Na2IrIVCl6: Spin–Orbital-Induced Semiconductor Showing Hydration

Oct 19, 2018 - Max Planck Institute for Solid State Research, D-70569 Stuttgart , Germany ... Reczyński, Nowicka, Näther, Kozieł, Nakabayashi, Ohkoshi...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Na2IrIVCl6: Spin−Orbital-Induced Semiconductor Showing Hydration-Dependent Structural and Magnetic Variations Song-Song Bao,†,# Di Wang,‡,# Xin-Da Huang,† Martin Etter,§ Zhong-Sheng Cai,† Xiangang Wan,*,‡,∥ Robert E. Dinnebier,*,⊥ and Li-Min Zheng*,†,∥

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State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ‡ National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China § Deutsches Elektronen-Synchrotron (DESY), P02.1 HRPD, Notkestraße 85, 22607 Hamburg, Germany ⊥ Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany ∥ Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: Iridium(IV) oxides have gained increased attention in recent years owing to the presence of competing spin−orbit coupling and Coulomb interactions, which facilitate the emergence of novel quantum phenomena. In contrast, the electronic structure and magnetic properties of IrIV-based molecular materials remain largely unexplored. In this paper, we take a fresh look at an old but puzzling compound, Na2IrCl6, which can be hydrated to form two stable phases with formulas Na2IrCl6·2H2O and Na2IrCl6·6H2O. Their crystal structures are well illustrated based on X-ray powder diffraction data. Magnetic studies reveal that Na2IrCl6 and Na2IrCl6·2H2O are canted antiferromagnets with ordering temperatures of 7.4 and 2.7 K, respectively, whereas Na2IrCl6·6H2O is paramagnetic down to 1.8 K. First-principle calculations on Na2IrCl6 reveal a Jeff = 1/2 ground state, and the band structures show that Na2IrCl6 is a spin−orbital-induced semiconductor with an indirect gap of about 0.18 eV.

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INTRODUCTION

In order to study the intrinsic magnetic properties of the basic [IrO6]8− units in iridates, Pedersen and co-workers synthesized molecular fluoride iridates incorporating isolated [IrF6]2− units as model compounds12,13 and found that the [IrF6]2− species possess the same electronic Jeff = 1/2 ground state. Noting that molecular systems of IrIV are extremely rare and their electronic structures and magnetic properties remain largely unexplored, herein, we take a fresh look at the old but puzzling compound Na2IrCl6. In addition to the anhydrate Na2IrCl6, two hydrated forms, Na2IrCl6·2H2O and Na2IrCl6· 6H2O, exist with dependence on the humidity of the environment. More interestingly, the hydration of Na2IrCl6 poses significant influences not only on the local environment of the {IrCl6} octahedra but also on their magnetic behaviors. Consequently, compounds Na2IrCl6 and Na2IrCl6·2H2O are canted antiferromagnet (or so-called weak ferromagnet) with ordering temperatures of 7.4 and 2.7 K, respectively, unlike compounds A2IrCl6 (A = NH4, K), which are antiferromagnets.14 In contrast, Na2IrCl6·6H2O is paramagnetic down to 1.8 K. The first-principle calculations were conducted for Na2IrCl6 to unravel the electronic and magnetic nature of the compound.

The family of IrIV hexahalides, A2IrX6 (A = NH4, alkaline metal ion; X = F, Cl, Br), has been known for more than 100 years. These molecular systems contain isolated octahedrally coordinated IrIV species and thus are paramagnetic.1,2 They can also be used as a one-electron oxidant for organic reactions.3,4 Although the crystal structures and magnetic properties of A2IrCl6 (A = NH4, K, Cs) have been well illustrated,5−7 those of Na2IrCl6 remain a puzzle, possibly due to its hygroscopic nature. The IrIV ion has a d5 configuration possessing strong spin− orbit coupling (SOC) and spatially extended valence orbitals. Recent works demonstrated that the SOC and the Coulomb interactions in iridates have comparable energy scales, which facilitate the emergence of novel quantum phenomena such as spin liquids,8 topological Mott insulators,9 and Weyl semimetals.10 The iridates usually contain corner-, edge-, or facesharing IrIVO6 octahedra. The strong SOC splits the band of t2g5 states into Jeff = 1/2, 3/2 bands. The half-filled Jeff = 1/2 band can be further split via relatively weak Coulomb repulsion. As a result of the competing interactions, the ground states of such systems are highly tunable, and their electronic structure and magnetic properties are sensitive to changes in local environment, symmetry, or dimensionality.11 © XXXX American Chemical Society

