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Preparation, Structure Evolution, and Metal−Insulator Transition of NaxRhO2 Crystals (0.25 ≤ x ≤ 1) Bin-Bin Zhang,†,‡ Cong Wang,† Song-Tao Dong,§ Yang-Yang Lv,† Lunyong Zhang,∥,⊥ Yadong Xu,‡ Y. B. Chen,*,# Jian Zhou,†,$ Shu-Hua Yao,*,†,$ and Yan-Feng Chen†,$ †

National Laboratory of Solid State Microstructures Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, People’s Republic of China ‡ Key Laboratory of Radiation Detection Materials and Devices, Ministry of Industry and Information Technology, School of Materials Science and Engineering State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, People’s Republic of China § Institute of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, People’s Republic of China ∥ Max Planck Postech Center for Complex Phase Materials, Max Planck Postech/Korea Research Initiative (MPK), Gyeongbuk 376-73, Korea ⊥ Max Planck Institute for Chemical Physics of Solids, Nöthnitzerstr., Dresden 40 01187, Germany # National Laboratory of Solid State Microstructures & Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China $ Collaborative Innovation Center of Advanced Microstructure, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *

ABSTRACT: The triangular lattice NaxRhO2 contains a 4d Rh element with large spin−orbit coupling, and the electron−electron correlation effect is expected to have some novel physical properties. Here we report NaRhO2 crystal growth by Na2CO3 vapor growth and a series of NaxRhO2 (0.25 ≤ x ≤ 1) crystals prepared using the chemical desodiation method. NaxRhO2 reveals a layer structure with the space group R3̅m, and the lattice parameter a evolves from 3.09 to 3.03 Å and c from 15.54 to 15.62 Å when x decreases from 1.0 to 0.2. Decreasing potassium concentration leads to a contraction of the RhO6 octahedral layers, which may be attributed to a higher covalency of Rh−O bonds. More important, the metal−insulator transition in NaxRhO2 was observed in resistivity along the ab plane. The conducting mechanism of NaxRhO2 is strongly dependent on x. Two-dimensional variable range hopping (VRH) mechanisms (0.67 ≤ x ≤ 1) and metallic behaviors (0.42 and 0.47) are observed in temperature-dependent resistivity. The origin of this metal−insulator transition was discussed on the basis of the Ioffe−Regel criterion. Our work demonstrates the strong correlation between sodium concentration and physical properties of NaxRhO2. relatively large Seebeck coefficients such as 46 μV/cm (K0.63RhO2)8 and 40 μV/cm (K0.49RhO2)9 were measured at room temperature. Except for the thermoelectric properties, some unconventional magnetic structures can be generated in AxRhO2. Zhou et al. predicted a noncoplanar spin structure in K0.5RhO2 and a resultant quantum topological Hall effect by first-principles calculation.10 The same conclusion can also be applied to NaxRhO2. However, in comparison to the plentiful theoretical explorations of AxRhO2, there have been very few reports

1. INTRODUCTION The layered compound NaxCoO2 with a triangular lattice has demonstrated rich phenomena with variable Na contents, including spin-entropy-induced large thermoelectric power,1 charge-ordering insulator behavior,2 and superconductivity.3 In compariso with 3d cobaltate, 4d Rh-based transition-metal oxides show a weak electron correlation and a strong spin− orbit coupling (SOC) effect.4 Accordingly, Rh-based oxides AxRhO2 (such as NaxRhO2) have been theoretically predicted to be good reference systems to study the novel physical properties under the synergetic effects of both SOC and electron correlation. For example, extremely large thermopower is induced by the strong correlation.5−7 In experiments, © XXXX American Chemical Society

Received: December 11, 2017

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DOI: 10.1021/acs.inorgchem.7b03110 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Schematic of experimental apparatus for NaRhO2 crystal growth from Na2CO3 vapor.

