Effects of Rb Intercalation on NbSe2: Phase Formation, Structure, and

Article Cite This: Inorg. Chem. 2019, 58, 7564−7570

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Effects of Rb Intercalation on NbSe2: Phase Formation, Structure, and Physical Properties Xiao Fan,†,‡ Hongxiang Chen,†,‡ Jun Deng,†,‡ Xiaoning Sun,†,‡ Linlin Zhao,†,‡ Long Chen,†,‡ Shifeng Jin,†,§ Gang Wang,*,†,∥ and Xiaolong Chen*,†,§,∥

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Research and Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinses Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 101408, China ∥ Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China S Supporting Information *

ABSTRACT: Here, we report the crystal structures and properties of RbxNbSe2, with 0 ≤ x ≤ 0.5. With Rb intercalation, RbxNbSe2 evolves from 2H (Phase I) for 0 ≤ x ≤ 0.025, to 6R (Phase II) for x ∼ 0.2 with space group R3m (no. 166), and finally to 2H (Phase IV) for 0.375 ≤ x ≤ 0.5 with space group P63/mmc (no. 194). In addition, Phase II is found to transform to a rare 6H structure (Phase III) with space group P63/mmc (no. 194) by annealing at a relatively low temperature. We show the first 6H phase in the intercalated transition-metal dichalcogenides (TMDs) family obtained through a solid-state reaction. Moreover, both 6R and 6H phases are new polymorphs in the NbSe2 system. For the range 0.2 ≤ x ≤ 0.5 in RbxNbSe2, we show metallic electronic transport behavior and a paramagnetic feature. The lack of superconductivity (SC) down to 2 K is most probably due to the decrease of hole carrier density with increasing Rb content. Through careful analysis of the structural data, we were able to assemble a phase diagram covering the range of 0 ≤ x ≤ 0.5 in RbxNbSe2.

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INTRODUCTION Layered transition-metal dichalcogenides (TMDs) have attracted a lot interest due to their rich physical and chemical properties.1−12 TMDs form from stacking MX2 layers with van der Waals (vdW) bonding and feature polymorphism, such as 1T, 2H, 3R, etc., owing to different stacking sequences of MX2 layers, where M and X represent a transition metal of groups 4−10 and a chalcogen (S, Se, and Te), respectively.13,14 The intercalation of an alkali, alkaline earth, transition metal, or ammonia-ion into the TMDs often leads to new phases. For example, Li intercalation results in a phase transition from 1TTaS2 to the 2H polymorph.15 A reverse transition from 2H to 1T was observed in 2H-MoS2 with Li intercalation,16,17 and more phases can be induced by further intercalation.18 The flexibility in composition and structure of TMDs makes them ideal candidates for designing new materials with novel crystal structures and properties19−22 and for designing the thermodynamics and diffusion kinetics through intercalant migration and ordering processes as well.19,21,23−26 The compound 2H-NbSe2, arguably the most well-studied of the TMDs, undergoes a charge density wave (CDW) transition at ∼33 K on cooling which is followed by a superconducting transition at 7.2 K.27 To date, the alkali metals Li,28 Na,29,30 and K29 and the first-row transition metals Mn,31 Fe,31 Co,31 and Ni31 have been intercalated into the 2H© 2019 American Chemical Society

NbSe2 structure. In all cases, the lattice parameter c was dilated or slightly shrank, indicating that the species intercalated into the vdW gap of NbSe2.28,29,31 The literature on Rb intercalation has been sparser in nature, with only the compositions Rb0.28NbSe2 and Rb0.56NbSe2 were reported without detailed crystal structures.32 Here, we carefully investigated the structural, magnetic, and electronic properties of Rb-intercalated NbSe2 prepared by a solid-state reaction. We observed three phases including 2H (Phase I), 6R (Phase II) with space group R3m (no. 166), and 2H (Phase IV) within 0 ≤ x ≤ 0.5. The structural evolution is different from the cases with Li, Na, and K intercalations. Moreover, Phase II transforms to 6H (Phase III) with space group P63/mmc (no. 194) by annealing at a relatively low temperature. To the best of our knowledge, the 6H phase we report is the first such example in a metal- or small-molecule-intercalated TMD, and the 6R phase we report is the first such example in a metal- or small-molecule-intercalated NbSe2. Using the structural data, we created a phase diagram of the RbxNbSe2 system. Received: March 26, 2019 Published: May 22, 2019 7564

DOI: 10.1021/acs.inorgchem.9b00862 Inorg. Chem. 2019, 58, 7564−7570

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Inorganic Chemistry

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EXPERIMENTAL AND CALCULATION METHODS