Received: June 25, 2018

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DOI: 10.1021/acs.inorgchem.8b01753 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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Rwp and R-Bragg values given in Table S1. A small amount of Na2IrCl6·6H2O in the powder pattern of Na2IrCl6·2H2O was included as second phase in the Rietveld refinement. The results, atomic coordinates, and selected bond distances and angles are given in Table S2, and the Rietveld fit of the whole pattern is shown in Figure S3. Further details of the crystal structure investigation may be obtained from FIZ Karlsruhe, 76344 EggensteinLeopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, on quoting the deposition number CSD433510 for Na2IrCl6, CSD-433511 for Na2IrCl6·2H2O, and CSD433512 for Na2IrCl6·6H2O). Resistivity Measurement. Method A: A pellet (2.5 mm diameter) of Na2IrCl6·xH2O was prepared under a pressure of ∼0.5 GPa. The thickness of the pellet was determined as 0.80 mm. Both round faces of the sample pellet were treated with silver paste. Then the pellet was placed in a CRX-4K cryogenic probe station. The anhydrate Na2IrCl6 was achieved by drying the pellet for 4 h under high vacuum before cooling, the purity of which was confirmed by PXRD measurement. After thorough drying, the temperaturedependent I−V curves of the pellet were collected by using a twoprobe method on a Keithley 2612b source meter. Method B: To verify the resistivity of anhydrate Na2IrCl6, another method was used by pressing the anhydrate Na2IrCl6 (76.7 mg), obtained by heating Na2IrCl6·xH2O for 2 h at 100 °C, between parallel circular Cu electrodes (4.0 mm diameter) with an O-shaped seal ring in a homemade insulated cell in the glovebox. After a pressure of 10 MPa was placed on the sample, the sample length of 1.706 mm was measured by distance changes between two Cu electrodes. The I−V curve was quickly collected using a two-probe method. The resistivity (3.1 × 106 Ω·cm) was close to the value obtained by method A (3.2 × 106 Ω·cm at 299 K). Computation method. The electronic band structure calculations have been carried out by using the full potential linearized augmented plane wave method as implemented in WIEN2K package.25 Local spin density approximation (LSDA) for the exchange-correlation potential has been used here. A 11 × 11 × 7 k-point mesh is used for the Brillouin zone integral. Using the secondorder variational procedure, we include the SOC,26 which has been found to play an important role in the 5d system. The self-consistent calculations are considered to be converged when the difference in the total energy of the crystal does not exceed 0.1 mRy and that in the total electronic charge does not exceed 10−3 electronic charge at consecutive steps. We utilize the LSDA+U (U = 2 eV) scheme27 to take into account the effect of Coulomb repulsion in the 5d orbital.

