1 Introduction

Recently, 5d transition metal oxide Sr2IrO4 with layered perovskite structure has attracted intensive attention as a candidate for a novel Mott insulator [1,2,3,4,5,6,7,8,9,10]. Sr2IrO4 forms in a reduced tetragonal I41/acd structure, similar to the tetragonal I4/mmm structure in isostructural La2CuO4 but with the Ir–O–Ir bond angle being significantly bended. Considering its odd number of electrons per unit formula (5 d 5) and the highly delocalized 5d electronic orbitals of the Ir ions, one expects a metallic state in a naive band picture. However, the Sr2IrO4 is unexpectedly found to be an insulator. The insulating behavior has been attributed to the strong spin-orbit coupling (ζ SO ∼ 0.4 eV) which is comparable to the Coulomb repulsion U = 0.5 ∼ 2 eV, giving rise to a novel J eff = 1/2 Mott-insulating ground state [1].

Taking into account the facts that Sr2IrO4 has a similar crystal lattice structure and the same Mott-insulating ground state with the high-temperature superconducting cuprate parent compound La2CuO4, it has been proposed that the novel superconducting phase can be expected when carriers are properly doped in this compound [11,12,13,14,15]. As a matter of fact, angle-resolved photoemission spectra of Sr2IrO4 reveal the existence of disconnected segments of zero-energy states, known as Fermi arcs, with a gap as large as 80 meV [16,17,18]. The occurrence of Fermi arcs in Sr2IrO4 is of particular importance for investigating possible unconventional superconductivity: it is still strongly controversial whether the Fermi arcs are a precursor signature of d-wave superconductivity. Different from the La2−x Sr x CuO4 and La2−x Ba x CuO4 where only a few percent of doping at the La site leads to a metal-insulator transition and superconductivity, the insulating phase in the Sr2IrO4 compound is very robust against Sr-site doping or application of high pressure [19,20,21,22].

Comparing to the nearly straightened Cu–O–Cu bond angle in La2CuO4, the Ir–O–Ir bond angle in Sr2IrO4 is greatly bended. As is well known, the conductance in K2NiF4-type compounds is quasi-two-dimensional. Thus, it is reasonable to conclude that the Ir–O–Ir bond angle could play an important role in the robustness of the insulating state. However, to our best knowledge, there is very little study on the role of the Ir–O–Ir bond angle in the Sr2IrO4 compound.

In this work, we perform a comparison investigation on the influence of the Ir–O–Ir bond angle and the transport and magnetic properties of two series of Ba- and La-doped Sr2IrO4 samples. It is found that the Ba doping in Sr2IrO4 indeed leads to straightening of the Ir–O–Ir bond angle, while the La doping alters the Ir–O–Ir bond angle very little. However, from the transport and magnetic data, we find that comparing to the Ba doping case, the La doping results in a much enhanced conductance and a decreased magnetic moment of the samples. The present results suggest that the Ir–O–Ir bond angle plays a relatively less important role in the conductance and magnetic properties of the Sr2IrO4 compound.

2 Materials and Methods

Polycrystalline samples of Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) and Sr2−x La x IrO4 (x = 0.05 and 0.1) were prepared by conventional solid-state reaction method. Stoichiometric powders of SrCO3 (99.9%), IrO2 (99.9%), BaCO3 (99.9%), and La2 O 3 (99.9%) were mixed, ground, and heated at 1050∘C for 1 day, followed by several times of calcining at 1250∘C for 2 days. The crystal structure and phase purity were checked by powder X-ray diffraction (XRD) on a Rigaku-TTR3 X-ray diffractometor using Cu K α radiation. Rietveld refinement was performed for XRD data using the GSAS software package. The resistivity was taken on a Sumitomo CNA-11 cryogenic system, Hall effect measurements were taken on an Oxford TeslatronPT cryogenic system, and the magnetic measurements were taken on a Quantum Design SQUID-VSM3.

3 Results and Discussion

Figure 1 shows the powder XRD patterns of Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) and Sr2−x La x IrO4 (x = 0.05 and 0.1) samples at room temperature. All the peaks can be indexed with the space group I41/acd. In order to check the change of lattice constants and Ir–O–Ir bond angles, the refinements of XRD data are further performed. As shown in Table 1, the Ba doping increases the lattice constants of both a and c, and straightens the Ir–O–Ir bond angle. The calculated parameters of Sr2−x Ba x IrO4 (x = 0.3) are a = 5.5060 Å and c = 25.8491 Å, which are both larger than those of the undoped Sr2IrO4, i.e., a = 5.4941 Å and c = 25.8026 Å. The enlargement of the lattice constants in the Ba-doped samples is consistent with the fact that the ionic radius of Ba2+ is larger than that of Sr2+. The Ir–O–Ir bond angle is increased from 157.3∘ for Sr2IrO4 to 178.3∘ for Sr2−x Ba x IrO4 (x = 0.3). On the other hand, La doping decreases the c parameter. The lattice parameters of Sr2−x La x IrO4 (x = 0.1) are a = 5.5026 Å and c = 25.7826 Å. The Ir–O–Ir bond angle of Sr2−x La x IrO4 (x = 0.1) is 159.1∘, which is only slightly larger than that of the undoped sample.