cooling rate of 1−5 °C/h and finally to room temperature naturally. Notably, a lid with several holes (see Figure 1) was used to control the speed of volatility of Na2CO3. After the flux was washed away with deionized water, we could obtain many brown crystals of millimeter size from the upper region of the alumina crucibles. The single-phase NaRhO2 powders were prepared by a similar process but with a short growth time (∼5 h). To get NaxRhO2 with low Na content, I2/ CH3CN solution was used as the oxidizing agent in the desodiation process. By control of the reaction time, samples with different Na concentrations could be obtained. A similar preparation process was reported previously.11 The composition of NaxRhO2 was determined by energy-dispersive spectroscopy (EDS) in a scanning electron microscope (SEM) using a standard reference sample. The accuracy/error bar for EDS is ±3%. We also averaged the EDS data taken from 20 different areas to avoid inhomogeneity of the sample. Its crystal structure was determined by powder X-ray diffraction (XRD, Rigaku). These results were analyzed by Rietveld refinement (the general structure analysis software package GSAS-EXPGUI).12 The temperature-dependent in-plane resistance was measured using a dc four-probe configuration (PPMS, Quantum Design, Inc.). The diffuse reflective spectra were recorded at room temperature with a UV-3150 spectrometer (Shimadzu UV−vis−NIR) using BaSO4 as the reference from 335 to 1320 nm. The band structures of NaRhO2 were calculated by density functional theory (DFT) in the generalized gradient approximation implemented in the Vienna ab initio simulation package (VASP) code,13,14 in which the projected augmented wave method and the Perdew−Burke−Ernzerhof exchange correlation are used.15−17 The plane-wave cutoff energy was 520 eV throughout the calculations. The effective Coulomb energy Ueff = 2.0 eV was used for the Rh element.10 Spin−orbit coupling (SOC) was also included in the calculation. We have used the total energy as the convergence criterion in the DFT method. Practically, the tolerance energy is 10−6 eV in the real calculations. Generally, this energy accuracy is high enough for most materials.

crystal growth. The starting materials were mixtures of Na2CO3 (99.99%, Sigma-Aldrich) and Rh2O3 (99.99%, Sigma-Aldrich). Single crystals were grown from a mixture of powdered Rh2O3 and Na2CO3 solvent with a typical mass ratio of 1:11. The mixtures placed in an alumina crucible with a lid were heated to 850 °C, held at this temperature for 5 h, and then heated to 1100 °C and sustained at this temperature for 3 days. The melt was slowly cooled to 850 °C at a

3. RESULTS AND DISCUSSION 3.1. Crystal Growth and Characterization. Figure 2 shows the microstructure of as-grown NaRhO2 crystals characterized by SEM. The planar-view SEM (Figure 2a) image shows that NaRhO2 crystals have a hexagonal shape. In Figure 2a, two large crystals cross each other and many small crystals are accumulated on the surfaces of large crystals. Figure

about NaxRhO2, to the best of our knowledge. In addition, the growth of NaxRhO2 crystals and electrical property characterizations are still unexplored. In this work, we have studied the preparation of NaxRhO2 single crystals and polycrystals by the vapor growth and successive chemical desodiation methods. On the basis of these samples, the evolution of the crystal structure and electrical properties of NaxRhO2 (x = 0.25−1.0) have been investigated. A metal−insulator transition is observed in NaxRhO2, which can be explained by the Ioffe−Regel criterion. Our work demonstrates the strong correlation between the sodium concentration and physical properties of NaxRhO2.

2. EXPERIMENTAL SECTION NaRhO2 crystals were grown from Na2CO3 vapor transport method in the temperature range 1000−1100 °C. Figure 1 illustrates the setup of

Figure 2. (a) Planar-view SEM image of the as-grown NaRhO2. (b) Enlarged image of the area highlighted by the yellow dotted box in (a). (c) Enlarged image of the yellow dotted box in (b). There exist many thin hexagonal platelike crystals nucleated on the surface of large crystals. (d) Cross-section SEM image of NaRhO2 crystals. (e) Enlarged image of the yellow dotted box in (d). (f) Enlarged image of the yellow dotted box in (e). It shows the clear layer-by-layer structure of NaRhO2. (g) Typical EDS spectrum of a NaRhO2 single crystal. B

DOI: 10.1021/acs.inorgchem.7b03110 Inorg. Chem. XXXX, XXX, XXX−XXX

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samples is shown in Figure 3b. The insets show the schematic diagram of the NaRhO2 crystal structure and the optical pictures of millimeter-sized NaRhO2 crystals. The electronic band structure of NaRhO2 was studied by the first-principles calculation and result is shown in Figure 4a.