EXPERIMENTAL SECTION

Materials and Physical Measurements. Na2IrCl6·xH2O was bought from Alfa Aesar. Na2IrCl6·xH2O was heated for 2 h at 100 °C under vacuum, and the resulting Na2IrCl6 was sealed in the capsule in the glovebox (Ar). Na2IrCl6·xH2O was kept in the atmosphere of saturated solution of LiBr (20 °C, ca. 7% RH) or NaBr (20 °C, 59% RH) for 1 day to obtain Na2IrCl6·2H2O and Na2IrCl6·6H2O, respectively. The colors of all compounds are dark brown. Thermal analyses were performed in nitrogen in the temperature range of 20−500 °C with a heating rate of 5 °C/min on a PerkinElmer Pyris 1 TGA instrument. The magnetic susceptibility data were carried out between 1.8 and 300 K using a vibrating sample magnetometer (VSM) of the Quantum Design MPMS SQUID-VSM system. The data were corrected for diamagnetic contributions of both the capsule and the compound obtained from Pascal’s constants.15 Powder Diffraction Measurement and Rietveld Refinement. Powder X-ray diffraction (PXRD) patterns of Na2IrCl6·6H2O and Na2IrCl6·2H2O were collected at room temperature in Debye− Scherrer mode on a laboratory powder diffractometer (Stadi PDiffraktometer (Stoe), Mo Kα1 radiation from primary Ge(111) Johannson-type monochromator, Mythen 1 K position sensitive detector (Dectris)). The samples were sealed in 0.5 mm diameter borosilicate glass capillaries (Hilgenberg glass No. 14), which were spun during the measurement. Data were taken for 12 h in steps of 0.012° (2θ) from 2 to 40° (2θ). Synchrotron PXRD pattern of Na2IrCl6 was collected at a temperature of 200 °C in Debye−Scherrer mode at the Petra III Synchrotron of DESY (Hamburg) at an energy of approximately 60 keV (λ = 0.2072 Å) at Beamline P02.1 using an unfocused and collimated X-ray beam with a size of ca. 0.6 × 0.9 mm2. Diffracted Xrays were detected using a XRD1621 PerkinElmer image plate detector at a distance of 1.11 m from the sample. For heating, a hot air blower was placed below the capillary of 0.5 mm in diameter. Data were taken for 60 s. The collected two-dimensional Debye−Scherrer rings were subsequently integrated with the program FIT2D16 to onedimensional powder diffraction patterns. Parameters for integration were determined from a LaB6 reference sample (NIST SRM 66Oa). The program TOPAS 5.017 was used to determine and to refine the crystal structures. The instrumental resolution (IRF) of both instruments was determined from Rietveld18 refinements of the LaB6 NIST SRM 66Oa line profile standard applying the modified Thompson−Cox−Hastings pseudo-Voigt (TCHZ-PV) profile function as defined in ref 19. Indexing of the three phases was carried out by an iterative use of the singular value decomposition (LSI),20 leading to primitive unit cells in P1̅ for Na2IrCl6·6H2O, Pca21 for Na2IrCl6·2H2O, and P21/n for Na2IrCl6 with lattice parameters given in Table S1. The space groups were estimated as the most probable space groups from the observed extinction rules and were later confirmed by the crystal structure refinement. The peak profile and the precise lattice parameters were determined by Pawley whole powder pattern fitting21 by convoluting a double Voigt function22 describing the microstructure of the samples to the IRF. The background was modeled by employing Chebyshev polynomials of the 6th to the 10th order. The refinements converged quickly. The crystal structures of the solid phases were solved by applying the global optimization method of simulated annealing (SA) in real space as implemented in TOPAS.23 Atoms located on identical positions and occupying special positions were identified by using a merging radius of 0.7 Å.24 After a few minutes, the positions of all iridium, sodium, and chlorine atoms were found. The remaining oxygen atoms were determined by a combination of difference Fourier analysis followed by Rietveld refinement17 and SA. For the final Rietveld refinement,17 all profile and lattice parameters were released iteratively, and all atomic positions were subjected to free unconstrained refinement. For less crystalline Na2IrCl6·2H2O, some Na−O bond lengths were slightly out of the expected range, so slack soft constraints were introduced. This did not change the overall weighted agreement factors. The final refinement led to reasonable

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RESULTS AND DISCUSSION Water Adsorption and Desorption of Na2IrCl6. Commercially available Na2IrCl6·6H2O was bought from Alfa Aesar. X-ray powder diffraction measurements showed a dependence on the humidity of the environment (Figure S1), suggesting that the compound is not a pure phase at ambient conditions. Thermal analysis revealed that only 3.8 H2O molecules were lost below 150 °C (Figure S2), much less than the expected six lattice water molecules. To determine the possible phase transition upon humidity variation, water adsorption and desorption isotherms were performed at 25 °C on Na2IrCl6·xH2O pretreated at 100 °C under vacuum for 2 h. As shown in Figure 1, the anhydrate phase of Na2IrCl6 experiences a clear two-step adsorption process in the relative humidity (RH) range of 0−85% with the number of water molecules per molecular unit being 2.0 (5− 13% RH) and ca. 6.0 (18−70% RH), respectively, corresponding to the hydrated phases of Na2IrCl6·2H2O and Na2IrCl6· 6H2O. When the relative humidity is above 70%, the compound would dissolve in water. Desorption isotherms display significant hysteresis, indicating that the lattice water molecules could be involved in the coordination with metal B

