The new ordered double perovskite SrLaCuIrO6

The new ordered double perovskite SrLaCuIrO6

Accepted Manuscript The new ordered double perovskite SrLaCuIrO6 Klaus K. Wolff, Liu Hao Tjeng, Martin Jansen PII: S0038-1098(18)30661-6 DOI: https...

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Accepted Manuscript The new ordered double perovskite SrLaCuIrO6 Klaus K. Wolff, Liu Hao Tjeng, Martin Jansen PII:

S0038-1098(18)30661-6

DOI:

https://doi.org/10.1016/j.ssc.2018.11.021

Reference:

SSC 13544

To appear in:

Solid State Communications

Received Date: 10 September 2018 Revised Date:

19 November 2018

Accepted Date: 27 November 2018

Please cite this article as: K.K. Wolff, L.H. Tjeng, M. Jansen, The new ordered double perovskite SrLaCuIrO6, Solid State Communications (2018), doi: https://doi.org/10.1016/j.ssc.2018.11.021. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT Klaus K. Wolffa,*1, Liu Hao Tjenga and Martin Jansena,b,**

The new ordered Double Perovskite SrLaCuIrO6 Abstract: The new fully rock-salt ordered Ir5+ double perovskite SrLaCuIrO6 was synthesized, and its structural, magnetic and electronic properties were investigated. The compound crystallizes tetragonally (space group I4/m). The Cu2+ cation leads to strong Jahn-Teller distortion of the CuO6 octahedra. The compound shows semiconducting behavior with an estimated band gap of approximately 0.2 eV. At around 10 K a magnetic transition is observed.

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Keywords: A. Double perovskite; D. Jahn-Teller distortion; D. Magnetic properties; E. High-temperature synthesis

1 Introduction

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The double perovskite (DP) structure type of general formula AA’BB’O6 continues to attract a lot of attention.[1] This is partly due to a marked tunability of the electronic properties, as magnetic effects, bandgap or transport behavior. Among this class of compounds are relaxor ferroelectrics,[2] half-metallic antiferromagnets,[3] high-Tc metallic ferromagnets,[4] and ferrimagnets,[5] or insulating high-Tc ferrimagnets.[6] For example, Bi2NiMnO6, CaMnTi2O6 and Bi2FeCrO6 are showing ferroelectric character, with the latter two having potential for applications as photovoltaic energy conversion.[7] The right choice of elements positioned on the B and B’ sites is the most important tool in realizing the desired physical properties. Variation of the A-type cations allows the adjustment of the oxidation states of the B and B’ cations. Presence of copper(II) as a B-type cation has been shown to generate particular structural and magneto-electronic features. Due to the Jahn-Teller distortion innate to Cu2+ in octahedral coordination, such DPs show considerable inclination to B-site ordering and significant deviations from cubic symmetry. The underlying cooperative elongation of the CuO6 octahedra with the elongated CuO-bonds pointing towards the same direction (z) results in pronouncedly anisotropic magnetic exchange patterns. Since the copper 3dz2, 3dxz and 3dyz orbitals are fully occupied and the 3dx2–y2 orbitals are half-filled, comparatively strong magnetic interactions within the xy plane result, driving the systems to quasi two-dimensional magnetic behavior. However, such an expectation has only been confirmed for DPs containing non-magnetic B’ cations. Examples are the DP series A2CuB’O6 (A = Sr, Ba; B’ = W, Te), which are antiferromagnetic with a reduced dimensionality of the magnetic interactions.[8,9] In contrast, in DPs based on combinations of copper(II) and open-shell B’ cations the low-dimensional character of magnetic response appears to be lifted. Sr2CuIrO6 with hexavalent iridium, for instance, shows a magnetic transition at 15 K with a considerable degree of frustration,[14] whereas the respective Ir(IV) compound La2CuIrO6 is antiferromagnetic with a TN of 70 K.[11] Against this background we were interested to explore if presence of iridium(V) will show effects similar to conventional closed shell B’ cations. Ir5+ (5d4 electron configuration) is expected to feature an effective nonmagnetic ground state with Jeff = 0,[14] driven by spin-orbit coupling. In fact, all iridates(V) reported so far show a van-Vleck-type temperature independent paramagnetism in combination with small temperature-dependent contributions.[14–17] In combining Cu2+ and Ir5+ as B and B’ cations, respectively, in targeted SrLaCuIrO6 we aim at tracking the influence of weakly paramagnetic Ir5+ on the magnetic ordering. Here we report on synthesis, structure refinement and basic physical characterization of the title compound.