2b,c displays these small crystals in the enlarged image of the area highlighted by the yellow dotted box in Figure 2a. These small crystals show a hexagonal shape as well. The cross-section SEM image of NaRhO2 is shown in Figure 2d. Figure 2e,f gives the amplified images of the area indicated by the yellow dotted box in Figure 2d, which demonstrates the layer-by-layer structure of NaRhO2 crystals. The composition of as-grown sodium rhodate crystals was determined by energy dispersive spectroscopy (EDS). As shown in Figure 2g, the ratio of Na and Rh is 1.01, which is very close to the stoichiometric NaRhO2. In order to determine the crystal structure of NaRhO2, X-ray diffraction (XRD) measurements of polycrystalline samples and single crystals were carried out. The XRD patterns are shown in Figure 3a,b. The crystal structure of layered NaRhO2 was

Figure 4. (a) Calculated energy band of NaRhO2 showing the semiconducting behavior with the energy gap around 1.75 eV. The inset gives the schematic of formation of the energy band gap in NaRhO2 through filling t2g orbitals only. (b) Diffuse reflection spectrum of NaRhO2 powder. The energy gap is determined at around 1.80 eV, which is quite close to the theoretical gap.

Obviously, NaRhO2 is a semiconductor with an indirect band gap about 1.75 eV, which is larger than the calculated bandgap 1.33 eV of NaRhO2 reported by Saeedet al.7 It may be due to different pseudopotential used in the calculation. This band gap can be straightforwardly understood by the crystal field splitting in the RhO6 octahedron, splitting of Rh′ d-orbital into the eg and t2g ones. In NaRhO2, the Rh ion has a +3 valence state and has six electrons, which can fill all three t2g orbitals and leave eg orbitals totally empty (see inset of Figure 4a). Therefore, the crystal field splitting between the Rh atom’s eg and t2g orbitals gives rise to the semiconductor property in NaRhO2. The semiconducting gap can be experimentally determined by diffuse reflection spectra. The absorption spectrum (Figure 4b), recorded in powdered polycrystalline NaRhO2 in diffuse reflectance mode, substantiates that the compound is a semiconductor with band gap Eg = 1.80 eV at room temperature, which is quite close to the predicted value (1.75 eV) described above. 3.2. Chemical Desodiation and Electrical Transport Properties. To obtain low Na content sodium rhodate samples, acetonitrile (CH3CN) solutions of I2 were used as oxidizing agents for the chemical desodiation process. The solutions with variable I2 concentrations show different colors (see Figure 5a). Due to the difference in electrode potentials between NaRhO2 and I2/CH3CN solutions, the redox reaction happens accompanied by the Na ions migration process. The chemical reaction of desodiation can be described as y NaxRhO2 + I 2 → Nax − yRhO2 + y Na + + y I− 2 When the starting material NaRhO2 reacts with I2, Na ions can migrate along the ab plane from the crystal to the solution. As shown in Figure 5a, the electrode potential will increase with a decrease in Na content in NaxRhO2 crystals. Finally, equal electrode potentials between NaxRhO2 and I2/CH3CN solutions are reached at a specific Na content. Accordingly, in order to obtain a series of NaxRhO2 samples, I2/CH3CN solutions with different concentrations were used.