DOI: 10.1021/acs.inorgchem.8b01753 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

neighboring {IrCl6} octahedra are 6.616, 6.691, 6.751, and 6.880 Å (Figure 2d). In Na2IrCl6·2H2O, two coordination sites of sodium ion are occupied by water molecules, forming {NaCl4O2} polyhedra. The {NaCl(H2O)2} layers composed of corner-sharing {NaCl4O2} polyhedra become undulated in the ab plane (Figure 2f). The {IrCl6} octahedra connect the layers via both edge- and corner-sharing with the {NaCl4O2} polyhedra (Figure 2g). As a result, the {IrCl6} octahedron is heavily distorted compared to the anhydrous one, and the Ir−Ir distances in the ac plane are significantly elongated (6.950, 7.137, 7.292 Å) (Figure 2h). In contrast, the Ir−Ir distance along the b-axis is slightly shortened to 6.391 Å, attributed to the parallel edge-sharing array of the {IrCl6} and {NaCl4O2} polyhedra in this direction. In Na2IrCl6·6H2O, each sodium ion is surrounded by five water molecules and one chloride anion, forming {NaClO5} polyhedra. This destroys the connection of the sodium polyhedra in the a-direction (Figure 2j). Therefore, chains of {Na2Cl2(H2O)6} made up of edge-sharing {NaClO5} polyhedra are found running along the b-axis (Figure 2k). Compared to those of Na2IrCl6·2H2O, the Ir−Ir distances along the c- and b-axes in Na2IrCl6·6H2O are remarkably enlarged to 8.399 and 7.088 Å, respectively (Figure 2l), whereas the Ir−Ir distances along the a-axis (6.728 Å) are slightly shortened due to the presence of O−H···Cl hydrogen bonds [O(2)···Cl(1), 3.227 Å and O(2)···Cl(2), 3.282 Å] (Figure S4f). The crystal structure of Na2IrCl6 is distinct from the other hexahalide iridates such as A2IrCl6 (A+ = NH4+, K+, Cs+).5−7 The latter crystallize in the cubic system with space group Fm3̅m and contain perfect {IrCl6} octahedra, whereas Na2IrCl6 crystallizes in the monoclinic space group P21/n and contains slightly distorted {IrCl6} octahedra. The symmetry lowering in the case of Na2IrCl6 is attributed to the smaller size of the sodium cations. The hydration of Na2IrCl6 causes slight changes of the {IrCl6} octahedra and also the elongation of the adjacent Ir−Ir distances. These structural differences should be reflected by their magnetic behaviors. Magnetic Properties. The magnetic susceptibility measurements of Na2IrCl6·xH2O (x = 0, 2, 6) were performed in the temperature range of 1.8−300 K under an external field of 1 kOe (Figure S6). The room temperature effective magnetic moments per Ir unit are 1.67, 1.74, and 1.57 μB for the three compounds, which are close to the spin-only value of 1.73 μB expected for one isolated spin-1/2 and g = 2.0. The susceptibility data above 100 K can be fitted to the Curie− Weiss law, giving Weiss constants (θ) of −39.4, −3.8, and −46.7 K for Na2IrCl6·xH2O (x = 0, 2, 6). The negative θ value is attributed to the presence of dominant antiferromagnetic

Figure 1. Water adsorption (filled circle) and desorption (open circle) isotherms of Na2IrCl6 at 25 °C.

ions and/or hydrogen bond interactions. By carefully controlling the condition of equilibrium of Na2IrCl6·xH2O and moisture, three pure phases of Na2IrCl6·xH2O (x = 0, 2, or 6) were successfully isolated. Their crystal structures could be determined from PXRD data using the global optimization method of simulated annealing and Rietveld refinements18 (Figure S3). Crystal Structures. Compounds Na2IrCl6, Na2IrCl6·2H2O, and Na2IrCl6·6H2O crystallize in space groups P21/n, Pca21, and P1̅, respectively. All contain discrete [IrCl6]2− species isolated by sodium cations, but the coordination geometries are slightly different. The Ir−Cl bond lengths, Cl−Ir−Cl angles, and neighboring Ir−Ir distances of the three compounds are listed in Table 1, together with other related hexachloroiridates. For Na2IrCl6 and Na2IrCl6·6H2O, there exist three kinds of Ir−Cl bonds with distances of 2.302(4)− 2.347(4) Å and 2.311(4)−2.332(4) Å, respectively (Figure 2a,i), whereas for Na2IrCl6·2H2O, all six Ir−Cl bond lengths are different [2.13(2)−2.461(19) Å] with the Cl−Ir−Cl angles ranging from 84.4(8) to 93.1(7)° (Figure 2e). Apparently, the IrCl6 octahedra are distorted to a different extent in the three cases. The values of continuous shape measure (CShM)28 are 0.0094, 0.3853, and 0.0367 for Na2IrCl6·xH2O (x = 0, 2, 6), respectively. The results indicate that the {IrCl6} octahedra in Na2IrCl6·2H2O are the most distorted, whereas those in Na2IrCl6 are closer to an ideal octahedron. The incorporation of lattice water molecules leads to the partial hydration of the sodium cations. In anhydrous Na2IrCl6, the sodium cations are fully surrounded by the chloride anions of the [IrCl6]2− species. {NaCl} layers made up of edgesharing {NaCl6} polyhedra are found in the ab plane, and the {IrCl6} octahedra reside between the {NaCl} layers through edge-sharing (Figure 2b,c). The Ir−Ir distances between the