2 Results and Discussion The new rock-salt ordered double perovskite oxide, SrLaCuIrO6, was synthesized by solid state reaction from thoroughly mixed binary constituents. The black polycrystalline product was stored and handled under dry argon due to its slight sensitivity towards humid air. ―― 1 * Dr. K. Wolff E-Mail: [email protected] ** Prof. Dr. M. Jansen E-Mail: [email protected] a Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany b Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany

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ACCEPTED MANUSCRIPT Rietveld refinement[18] of the obtained powder X-ray diffraction pattern (Figure 1) shows that the title compound crystallizes tetragonal in the space group I4/m (no. 87). Details of the structural refinement are depicted in Table 1. From the X-ray diffractogram it is obvious that there is a small amount of unknown side phase present in the sample.

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Table 1. Structure parameters from Rietveld refinements of the powder X-ray diffraction patterns at ambient temperature for SrLaCuIrO6. Atom Site x/a y/b z/c Beq / Å2 3 –3 I4/m, a = 5.4781(2) Å, c = 8.2673(4) Å, Z = 2, Vcell = 248.10(2) Å , Dcalc = 7.7412(6) g cm , Rexp = 2.08, Rp = 2.24, Rwp = 3.50. 1 1 Sr/La 4d 0 /2 /4 1.7(2) 1 Cu 2b 0 0 /2 2.5(3) 1 Ir 2a 0 0 /2 1.5(2) O1 8h 0.208(2) 0.294(2) 0 6.4(4) O2 4e 0 0 0.231(1) 6.4(4)

Figure 1. Powder X-ray diffractogram of SrLaCuIrO6. Red dots represent experimental data, black solid line is the calculated Rietveld fit corresponding to the double perovskite model (space group I4/m), the blue curve depicts the difference plot between experimental and calculated pattern, and green bars indicate positions of the Bragg reflections for the targeted phase.

A perspective view of the crystal structure of SrLaCuIrO6 is shown in Figure 2. According to Glazer’s notation[19] a rock-salt ordered A2BB’O6 double perovskite with the space group I4/m (no. 87), as found for SrLaCuIrO6, corresponds to a tilting pattern of a0a0c–[20]. Starting from the undistorted Elpasolite lattice in Fm–3m, the symmetry reduction caused by Jahn-Teller distortion and tilting about [001] leads to the observed tetragonal structure in I4/m.[21]

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Figure 2. Perspective view of the crystal structure of SrLaCuIrO6. Thick black lines indicate the margins of the unit cell.

Relevant distances for SrLaCuIrO6, obtained from the Rietveld refinement, are listed in Table 2. Table 2. Selected interatomic distances for SrLaCuIrO6. Distance d/Å Cu–O1 1.958(9) Cu–O2 2.225(10) Ir–O2 1.906(10) Ir–O1 1.975(9)

(4×) (2×) (2×) (4×)

Due to the pronounced Jahn-Teller distortion as to be expected for the Cu2+ ion (3d9), the CuO6 octahedra are strongly elongated along the c-axis. This induces some compression of the IrO6 octahedra along the same direction, and the respective observed distances deviate slightly from the values found in related compounds not containing any Jahn-Teller-active ions. In contrast, the Ir-O distances within the ab plane (Ir–O1 = 1.975(9) Å) are in the range of other published iridium-containing double perovskites. (For example, oxygen to iridium distances are reported to be 1.974, 1.966, 1.970 and 1.99 Å in Sr2YIrO6[12,14,15], Ba2YIrO6[14], Sr2FeIrO6[22] and Sr2CoIrO6[23], respectively). In the Ir6+ compound Sr2CuIrO6[10] the Ir-O distances are 1.940(6) and 1.970(8) Å within and perpendicular to the ab plane,