Figure 3. (a) Rietveld refinement of the XRD data from polycrystalline NaRhO2. Circles (○) represent the experimental data, and the solid red curve denotes the refinement result. The difference between the experimental and calculated results is shown by the blue curve. The pink tick marks indicate the expected Bragg peak positions for the NaRhO2 phase. (b) XRD pattern for single-crystal NaRhO2, which shows a set of (00l) peaks. The insets show the layered crystal structure of NaRhO2 with a triangular lattice and an optical picture of as-grown crystals.

determined by powder XRD together with the Rietveld refinement, as shown in Figure 3a. The calculated patterns (solid lines) agree very well with the experimental patterns. The fitting factors are Rwp = 9.8% and Rp = 7.4%. Table 1 summarizes the structural parameters of NaRhO2. The unit cell parameters obtained from the Rietveld refinement are a = b = 3.09 Å and c = 15.54 Å, respectively. The crystal belongs to space group R3m ̅ , which is in agreement with previous reports.18,19 Only one set of (00l) peaks for single-crystal Table 1. Structural Parameters of NaRhO2a atom

x

y

z

occ

Uiso (Å2)

Rh Na O

0 0 0

0 0 0

0.5 0 0.234(9)

1.0 1.0 1.0

0.007(8) 0.015(1) 0.014(1)

a Crystal dataa: space group R3̅m, a = 3.0936(9) Å, c = 15.5389(2) Å, Rwp = 9.8%, Rp = 7.4%.

C

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Figure 5. (a) Pictures of I2/CH3CN solutions with different I2 concentrations, diagram of the migration process, and changes in chemical potentials between NaxRhO2 and I2/CH3CN solutions during the desodiation process. C is the saturated concentration of I2 acetonitrile solution. (b) XRD patterns of NaxRhO2 powder (x = 1.01, 0.91, 0.73, 0.61, 0.25) plotted with the space group R3̅m. The asterisks in the XRD patterns of x = 0.61, 0.25 samples indicate the impurity phase, which may be the water interacted NaxRhO2 phase.

Figure 6. (a) Schematic of atomic structures of NaRhO2, highlighting the lattice parameters a and c, the cell volume, the Rh−Rh, and Rh−O bonds lengths, and the Rh−O−Rh angular degree. (b) Decrease in the lattice constant a but increase in c with a decrease in the value of x. Circles, squares, and stars indicate the data of this work and those of Varela et al.20 and Mendiboure et al.,21 respectively. Also shown is the x dependence on the volume of the unit cell, the Rh−Rh and Rh−O bond lengths, and the Rh−O−Rh angular degree in NaxRhO2.

NaxRhO2 phase, when the lower Na content powder samples were exposed to the air. A diagram of the crystal structure of NaxRhO2 is shown in Figure 6a, which highlights the lattice parameters (a and c), the Rh−O bond distance, and the Rh−O−Rh bond angle. Through the powder XRD refinements of desodiated NaxRhO2, the evolution of the lattice constants of NaxRhO2 with x are summarized in Figure 6b, and some data from Varela et al.20 and Mendiboure et al.21 are also included. With a decrease in Na concentration, the lattice constant a decreases but c increases. In addition, the corresponding volume of NaxRhO2 unit cell increases. The increase in c may be due to the uncompensated repulsion of the oxygen layers and decreased

A series of XRD patterns of the obtained desodiation NaxRhO2 (x = 1.00, 0.91, 0.73, 0.61, 0.25) powder samples are shown in Figure 5b. The contents of Na were also determined by EDS. They all belong to the space group R3̅m. With decreasing Na contents, all (003) peaks move to the low degree side, indicating that the lattice constant c or the distance between two neighboring RhO6 octahedron layers increased. However, with a decrease in Na content by the desodiation process, extra peaks were generated in the XRD patterns of x = 0.61 and 0.25 samples, indicating an impurity phase or another phase of NaxRhO2. On careful comparison to the XRD patterns of Na0.3RhO2·0.6H2O reported by Parket al.,18 we inferred that this impurity phase maybe come from the water interacted D