Table 1. Comparison of Crystal Structures and Magnetic Properties in Na2IrCl6·xH2O (x = 0, 2, 6) and Related IrIV Hexahalides

Na2IrCl6 Na2IrCl6·2H2O Na2IrCl6·6H2O K2IrCl6 (NH4)2IrCl6 Cs2IrCl6 (PPh4)2[IrCl6]

Ir−Cl (Å)

Cl−Ir−Cl (deg)

Ir−Ir (Å)

2.302, 2.321, 2.347 2.13, 2.226, 2.269, 2.343, 2.440, 2.461 2.311, 2.312, 2.332 2.374 2.468 2.331 2.321−2.337

89.4−90.6 84.4−93.1 87.7−92.3 90.0 90.0 90.0 88.5−91.5

6.616, 6.691, 6.751, 6.880 6.390, 6.950, 7.137, 7.292 6.728, 7.688, 8.399 6.872 6.979 7.221 10.1 C

magnetic properties θ= θ= θ= AF, AF,

−39.4 K, Tc = 7.4 K, α = 16.3° −3.8 K, Tc = 2.7 K, α = 5.8° −46.7 K θ ≈ −30 K, TN = 3.08 K θ ≈ −20 K, TN = 2.16 K

slow relaxation of the magnetization

ref this work this work this work 7,14 6,14 5 12

DOI: 10.1021/acs.inorgchem.8b01753 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Projections of the crystal structures of Na2IrCl6·xH2O (x = 0, 2, 6). The coordination environment of Ir atoms, packing diagrams, layer structures, and the Ir···Ir distances between adjacent Ir atoms in compounds Na2IrCl6 (a−d), Na2IrCl6·2H2O (e−h), and Na2IrCl6·6H2O (i−l) are shown. For clarity, the equatorial planes consisting of four Cl atoms with shorter Ir−Cl bonds (j−l) are highlighted with distinct color in the {IrCl6} distorted octahedra. Color codes: {IrCl6} brown, {NaCl6} or {NaCl4O2} or {NaClO5} gray.

(AFM) interactions and/or the strong spin−orbital coupling of the single IrIV ion. Notably, the Weiss constant of Na2IrCl6· 2H2O is much smaller than those of the other two compounds, indicating that the AFM interaction could be weaker in this compound. Upon cooling, the χMT product of Na2IrCl6 decreases with decreasing temperature until a minimum of 0.24 cm3·K·mol−1 is reached at ca. 20 K. A sharp increase is then observed up to a maximum of 6.86 cm3·K·mol−1 at 6.0 K, followed by a steep decline to 2.66 cm3·K·mol−1 at 1.8 K (Figure 3a). Such a behavior is typical for a canted antiferromagnetic or ferrimagnetic system. However, considering that compound Na2IrCl6 contains only one crystallographically independent magnetic center, canted AFM behavior should be operative. To determine whether the magnetic ordering occurs at low temperature, zero-field-cooled (ZFC) and low-field-cooled (FC) (50 Oe) magnetic susceptibility measurements were performed. The bifurcation at approximately 7.4 K suggests the onset of a magnetic phase transition below this temperature (Figure 3b, left). The occurrence of long-range magnetic ordering is confirmed by the ac magnetic susceptibility measurements, performed in a zero static field and 2 Oe oscillating field at frequencies of 72, 303, and 999 Hz. Both inphase (χ′) and out-of-phase (χ″) signals are frequency independent and exhibit peaks at 7.0 and 6.6 K, respectively (Figure S7). The χ″ signal becomes nonzero below 7.4 K, indicating that the ordering temperature (Tc) is 7.4 K. The isothermal magnetization of Na2IrCl6 reveals a hysteresis loop below the Curie temperature (Figure 3b, right). The coercive fields (Hc) are 867 and 424 Oe at 2.0 and 5.0 K, respectively. The magnetization at 70 kOe is close to 0.45 μB at 2.0 K, far from the expected saturation value of 1.0 μB for one Jeff = 1/2