A series of polycrystalline RbxNbSe2 samples were prepared by a solid-state reaction. High purity Nb powder (Alfa Aesar, 99.99%) and Se shot (Alfa Aesar, 99.999%) in a 1:2 ratio were first heated at 973 K for 5 h in Al2O3 crucibles sealed in evacuated quartz ampules to synthesize NbSe2. Then, mixtures of Rb metal pieces (Alfa Aesar, 99.75%) and preprepared NbSe2 powder with different x in the nominal formula of RbxNbSe2 (x = 0, 0.025, 0.05, 0.2, 0.3, 0.375, 0.4, 0.45, 0.5, and 0.75) were heated at 923 K for 60 h in a similar vacuum environment. As-prepared RbxNbSe2 products were ground, pelletized, and sintered at 1023 K for 48 h to yield Phases I, II, and IV. Phase III was synthesized by further annealing the product of Phase II at 750 K with multiple ground, palletization, and annealing cycles. Powder X-ray diffraction (PXRD) was carried out using a Panalytical X’pert PRO diffractometer (Cu Kα radiation) with a graphite monochromator in the 2θ range from 3.5° < 2θ < 125°. We performed Rietveld refinements using the FullProf software suite.33 The chemical compositions determined by the inductively coupled plasma-atomic emission spectrometry (ICP-AES) revealed that the real and nominal compositions are close to each other. DC magnetizations were measured in a Quantum Design (QD) physical properties measurement system (PPMS) using finely grounded powders. Electronic resistances were measured in a QD PPMS using the standard four-probe configuration. We performed the first-principles calculations using the Vienna ab initio simulation package (VASP) code.34,35 The projector augmented-wave (PAW) pseudopotential36 was used with a plane-wave energy cutoff of 500 eV. The generalized gradient approximation (GGA) in the form of Perdew−Burke−Ernzerhof (PBE) was used as the exchange-correlation potentials.37 The electron localization function (ELF) was used to describe the chemical bonds, where ELF = 1 indicates full localization and ELF = 0.5 represents uniform electron gas. More calculation details are shown in the Supporting Information.

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RESULTS For RbxNbSe2 with 0 ≤ x ≤ 0.025, there is only one pure phase with the space group P63/mmc (no. 194) (Phase I), isostructural to 2H-NbSe2, with lattice parameters increasing from a = 3.44515(3) Å and c = 12.5507(1) Å (x = 0) (see Figure S1) to a = 3.44719(9) Å and c = 12.5864(5) Å (x = 0.025).38 At x = 0.2, a new phase emerges. All the diffraction peaks of it can be attributed to a single phase, RbxNbSe2 (Phase II). This phase can be indexed with lattice parameters a = 3.4575(1) Å, c = 47.196(2) Å, and possible space group R3m (no. 166) by referring to cases with other intercalated TMDs.39,40 Based on the lattice parameter c, we deduced that there are six NbSe2 layers and three Rb layers along the c axis within one unit cell. The Rb atoms intercalate into every two NbSe2 layers forming a second staging 6R phase. The term “staging” is used to assign a periodic sequence of intercalated layers in the host matrix. The stage number n represents the number of host layers separating two intercalant layers.41,42 The structure was determined by treating NbSe2 layers as independent segments in a simulated annealing approach with space group R3m (no. 166). Subsequent Fourier difference analysis revealed the location of the Rb site. The Rietveld refinement in Figure 1(a) using the Fullprof program converged to Rp = 5.48% and Rwp = 7.65%, indicating the validity of the 6R structure model. The crystal structure is shown in Figure 3(b), and the refined structure parameters are given in Table S2. We note that annealing the 6R phase at 750 K leads to the formation of a new structure (Rb0.2NbSe2) with peak reflections that can be indexed based on lattice parameters a = 3.4528(1) Å and c = 44.000(2) Å with the space group P63/

Figure 1. Rietveld refinements of Rb0.2NbSe2 (Phase II), Rb0.2NbSe2 (Phase III), and Rb0.4NbSe2 (Phase IV). (a) Refinement against PXRD data for Phase II. (b) Refinement against PXRD data for Phase III. (c) Refinement against PXRD data for Phase IV.

mmc (no. 194), as shown in Figures 1(b) and S2. We used a similar approach for the determination of the 6R phase to determine the structure of the Phase III (see Figure 3c). The Rb atoms intercalate into every three NbSe2 layers, forming the third staging 6H phase. Within one unit cell, there are six NbSe2 layers and two Rb layers along the c axis. The Rietveld fitting patterns and the refined parameters are given in Figure 1(b), Table 1, and Table S3. Minor amounts of Nb1+xSe2 (∼0.4%) and Rb4.88Nb14.62O39 (∼2.5%) were observed in the sample due to oxidation. The 6H phase is, to our knowledge, the first example of the odd staging other than the first staging in intercalated TMDs synthesized via a solid-state reaction. Other examples of odd staging formed upon intercalation have been obtained in intercalation via electrochemistry 43 (LixNbSe2) or hydration.44 Moreover, both 6R and 6H phases represent new polymorphs in the NbSe2 system. The firstprinciples calculations show that the value of ELF around Rb 7565