DOI: 10.1021/acs.inorgchem.7b03110 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) Temperature-dependent ab plane resistivities of NaxRhO2 crystals (x = 1.01, 0.88, 0.80, 0.64, 0.47, 0.42). (b) Curves of the temperature-dependent resistivity for NaxRhO2 (x = 1.01, 0.88, 0.80, 0.64), which obey ρ(T)= ρ0 exp(T0/T)v. In addition, v = 1/3 indicates that the carriers transport in two dimensions through variable ring hopping. (c) Metal−insulator transition following the Ioffe−Regel criterion kFl ≈ 1.

electrostatic attraction (decreased distance of Na−O bonds). The change in the lattice constant a corresponds to a variation of the O−O distances within the RhO2 sheets. Conceptually, during the depotation process, the relative amount of Rh4+ ions increases and leads to a higher covalency of Rh−O bonds, resulting in a contraction of the RhO6 octahedra and a decreased lattice constant a. A decrease in a (or Rh−Rh bond) is advantageous for electrical carriers transported along the ab plane because electrons hop along Rh−O−Rh in this material. The large difference in the transport properties of NaxRhO2 can be expected due to the changes in crystal structure and carrier concentration. The temperature-dependent ab plane resistivity ρab for NaxRhO2 (x = 1.00, 0.88, 0.80, 0.47, 0.42) single crystals are summarized in Figure 7a. The ρ−T plot of NaRhO2 verifies the normal insulator behavior. With a decrease in x, the resistivity is decreased gradually. The slopes of the curves change from a negative value to a positive value (x = 0.47, 0.42 for NaxRhO2), which suggests a transition from an insulating to a metallic state. The variable-range hopping (VRH) behavior was applicable to fit the resistivity temperature curves of NaxRhO2 (x = 1.00, 0.88, 0.80). The VRH here for NaxRhO2 (x = 1.00, 0.88, 0.80) obeys ρ(T) = ρ0 exp(T0/T)v,22 where T0 is the fitted localization temperature and v is related to the dimensionality of the system. v = 1, v = 1/3, and v = 1/4 were attempted to fit the temperature-dependent resistivity curves of insulative NaxRhO2 samples (see the Supporting Information). In addition, v = 1/3 shown in Figure 7b was well fitted, which indicates that the carriers hop in two-dimensional RhO2 planes. VRH behavior is the feature of an Anderson insulator. To clarify the origin of the metal−insulator transition observed in NaxRhO2, a carrier-filling controlled Mott transition is discussed below. According to the Drude model σ=

1 ρ

σ=

we can write l=

ℏ(3π 2n)1/3 nq2ρ

where ρ and ℏ are the resistivity and the reductive Planck constant. Thus, the Ioffe−Regel criterion can be written as kFl =

ℏ 1 (3π 2)2/3 1/3 q2 n ρ

where kF is the Fermi wave vector. In accordance with the IoffeRegel criterion, kFl is larger than 1 in the metallic state but smaller than 1 in the insulative state. Finally, the formula was obtained as kFl =

1.35 × 10−5 (1 − x)1/3 ρ

where x is the content of Na. The carrier concentration n is written as 1 − x because the carriers in NaxRhO2 are holes. This assertion is reasoned as follows: in NaRhO2 and RhO2, the valence states of Rh are +3 and +4 in the ionic picture. Accordingly, there are six and five electrons in 4d bands. These electrons are distributed at the t2g band (degeneracy is 6). Then the hole carrier concentrations in Rh3+ (six electrons) and Rh4+ (five electrons) are 0 and 1.0, respectively. Therefore, the hole concentration is 1 − x in each unit cell volume in NaxRhO2. This picture can also be seen by the electronic band structure shown in Figure 4a; NaRhO2 is an insulator, whose Fermi level is in the upmost of the t2g band. In NaxRhO2, the Fermi level is decreased and penetrates the t2g band, which gives rise to hole carriers. Using the values of x and ρ of NaxRhO2, the phase diagram of NaxRhO2 can be drawn in Figure 7c. Obviously, kFl in metallic NaxRhO2 (x = 0.47, 0.42) is larger than 1, but that in insulative NaxRhO2 (x = 0.64, 0.80, 0.88, 1.00) is smaller than 1. This clearly substantiates that the metal−insulator transition observed in NaxRhO2 (x = 0.42−1.0) belongs to the carrierfilling controlled Mott transition.

