New Insulating Antiferromagnetic Quaternary Iridates MLa10Ir4O24 (M = Sr, Ba)

Recently, oxides of Ir4+ have received renewed attention in the condensed matter physics community, as it has been reported that certain iridates have a strongly spin-orbital coupled (SOC) electronic state, Jeff = ½, that defines the electronic and magnetic properties. The canonical example is the Ruddlesden-Popper compound Sr2IrO4, which has been suggested as a potential route to a new class of high temperature superconductor due to the formal analogy between Jeff = ½ and the S = ½ state of the cuprate superconductors. The quest for other iridium oxides that present tests of the underlying SOC physics is underway. In this spirit, here we report the synthesis and physical properties of two new quaternary tetravalent iridates, MLa10Ir4O24 (M = Sr, Ba). The crystal structure of both compounds features isolated IrO6 octahedra in which the electronic configuration of Ir is d5. Both compounds order antiferromagnetically despite the lack of obvious superexchange pathways, and resistivity measurement shows that SrLa10Ir4O24 is an insulator.

Electrons in 3d transition metal oxides exhibit correlated behavior because the bandwidth, W, is relatively narrow, while the electron-electron repulsion, parametrized by a Hubbard U, is significant. The result is a set of collective phenomena including high temperature superconductivity 1 and colossal magnetoresistance 2,3 . On the other hand, 5d transition metals are characterized by more extended orbitals, and in general, are expected to be uncorrelated metals, as for example IrO 2 4 and Bi 2 Ir 2 O 7 5 . Recently, however, Kim et al. found that strong spin-orbital coupling in the layered compound Sr 2 IrO 4 leads to a relatively narrow J eff = ½ band whose width is of the same scale as electron correlation, yielding a Mott insulating state 6,7 . With a square IrO 2 network, a U/W ~ 1, and a magnon dispersion qualitatively the same as cuprates 8 , Sr 2 IrO 4 has been studied intensely as a potential route to a new class of high temperature superconductor [8][9][10] . Some known iridates, including Ba 2 IrO 4 11 and Ca 4 IrO 6 12 , have already shown behavior consistent with a J eff = ½ description by resonant inelastic X-ray scattering, indicating that the phase space of J eff = ½ materials extends beyond Sr 2 IrO 4 . These discoveries underscore the importance of identifying new iridates with J eff = ½ states to better understand the phenomenology of these unusual correlated oxides.
The scope of the present work lies firmly in the regime of discovery synthesis of new compounds in a relatively unexplored regime of crystal chemistry as a first step on the way to classifying and understanding the breadth of spin-orbit driven physics in iridates. Toward this end, we have synthesized two new isostructural tetravalent iridates, MLa 10 Ir 4 O 24 (M = Sr, Ba) and characterized their crystal structures and magnetic, transport, and thermodynamic signatures. Each is comprised of isolated IrO 6 octahedra in which the nominal electronic configuration of Ir is d 5 . They both order antiferromagnetically, and resistivity measurement shows that SrLa 10 Ir 4 O 24 exhibits insulating behavior. , with a reported μ eff = 1.79 μ B /Ir, follow more closely the expected behavior. Discrepancies such as these suggest an 'effective g-factor' significantly reduced from the free electron value, deriving potentially from non-cubic symmetry, an admixture of configurations other than t 2g 5 , or the effect of hybridization with the O sublattice network.
Similarly, the temperature dependent magnetic measurement of BaLa 10 Ir 4 O 24 shows antiferromagnetic ordering at a somewhat lower temperature, T N = 6 K (Fig. 2, bottom), and also follows Curie-Weiss behavior above this temperature, with μ eff = 1.35 μ B , slightly larger than that of SrLa 10 Ir 4 O 24 . The Weiss constant obtained is − 5.1 K, close to the measured T N . For both SrLa 10 Ir 4 O 24 and BaLa 10 Ir 4 O 24 , at temperatures below the T N , the susceptibility has a small upturn, which may arise from a small amount of paramagnetic impurity spins.
Heat capacity measurements were carried out on both compounds. Above the magnetic transition, the data can be described by the expression C = C electron + C phonon = γ T+ β T 3 (Fig. 3a). The v a l u e s of γ an d β extracted from the fits for SrLa 10 (Fig. 3b), where C mag = C tot -C electron -C phonon , yielding S mag = 6.31 J mol −1 K −1 for SrLa 10 Ir 4 O 24 and 6.17 J mol −1 K −1 for BaLa 10 Ir 4 O 24 (Fig. 3b). The expected entropy of the magnetic transition (S mag ) equals to Rln(2 J+ 1), where R is the gas constant, and J is the total angular momentum. For J eff = ½, the expected value is 5.76 J mol −1 K −1 , in fair agreement with that measured here.
Resistivity measurement shows that SrLa 10 Ir 4 O 24 exhibits insulating behavior (Fig. 4), which is expected given that the crystal structure features isolated IrO 6 octahedra. With the caveat that the behavior is evaluated in a narrow temperature range of 260 K to 350 K, it was found that the resistivity is best modeled by simple thermally activated hopping, with Ea ~0.26 eV, while three-dimensional and two-dimensional variable range hopping and small polaron models yield poorer agreement with the measured data. Unfortunately, BaLa 10 Ir 4 O 24 crystal specimens are too small for a conductivity measurement at this time. Due to the similar crystal structure, one may expect similar electronic transport behavior to that of the Sr analogue.