Fig. 1
figure 1

Powder XRD patterns of Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) and Sr2−x La x IrO4 (x = 0.05 and 0.1) samples

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Table 1 Structural parameters and Hall coefficients of Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) and Sr2−x La x IrO4 (x = 0.05 and 0.1) samples at 300 K
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Figure 2 shows the temperature dependence of the electrical resistivity for the Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) and Sr2−x La x IrO4 (x = 0.05 and 0.1) samples. All the samples exhibit an insulating behavior from room temperature down to low temperature. The Ba doping seems to induce little change in the insulating nature, despite the fact that the Ir–O–Ir bond angle is significantly straightened. Differently, the La doping greatly decreases the electrical resistivity. For instance, the room-temperature resistivity decreases from about 20 Ωcm for Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) to 1.15 Ωcm for x (La) = 0.05 sample and further to 0.5 Ωcm for x (La) = 0.1 sample.

Fig. 2
figure 2

Temperature dependence of resistivity of Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) and Sr2−x La x IrO4 (x = 0.05 and 0.1) samples

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In order to better understand the changes in resistivity, Hall resistivity measurements were carried out at 300 K. As shown in Table 1, the calculated Hall coefficient R H of Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) samples at room temperature are all positive, implying that their dominant carriers are holes. However, R H turns into negative when a small proportion of La is introduced, showing an electron-dominant nature for Sr2−x La x IrO4 (x = 0.05 and 0.1) samples. Moreover, the carrier concentration n H changes from 1.84 × 1016 cm−3 for Sr2IrO4 to −3.75 × 1017 cm−3 for x (La) = 0.05 sample and further to −6.54 × 1017 cm−3 for x (La) = 0.1 sample. These facts suggest that effective electron carriers are substantially introduced in La-doped samples, which leads to the greatly decreased resistivity. This is consistent with previous researches [23]. It is also reasonable that the Ba doping does not affect the electrical transport too much, since it does not apparently change the carrier type and carrier concentration.

According to previous researches [23,24,25], the chemical doping at the A site influences both the electrical transport and the magnetic properties. Figure 3 presents the field cooling magnetizations as a function of temperature taken at a field of 5000 Oe for both the undoped and doped samples. All the curves show a ferromagnetic-like behavior at low temperature. Actually, the magnetic ground state of the Sr2IrO4 parent compound is canted antiferromagnetism with a net moment of 0.06 μ B/Ir [26, 27]. The transition temperature T C stays at 240 K for Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) samples and decreases to 210 K for Sr2−x La x IrO4 (x = 0.05 and 0.1) samples. Similar to the change of transition temperature, the decrease of the saturated moment in La-doped samples is much larger than that in Ba-doped samples. As La doping offers electrons and turns Ir4+ ions into Ir3+ ions (5 d 6), which do not possess an uncompensated moment in the J eff = 1/2 picture, the overall magnetic moment is thus reduced and the exchange interactions (i.e., transition temperature) are weakened in the La-doped samples.

Fig. 3
figure 3

Temperature dependence of field cooling magnetizations taken at a field of 5000 Oe for Sr2−x Ba x IrO4 (x = 0, 0.05, 0.1, 0.2, and 0.3) and Sr2−x La x IrO4 (x = 0.05 and 0.1) samples from 1.8 to 300 K

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In order to understand the magnetic ground states in these samples, the magnetization vs. magnetic field (M- H) measurements are further performed. Figure 4 shows the M-H curves taken at 1.8 K. The hysteresis loops, with a linear-like increase in magnetization at high fields, indicate a canted antiferromagnetic ground state. Both the Ba doping and La doping do not change the magnetic ground state. However, the doping results in a reduction of saturated moment M S in both systems. The M S decreases more quickly in Sr2−x La x IrO4 than in Sr2−x Ba x IrO4, i.e., 0.055 μ B/Ir for Sr2IrO4, 0.048 μ B/Ir for x (Ba) = 0.3 sample, and 0.016 μ B/Ir for x (La) = 0.1 sample. All the saturated moments are calculated after subtracting the linear antiferromagnetic part.

Fig. 4
figure 4

Magnetization vs. magnetic field at 1.8 K in the field range of ±7 T

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The much more decrease in both the resistivity and the magnetic moment in La-doped samples than that in Ba-doped samples could be due to the difference in the valance states. Though the isovalent Ba doping straightens the Ir–O–Ir bond angle, it does little change to the carrier type and low carrier concentration which plays an important role in the electrical transport. By combining the electrical and magnetic measurements, we could find that the straightened Ir–O–Ir bond angle by Ba doping does not give rise to any significant changes in either electrical conductance or magnetic properties. In contrast, the doped electrons and Ir3+ ions caused by La doping are responsible for both the enhanced electrical conductance and the decreased magnetic moment and transition.

4 Conclusion

In conclusion, we have synthesized two series of Ba- and La-doped Sr2IrO4 samples and investigated their lattice structure, magnetic, and transport properties. The Ba doping leads to significant straightening of the Ir–O–Ir bond angle while it does not introduce too much extra charge carriers. On the other hand, the La doping has less influence on the Ir–O–Ir bond angle but it efficiently introduces electron charge carriers into the Sr2IrO4 compound. Accordingly, the La doping in the Sr2IrO4 system results in an enhanced conductance and a reduced magnetic moment compared with those in Ba-doped samples. The present results suggest that the bended Ir–O–Ir bond angle plays a less important role in the robustness of the insulating state. The insulating ground state cannot be changed by the mere regulation of the Ir–O–Ir bond angle. The introduction of efficient charge carriers is the reason for the enhanced conductance and the reduced magnetic moment.