nq2τ nq2l nq2l = = m mvF pF

where σ, n, q, τ, m, l, vF, and pF are the conductivity, carrier concentration, charge of the carrier, relaxation time of the carriers, effective mass of the carriers, mean free path, Fermi velocity, and Fermi momentum, respectively.23 Because

4. CONCLUSIONS NaRhO2 single crystals have been successfully grown by a vapor transport method. The morphology and crystal structure of NaRhO2 characterized by SEM and XRD reveal a layered structure with the space group R3m ̅ and lattice parameters a =

⎛ p ⎞3 n = 3π 2⎜ F ⎟ ⎝ℏ⎠

and E

DOI: 10.1021/acs.inorgchem.7b03110 Inorg. Chem. XXXX, XXX, XXX−XXX

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(3) Takada, K.; Sakurai, H.; Muromachi, E. T.; Izumi, F.; Dilanian, R. A.; Sasaki, T. Superconductivity in two-dimensional CoO2 layers. Nature 2003, 422, 53. (4) Okazaki, R.; Nishina, Y.; Yasui, Y.; Shibasaki, S.; Terasaki, I. Optical study of the electronic structure and correlation effects in K0.49RhO2. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 075110. (5) Chen, S.-D.; He, Y.; Zong, A.; Zhang, Y.; Hashimoto, M.; Zhang, B.-B.; Yao, S.-H.; Chen, Y.-B.; Zhou, J.; Chen, Y.-F.; Mo, S.-K.; Hussain, Z.; Lu, D.; Shen, Z. X. Large thermopower from dressed quasiparticles in the layered cobaltates and rhodates. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 96, 081109. (6) Saeed, Y.; Singh, N.; Schwingenschlögl, U. Colossal Thermoelectric Power Factor in K7/8RhO2. Adv. Funct. Mater. 2012, 22, 2792− 2796. (7) Saeed, Y.; Singh, N.; Schwingenschlögl, U. Superior thermoelectric response in the 3R phases of hydrated NaxRhO2. Sci. Rep. 2015, 4, 4390. (8) Yao, S. H.; Zhang, B. B.; Zhou, J.; Chen, Y. B.; Zhang, S. T.; Gu, Z. B.; Dong, S. T.; Chen, Y. F. Structure and physical properties of K0.63RhO2 single crystals. AIP Adv. 2012, 2, 042140. (9) Shibasaki, S.; Nakano, T.; Terasaki, I.; Yubuta, K.; Kajitani, T. Transport properties of the layered Rh oxide K0.49RhO2. J. Phys.: Condens. Matter 2010, 22, 115603. (10) Zhou, J.; Liang, Q. F.; Weng, H. M.; Chen, Y. B.; Yao, S. H.; Chen, Y. F.; Dong, J. M.; Guo, G. Y. Quantum topological Hall effect and noncoplanar antiferromagnetism in K0.5RhO2. Phys. Rev. Lett. 2016, 116, 256601. (11) Zhang, B. B.; Lv, Y. Y.; Dong, S. T.; Zhang, L. Y.; Yao, S. H.; Chen, Y. B.; Zhang, S. T.; Zhou, J.; Chen, Y. F. Depotassiation of K0.62RhO2 and electronic property of the end-product K0.32RhO2 single crystal. Solid State Commun. 2016, 230, 1−5. (12) Toby, B. H. EXPGUI, a graphical user interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210−213. (13) Kresse, G.; Hafner, J. Ab initio molecular dynamics for openshell transition metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 13115. (14) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15−50. (15) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953. (16) Kresse, G.; Joubert, D. From ultrasoft pseudo potentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758. (17) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (18) Park, S.; Kang, K.; Si, W.; Yoon, W.-S.; Lee, Y.; Moodenbaugh, A. R.; Lewis, L. H.; Vogt, T. Synthesis and characterization of Na0.3RhO2·0.6H2O  a semiconductor with a weak ferromagnetic component. Solid State Commun. 2005, 135, 51−56. (19) Krockenberger, Y.; Reehuis, M.; Cristiani, G.; Ritter, C.; Habermeier, H. U.; Alff, L. Neutron scattering and magnetic behavior of NaxRhO2. Phys. C 2007, 460-462, 468−470. (20) Varela, A.; Parras, M.; González-Calbet, J. M. Influence of Na Content on the Chemical Stability of Nanometric Layered NaxRhO2 (0.7 ≤ x ≤ 1.0). Eur. J. Inorg. Chem. 2005, 2005, 4410−4416. (21) Mendiboure, A.; Eickenbusch, H.; Schollhorn, R.; Subba Rao, G. V. Layered alkali rhodium oxides AxRhO2: Topotactic solvation, exchange, and redox reactions. J. Solid State Chem. 1987, 71, 19−28. (22) Sheng, P. Introduction to wave scattering, localization, and mesoscopic phenomena; Academic Press: New York, 1995. (23) Mott, N. Metal-insulator transitions; Taylor & Francis: Oxford, U.K., 1974.