Discussion
Flux crystal growth is an important approach to grow single crystals of new materials [21][22][23][24][25][26] . For exploratory crystal growth of new iridates, KOH and K 2 CO 3 fluxes have typically been used 27,28 . Remarkably, for the synthesis of SrLa 10 Ir 4 O 24 , Ir metal was used as the source of Ir and was oxidized to Ir(IV) in the SrCl 2 flux. It is known that some fluxes like KOH can dissolve O 2 from the atmosphere to provide an oxidizing environment; mostly Ir(V) compounds have been synthesized from KOH flux, but some Ir(VI) and Ir(V) oxides are also reported 29,30 . Evidently SrCl 2 also dissolves sufficient O 2 from the atmosphere to oxidize Ir metal to Ir(IV). However, under the conditions of our synthesis, SrCl 2 apparently provides a less oxidizing environment compared to KOH, as we found no higher oxidation state products. EDS shows no evidence of chlorine incorporation in the crystals.   that such a reduced effective moment may arise from the d electron hybridization with oxygen p states 18 . Putting any such argument on a stronger, more quantitative footing calls for a broader materials search and theoretical input beyond the scope of this report.
As mentioned earlier, the J eff = ½ state has been implicated as foundational to the understanding of iridate physics, although this description rigorously applies only in the case of an isolated and ideal octahedral crystal field 6,7 . The former criterion eliminates band structure effects and super-exchange, while the latter guarantees the symmetry of the J eff = ½ wavefunction (assuming that the e g states lie at sufficiently high energy that the contribution from excited configurations such as t 2 g 4 e g 1 are negligible. This latter assumption has been questioned recently by Katakuri et al. from quantum chemical calculations 34 . By isolating the octahedra and thus eliminating bandwidth and super-exchange as a competing influence on the electronic structure 35 , compounds such as MLa 10 Ir 4 O 24 offer a platform for testing the intrinsic nature of the J eff = ½ description.

Conclusion
In summary, we report the discovery and characterization of two new quaternary iridates, SrLa 10  . By isolating the octahedra and thus eliminating bandwidth and super-exchange as a competing influence on the electronic structure, compounds such as these can provide a platform for testing the detailed nature and range of applicability of the J eff = ½ description using, for example, resonant inelastic x-ray scattering. More generally, the synthetic approaches reported here provide valuable insights that can stimulate the efforts in crystal growth of new iridates, particularly quaternary iridates, that will be essential to achieving a broader understanding of correlated electron physics in the presence of strong spin-orbit coupling. During the proofing process of this manuscript, we became aware of a paper reporting the synthesis and magnetic properties of SrxLa11−xIr4O24 (30 mmol) were loaded into a platinum crucible. The crucible was placed into a box furnace, heated to 900 °C at 300 °C/hour, then heated to 1200 °C at 12 °C/hour, held at 1200 °C for 12 h, cooled to 950 °C at 10 °C/hour, and finally cooled to room temperature by turning off the furnace. For both compounds, the crystals were separated from the flux by dissolving the flux in water aided by sonication, and then isolated with vacuum filtration and rinsing with acetone. The crystals are stable in air and water. They are black in color with an irregular polyhedral shape, and the crystal sizes are about 100 microns from one face to the face across.
Single Crystal X-ray Diffraction and EDS. Single crystals with irregular polyhedral shape were selected and mounted on tips of glass fibers for X-ray diffraction. Intensity data were collected at room temperature on a STOE imaging plate diffraction system (IPDS-II) using graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å) operating at 50 kV and 40 mA with a 34 cm diameter imaging plate. For SrLa 10 Ir 4 O 24 , individual frames were collected with a 15 min exposure time and a 1° ω rotation at a ϕ angle of 98°, while for BaLa 10 Ir 4 O 24 , individual frames were collected with a 5 min exposure time and a 1° ω rotation at a ϕ angle of 78°. Data reduction and integration absorption correction were performed using X-Area software provided by STOE, and the crystal structures were solved with SHELXL 97 software package 36 . The parameters for data collection and the details of the structure refinement are given in Table 1. Atomic coordinates, isotropic thermal displacement parameters (U eq ) and occupancies of all atoms are given in Table 2, and selected bond lengths are given in Tables 3 for both compounds. Anisotropic displacement parameters are given in the supplemental material. The isotropic thermal parameter for Sr1 in SrLa 10 Ir 4 O 24 is relatively large, and the anisotropic thermal parameters for Sr1 have an elongated ellipsoid shape. This may be a sign of disordering for this site. The Ba in BaLa 10 Ir 4 O 24 , on the other hand, is well behaved. Electron dispersive X-ray spectroscopy data were collected on Oxford INCA Model 6498 and no discernible chlorine peaks were detected.
Magnetism. The DC magnetic susceptibilities of the ground samples were measured using a Quantum Design MPMS XL SQUID magnetometer. Samples were measured under zero-field-cooled (ZFC) and field-cooled (FC) conditions in an applied field of 5000 G. For SrLa 10 Ir 4 O 24 , the magnetization was measured upon warming the samples from 1.8 to 300 K. For BaLa 10 Ir 4 O 24 , the magnetization was measured upon warming the samples from 2 to 300 K. The very small diamagnetic contribution of the gelatin capsule had a negligible contribution to the overall magnetization and was not subtracted.
Electrical Conductivity and Heat Capacity. Electrical conductivity of a single crystal of SrLa 10 Ir 4 O 24 was measured on a Quantum Design PPMS with a four-probe method. It was found that below 260 K the resistance is too large to be measured, thus data between 260 K and 350 K were measured. Heat capacity for both compounds was measured on the PPMS from 2 K to 30 K.