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respectively. In this case the [IrO6]6– octahedra show a slightly lower degree of compression compared to the present compound. The monoclinic Ir4+ compound La2CoIrO6[24] exhibits considerably larger Ir-O distances (1.987(13) to 2.069(9) Å). This fact can be interpreted as a sign that the iridium stoichiometry of SrLaCuIrO6 is close to +5, with more probable deviation towards +6 than +4. The Cu-O distances (1.958(9) and 2.225(10) Å, respectively) are in agreement with other JT-distorted perovskites including Cu2+. (Which have been described to be in the range of 2.00 and 2.30 Å for Sr2CuMoO6 and Sr2CuWO6[25] as well as 1.951(6) and 2.237(8) Å for Sr2CuIrO6[10].) A chemical analysis on the sample gave the following results: SrLaCuIrO6 (578.288 g mol–1); Sr 15.04(6) (calc. 15.15), La 24.62(8) (24.02), Cu 11.24(3) (10.99), Ir 32.5(2) (33.24), O 16.47(3) (16.60) wght-%. There seems to be a small systematic deficiency of iridium of about 0.7 weight-%, probably due to some volatility of IrO2 at the high reaction temperature applied. As stated earlier,[17] the resulting slightly elevated oxygen to iridium ratio is an indication of rather a higher than a lower oxidation state of some portion of iridium in the compound, which makes the presence of a low amount of Ir6+ or a coexistence of Ir6+ besides Ir4+ likely. On a sintered polycrystalline pellet of SrLaCuIrO6 electrical transport properties were measured (see Figure 3). The plot shows semiconductor-like temperature characteristics. At room temperature, the resistivity value is in the range of 0.1 Ω m. The resistivity of the compound exceeds the instrumental limit below a temperature of about 75 K. Fitting the Arrhenius equation to the resistivity data results in an approximately linear relation of ln(ρ) vs. T–1 and results in an estimated band gap of Eg ≈ 0.2 eV. Obviously, rather a variable-range hopping transport model[26] applies instead of a simple thermally activated process, as fitting of ln(ρ) on a T−1/4 scale results in a better match compared to the Arrhenius plot.

Figure 3. Plot of the electrical resistivity versus temperature for SrLaCuIrO6. Insets show the fit (blue line) applying the simple thermal activation (top) and variable-range hopping transport model (bottom), respectively.

In Figure 4 the temperature-dependent molar magnetic susceptibility χmol (black points) and inverse susceptibility (χ – χ0)mol–1 (red points) of SrLaCuIrO6 are displayed. The ZFC curve of χmol vs. T shows a maximum at a temperature of around 10 K. Sr2CuIrO6 displays a magnetic transition at 15 K, which has been interpreted to be most likely antiferromagnetic in nature.[10] In contrast, Sr2Cu(W,Mo)O6,[25] as well as A2CuB’O6 (A = Sr, Ba; B’ = W, Te) do not have clear signs of long-range magnetic order.[8,9] Due to broad maxima in the susceptibility curves, low-dimensional anti-

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ferromagnetic interactions have been proposed for Sr2Cu(W,Mo)O6, instead.[25] ZFC and FC susceptibilities diverge below the transition temperature (see inset of Figure 4), indicating that there are some weak ferromagnetic interactions at low temperature. For the van-Vleck paramagnetic contribution a value of χ0,mol = 10×10–4 emu mol–1 Oe–1 was determined experimentally. The sum of the diamagnetic susceptibilities of the individual ions[27] amounts to temperatureindependent diamagnetic contribution of about −1.4×10−4 emu mol–1 Oe–1, which leads to a total of 11.4×10−4 emu mol–1 Oe–1. χ0,mol values for similar compounds vary in the literature. For Sr2YIrO6[12] and Ba2YIrO6[13] with 10.1×10–4 and 7.5×10–4 emu mol–1 Oe–1 relatively close numbers are found. However, the values for SrLaNiIrO6, SrLaMgIrO6 and SrLaZnIrO6 samples are smaller (5×10–4, 3.5×10–4 and 3.9×10–4 emu mol–1 Oe–1).[17] By performing Curie-Weiss analysis in the region well above the transition, we extracted an effective magnetic moment µeff of 1.9 µB and a Weiss constant θW of –60 K. The obtained value for the experimental magnetic moment is relatively close to the spin-only value of 1.73 µB for a 3d9 (t2g6eg3) S = 1/2 system as represented by Cu2+. θW deviates considerably from the observed transition temperature. Similar compounds like Sr2CuIrO6[10] and La2CuIrO6[11] are showing Weiss constants of –374 and – 49 K, respectively. The rather small value of |θW|/Ttransition of 6 compared to a value of 25 for Sr2CuIrO6[10] leads to the assumption that the degree of frustration of the exchange interactions is low. Assuming that there are only Ir6+ magnetic impurities with an effective magnetic moment of 3.1 µB[17] contributing to the Curie signal, we estimated this amount to be about 7 %. Obviously, this value has to be considered with caution since also Ir4+ can be present.

Figure 4. Temperature dependence of the molar and inverse molar magnetic susceptibilities of SrLaCuIrO6 at fields of 10 kOe in the temperature range of 2–380 K. The inset shows a magnification of the low-temperature region of χmol.