Figure 3. Magnetic properties of Na2IrCl6·xH2O (x = 0, 2, 6). (a) χMT vs T plots. Inset shows a zoomed-in image on low and room temperature χMT products. (b) ZFC/FC curves (left) and magnetization curves (right).

D

DOI: 10.1021/acs.inorgchem.8b01753 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry IrIV in the ionic limit. The smaller magnetization value could be due to the hybridization of the 5d orbital with the neighboring chloride p orbital. The remnant magnetization (Mr) is 0.28 μB at 2.0 K. The canting angle (α) is estimated as 16.3° based on equation sin(α) = Mr/Ms, assuming that the saturation value Ms is 1.0 μB. This angle is quite large, possibly due to the combined effects of spin−orbit coupling and spin frustration, as indicated in the theoretical part given below. For Na2IrCl6·2H2O, the magnetic behavior is very similar to that of Na2IrCl6, except that the magnetic ordering temperature is much lower (Tc = 2.7 K). The magnetization hysteresis is again observed with the coercive fields of 395 and 12 Oe at 2.0 and 2.6 K, respectively (Figures 3b and S7). The magnetization at 70 kOe is 0.73 μB at 2.0 K, much closer than that of Na2IrCl6 to 1.0 μB (Figure S8). The remnant magnetization (Mr) is 0.10 μB at 2.0 K. The canting angle is estimated as 5.8°. For Na2IrCl6·6H2O, the χMT decreases continuously upon cooling and approaches zero at 1.8 K. No hysteresis appears at 2 K. The ac susceptibility shows neither χ′ nor χ″ peaks (Figure S11), indicating that Na2IrCl6·6H2O is paramagnetic down to 1.8 K. The dominant AFM exchange couplings in hexachloroiridates are mediated through Ir−Cl···Cl−Ir pathways. Because the Ir−Ir distances in (NH4)2IrCl6 (6.979 Å) and K2IrCl6 (6.872 Å) are close to those found for Na2IrCl6·2H2O, the observation of AFM ordering in (NH4)2IrCl6 (TN = 3.08 K) and K2IrCl6 (TN = 2.7 K)14 but canted AFM ordering in Na2IrCl6 (Tc = 7.4 K) and Na2IrCl6·2H2O (Tc = 2.7 K) is unexpected. It is well-known that the spin-canting effect arises from single-ion anisotropy and/or antisymmetric interactions related to the symmetry of the exchange pathways, such as Dzyaloshinsky−Moriya (DM) interaction.29 For Na2IrCl6· xH2O (x = 0, 2) with spin 1/2, the weak ferromagnetism must originate from their structures, which contain 21 helical axes, and thus a perfect antiparallel alignment of the spins on neighboring iridium(III) ions cannot be achieved. The presence of the spin frustration could also contribute to a complex magnetic structure with sizable net magnetization. The lower ordering temperature in Na2IrCl6·2H2O compared to that in the anhydrous one can be explained by the fact that the Ir−Ir distances in Na2IrCl6·2H2O are longer than those in Na2IrCl6, except along the b-direction, and thus the AFM interactions could be weaker. For comparison, compound Na2IrCl6·6H2O does not show weak ferromagnetism at low temperature because it crystallizes in a centrosymmetric space group of P1̅ and the Ir−Ir distances are much longer. Canted AFM ordering was also observed in iridium oxides with structural distortion.30 Notably, compound (PPh4)2IrCl6 with much longer Ir−Ir distance 10.1 Å shows slow relaxation of magnetization at low temperature,12 indicating that a longrange magnetic ordering cannot be achieved when the Ir−Ir distance is sufficiently long. Density Functional Theory Calculations. To clarify the basic electronic feature, we performed first-principle calculations for compound Na2IrCl6. Unfortunately, similar calculations on hydrated compounds Na2IrCl6·2H2O and Na2IrCl6·6H2O were not successful. The negative Curie− Weiss θ value reveals that the AFM interaction is dominant in Na2IrCl6 at low temperature. Therefore, we consider an AFM state where Ir atoms at the body center and corners have opposite spin orientations. The calculated total energy of AFM state is 10.0 meV lower than the one of ferromagnetic (FM)

