} {La}_{x} {FeIrO}_{6}$ compounds were synthesized at the same temperatures, as reported

Results and analysis

The Sr₂FeIrO₆ XRD pattern and its corresponding Rietveld analysis with the $P 2_{1} / n$ space group are shown in Fig. 1. For comparison, some XRD data were also refined in $I \bar{1}$ (reported in Battle et al. neutron powder diffraction (NPD) and XRD studies [17]) and in $I 2 / m$ (reported in Qasim et al. synchrotron XRD and NPD studies [18]) space groups. The main results are listed in Table 1, where one can see that despite the fact that XRD data could be fitted well in three symmetries, the $χ^{2}$ , R_p and R_wp

Conclusions

We have successfully synthesized polycrystalline samples of the ${Sr}_{2 - x} {La}_{x} {FeIrO}_{6}$ series. We shown the results of XRD, magnetic, thermal and electrical measurements for this series and also for ${Ca}_{2 - x} {La}_{x} {FeIrO}_{6}$ . The XRD patterns can be fitted well in a monoclinic structure and despite the fact that our XRD results were slightly better fitted in the $P 21 / n$ space group, other studies using NPD and synchrotron XRD reported the Sr₂FeIrO₆ structure to have $I \bar{1}$ [17] and $I 2 / m$ [18] symmetries. It would be

Acknowledgments

This work was supported by CAPES, FAPESP-SP and CNPq (Brazil).

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Effects of Nd doping on the magnetocaloric properties of double perovskite Sr<inf>2</inf>FeMoO<inf>6</inf>
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In this study, double perovskite oxide Sr₂FeMoO₆ doped with Nd was prepared through sol–gel method. SNFMO belongs to the I4/m space group. The Fe/Mo order of SNFMO obtained from Rietveld refinement are 89.3%, 88.6%, and 88.2% for x = 0.01, 0.03, and 0.05, respectively. For SNFMO (x = 0.01, 0.03, 0.05) samples, the values of Fe³⁺/Fe²⁺ ratio are 1.41, 1.51, and 1.53, and the values of Mo⁵⁺/Mo⁶⁺ ratio are 0.37, 0.48, and 0.54. According to the minimum dM/dT of the FC curves, the Curie temperatures of SNFMO are 378, 367, and 380 K for x = 0.01, 0.03, and 0.05, respectively. H/M-M² curve analyses showed that all the samples have a second-order magnetic transition. In the magnetic field of 2.5 T, the maximum ΔS_M of SNFMO (x = 0.01, 0.03, 0.05) samples were 0.36, 0.63, and 1.23 J kg⁻¹ K⁻¹, respectively.
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Structural, electronic and magnetic properties of the series of double perovskites (Ca,Sr)2−xLaxFeIrO6

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental details

Results and analysis

Conclusions

Acknowledgments

Physica

J. Solid State Chem.

Physica B

J. Solid State Chem.

J. Solid State Chem.

J. Solid State Chem.

Mater. Res. Bull.

Rev. Mod. Phys.

Nature

Phys. Rev. B

Phys. Rev. Lett.

Appl. Phys. Lett.

Science

Structural, electronic and magnetic properties of the series of double perovskites ${(Ca, Sr)}_{2 - x} {La}_{x} {FeIrO}_{6}$