Isothermal magnetization curves of SrLaCuIrO6 at five different temperatures in the range between 2 and 60 K can be seen in Figure 5. Below 30 K, a hysteresis is clearly visible. Despite its quite pronounced appearance, magnetic impurities cannot be completely ruled out to be the origin of this ferromagnetic response. To gain closer insights into the magnetism of this compound, neutron diffraction experiments would be required, which will also give a more precise value for the transition temperature.

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3 Conclusions

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Figure 5. Isothermal magnetization for SrLaCuIrO6 at temperatures of 2, 20, 30, 40 and 60 K in fields from –50 to +50 kOe.

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The new double perovskite SrLaCuIrO6, containing Ir5+ (5d4), was synthesized and characterized. The fully rocksalt ordered compound crystallizes in the tetragonal space group I4/m (Glazer notation of the tilted perovskite system: a0a0c–). Due to the presence of the Cu2+ (3d9) cation, strong Jahn-Teller distortion of the CuO6 octahedra is observed, which leads to elongation along the c-axis. Electrical transport measurement of the polycrystalline sample shows semiconducting behavior with an estimated band gap of around 0.2 eV. SrLaCuIrO6 shows a weak magnetic transition at a temperature of about 10 K. However, contrary to the DPs A2CuB’O6 (A = Sr, Ba; B’ = W, Te) and Sr2Cu(W,Mo)O6 with closed-shell B’ cations, weakly paramagnetic Ir5+ does not trigger low-dimensional magnetism in the present copper double perovskite.

4 Experimental Section SrLaCuIrO6 was synthesized by solid-state reaction in a capped alumina crucible. Stoichiometric amounts of the reactants SrO, La2O3 (99.99 %, Alfa Aesar), IrO2 (99.9 %, Sigma-Aldrich), and CuO (99.995 %, Alfa Aesar) were weighed inside a glove box and pressed into pellets after careful grinding. La2O3 was fired overnight at 1000 °C to obtain a water-free material. SrO was synthesized by decomposition of pure SrCO3 (99.99 %, Alfa Aesar) in a highvacuum furnace (p ≈ 5×10–5 mbar) at 1050 °C. The final product was obtained after sintering for periods of about 48 h at 1000–1100 °C in air interrupted by intermediate grinding. Heating and cooling rates were in the range of 100 °C h–1. Powder X-ray diffraction on the sample was conducted with a HUBER G670 imaging plate Guinier camera at room temperature equipped with a germanium monochromator using Cu-Kα1 radiation (λ = 1.5406 Å) in a 2θ range of 5−100° (step width: 0.005°). The Rietveld refinement of the obtained diffractogram was performed with the program TOPAS.[28] Chemical analyses of the metallic elements Sr, La, Cu and Ir were done by the use of a ICP-OES 5100

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SVDV (Agilent) after chemical digestion of the sample. The percentage of oxygen was determined by carrier gas hot extraction with the machine TCH600 (LECO), in which each experiment was repeated three times to achieve reliable statistics. Temperature and field-dependent magnetization measurements were performed using a MPMS XL-5 SQUID magnetometer (Quantum Design) in magnetic fields up to 5 T. The measurement was done in the temperature range 2−380 K under an applied magnetic field of 10 kOe in warming after zero-field cooling (ZFC) as well as during fieldcooling (FC). Isothermal magnetization curves were recorded at temperatures between 2 K and 60 K for fields from – 50 to +50 kOe. The electrical resistivity was measured by conventional DC four-point method with the resistivity option of a PPMS-9 (Quantum Design) by use of a small piece of the obtained powder pellet. (Electrical contacts were made with Au wires and conducting silver paste.)

5 Acknowledgements

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This work was supported by the Deutsche Forschungsgemeinschaft through SFB 1143. We thank Dr. Horst Borrmann and Steffen Hückmann for performing powder X-ray measurements. We are further grateful to Dr. Gudrun Auffermann and Ulrike Schmidt for conducting the chemical analyses on the sample.

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6 Bibliography

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The new ordered Double Perovskite SrLaCuIrO6 Klaus K. Wolffa,*, Liu Hao Tjenga and Martin Jansena,b,** Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany

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Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany

* Dr. K. Wolff, E-Mail: [email protected] ** Prof. Dr. M. Jansen, E-Mail: [email protected]

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New fully rock-salt ordered double perovskite was synthesized and characterized Jahn-Teller distortion leads to elongated copper oxygen octahedra The compound shows semiconducting behavior and a weak magnetic transition

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Highlights

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a