state per unit cell. Thus, the nearest-neighbor exchange interactions are recognized as AFM interactions. Further, in Na2IrCl6, three kinds of Ir−Ir distances have very similar values of 6.617, 6.692, and 6.881 Å, and they form a triangular lattice, indicating that there is huge magnetic frustration here. Moreover, the experimental data suggest the presence of a sizable net magnetization, indicating a probable complex magnetic structure. Therefore, the magnetic ground state is difficult to be determined. Anyway, as demonstrated in the structural description part, the {IrCl6} octahedra are quite well isolated by the {NaCl6} polyhedra (Figure 2), evidenced by the long nearest-neighbor Ir−Ir distance (6.6 Å). Thus we believe that within FM configuration, the calculations can also present fundamental electronic properties, such as a crystalfield splitting pattern and electronic occupation.31 Figure 4

Figure 4. Band structures of Na2IrCl6 from (a) LDA, (b) LSDA +SOC, and (c) LSDA+U+SOC calculations for ferromagnetic configuration. The Fermi level is set to zero. (d) Band structure in a narrow energy range to illustrate the dispersion around the Fermi level clearly from LSDA+U+SOC calculations.

gives the band structures of Na2IrCl6 from LDA, LSDA+SOC, and LSDA+U+SOC calculations for FM configuration. The Coulomb U was set to be 2 eV, at which the value is commonly used in Ir oxides.9a It is clear that in LDA calculations, the energy range, −6.0 to −1.0 eV, is dominated by Cl 3p states. The 6 bands located from −1.0 to 1.0 eV are basically coming from Ir t2g sates, whereas the 4 Ireg states distribute from 2.0 to 3.0 eV. The LDA band structures yield a metal with a wide t2g band. As SOC is included, four narrow bands crossing Fermi level are split off from the rest of the eight bands due to the splitting of the half-filled Jeff = 1/2 and filled Jeff = 3/2 bands (there are 2 Ir atoms per unit cell). Within LSDA+U+SOC calculations, the Jeff = 1/2 bands split and the band structures show that Na2IrCl6 is a semiconductor with an indirect gap of about 0.18 eV. The semiconductor behavior is supported by the temperature-dependent resistivity measurements on a pellet of Na2IrCl6. The resistivity at 299 K is 3.2 × 106 Ω· cm, which increases almost linearly upon cooling and reaches a value of 2.1 × 109 Ω·cm at 179 K (Figure S12). These properties suggest the Ir 5d5 state as standard Jeff = 1/2 configuration. The calculated magnetic moment of Ir atom is 0.99 μB with 0.51 μB spin and 0.48 μB orbital contributions, E

DOI: 10.1021/acs.inorgchem.8b01753 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry which are quite close to the ionic values of 1.0 μB for a pure Jeff = 1/2 system.

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CONCLUSIONS We have successfully isolated pure phases of Na2IrCl6, Na2IrCl6·2H2O, and Na2IrCl6·6H2O. The former two are weak ferromagnets with ordering temperatures of 7.4 and 2.7 K, respectively, owing to the presence of nonsymmetric exchange couplings between the neighboring IrIV ions, whereas the latter is paramagnetic down to 1.8 K. The results demonstrate that the partial hydration of the sodium ion in Na2IrCl6 causes structural changes not only in the local environment of the {IrCl6} octahedra and the Ir−Ir distances but also in their magnetic behaviors. The first-principle calculations on compound Na2IrCl6 reveal that it is a spin− orbit-induced semiconductor with a Jeff = 1/2 ground state.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01753. X-ray powder diffraction analysis (data collection and refinement details), thermal analyses, computation method, selected bond lengths and angles, additional structures, and additional magnetic susceptibility measurements (PDF) Accession Codes

CCDC 1853107−1853109 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Li-Min Zheng: 0000-0003-4437-1105 Author Contributions #

S.-S.B. and D.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Key R&D Program of China (2017YFA0303203, 2018YFA0306004). The authors thank Prof. Xi-Zhang Wang in Nanjing University for the assistance with the resistivity measurements.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01753 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01753 Inorg. Chem. XXXX, XXX, XXX−XXX