3.09 Å and c = 15.54 Å. First-principles calculations and diffuse reflection spectra substantiate NaRhO2 being a semiconductor with energy band gap of about 1.75 eV, which originates in the crystal-field splitting between the Rh’s eg and t2g orbitals. A series of NaxRhO2 (0.25 ≤ x ≤ 1) powders and crystals were obtained using a chemical desodiation method. The evolution of lattice parameters of NaxRhO2, determined by powder XRD refinements, demonstrates that the lattice constant c is increased with decreased x, but a is reduced. Experimentally, the metal−insulator transition in NaxRhO2 was also observed in ab plane resistivity with decreased x. The ab plane resistivity of NaxRhO2 (x = 1.00, 0.88, 0.80, 0.64) can be fitted by a twodimensional variable range hopping (VRH) mechanism. The metal−insulator transition observed in NaxRhO2 (x = 0.42− 1.0) can be explained by an Ioffe−Regel criterion. This work demonstrates the strong correlation between sodium concentration and electrical properties of NaxRhO2.

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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.7b03110. Detailed information on 2D VRH (T−1/3), 3D VRH (T−1/4), and Arrhenius (T−1) plots for NaxRhO2 samples and their evaluated carrier concentrations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Y.B.C.: [email protected]. *E-mail for S.-H.Y.: [email protected]. ORCID

Bin-Bin Zhang: 0000-0002-1874-1881 Lunyong Zhang: 0000-0002-1193-5966 Yadong Xu: 0000-0002-1017-9337 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (9162212, 11374140, 11374149, U1631116, and 51202197), the major State Basic Research Development Program of China (973 Program) (2015CB921203, 2014CB921103, and 2015CB659400), the National Key Research and Development Program of China (2016YFF0101301 and 2016YFE0115200), the Natural Science Basic Research Plan in Shaanxi Province of China (2016KJXX-09), the Fundamental Research Funds for the Central Universities (3102017zy057), and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51721001).

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REFERENCES

(1) Wang, Y.; Rogado, N. S.; Cava, R.; Ong, N. Spin entropy as the likely source of enhanced thermopower in NaxCo2O4. Nature 2003, 423, 425. (2) Foo, M. L.; Wang, Y.; Watauchi, S.; Zandbergen, H.; He, T.; Cava, R.; Ong, N. Charge Ordering, Commensurability, and Metallicity in the Phase Diagram of the Layered NaxCoO2. Phys. Rev. Lett. 2004, 92, 247001. F

DOI: 10.1021/acs.inorgchem.7b03110 Inorg. Chem. XXXX, XXX, XXX−XXX