DOI: 10.1021/acs.inorgchem.9b00862 Inorg. Chem. 2019, 58, 7564−7570

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content is not enough to separate the NbSe2 layer in Phase I,38 different from the separated NbSe2 layer in Phase IV. Using the results presented above, we can construct the structural evolution and the various phase relationships in the RbxNbSe2 system. As shown in Figure 3, with x increasing from 0 to 0.5, RbxNbSe2 transforms from the 2H phase (0 ≤ x ≤ 0.025), characterized by randomly distributed Rb in the vdW gap, through the 6R phase, and, finally, to the second 2H phase (0.375 ≤ x ≤ 0.5). Moreover, annealing the 6R phase at 750 K induces the 6H phase. It is noticed that the Nb atoms occupy the same coordinates in the ab plane between the two nearest Rb layers for Phase II and III. The results of ICP-AES indicate that the real composition of the samples is close to the nominal one as shown in Tables 1 and S6. Phases II, III, and IV all feature a trigonal prismatic rearrangement of Se−Nb−Se and Se−Rb−Se layers. For Phase IV, deintercalation of Rb atoms happens upon exposure to oxygen and moisture, thereby inducing a structural transformation from the high-Rb 2H phase (Phase IV) through the 6R phase (Phase II) to the 2HNbSe2 (Phase I) (see Figure S6). In Figure 4(a), we present the temperature dependence of normalized resistance of RbxNbSe2 (0 ≤ x ≤ 0.4). For Phase I, the superconductivity (SC) is suppressed with the superconducting transition temperature decreasing from 7.2 K for x = 0 to 4.2 K for x = 0.025.38 No superconducting transition was observed down to 2 K for the 6R, 6H, and 2H (Rb0.4NbSe2) phases, which all evince metallic behavior. Band structures of 6R and 6H phases show metallic behavior (see Figures S7a and S8a), consistent with the resistance results. As shown in Figures S7(b) and S8(b), the partial density of states of 6R and 6H phases reveal that the states near the Fermi surface mainly come from Nb and Se atoms, which contributes to the electronic conductivity and is consistent with the ELF results. At relatively high temperatures, the resistance declines nearly linearly with decreasing temperature. The estimated Hall carrier number nH (nH = 1/ eRH) is about 3.5(3) × 1021 cm−3 (90 K) in Phase II and 1.61(5) × 1021 cm−3 (90 K) in Phase IV (Rb0.4NbSe2), which is lower than that in NbSe2 of 5.4(4) × 1021 cm−3 (90 K) (see Figure 4b). The magnetic field dependence of Hall resistivity ρxy at different temperatures for NbSe2 (Phase I), Rb0.2NbSe2 (Phase II), and Rb0.4NbSe2 (Phase IV) is shown in Figure S9.

Table 1. Crystallographic Data from the Rietveld Refinements and the Rb Composition Obtained from ICPAES for RbxNbSe2 (0 ≤ x ≤ 0.4) nominal x in RbxNbSe2 space group staging a (Å) c (Å) Rp (%) Rwp (%) Rb composition by ICP-AES

0 (Phase I)

0.2 (Phase II)

0.2 (Phase III)

0.4 (Phase IV)

P63/mmc (no. 194) − 3.44515(3) 12.5507(1) 5.02 7.06 −

R3m (no. 166) 2 3.4575(1) 47.196(2) 5.48 7.65 0.21(1)

P63/mmc (no. 194) 3 3.4528(1) 44.000(2) 5.1 7.29 0.19(1)

P63/mmc (no. 194) 1 3.4712(2) 18.9042(8) 6.08 8.59 0.40(1)

approximates zero and that the electrons in Rb layers are delocalized in both 6R and 6H phases (see Figures S3 and S4). The charge density difference shows that the electrons around Rb deplete in both phases. The delocalization and depletion of the electrons around Rb indicate that electrons transfer from Rb to NbSe2 layers and that the interaction between Rb and NbSe2 layers is electrostatic. For the composition range 0.375 ≤ x ≤ 0.5, we obtained only the 2H phase (Phase IV), characterized by the P63/mmc (no. 194) space group in which Rb atoms intercalate into every NbSe2 layer (see Figure 3d). With increasing x, the lattice parameter a increases, which is ascribed to the decline of the average oxidation state of Nb as more Rb is added; meanwhile, the lattice parameter c shrinks due to the stronger electrostatic interaction between the Rb+ layers and negatively charged NbSe2 layers,45 as shown in Figures 2(b) and S5. For x up to 0.75, Rb2Se3 and Nb3Se4 were found in the sample. The Rietveld fitting patterns and the refined parameters of Rb0.4NbSe2 are given in Figure 1(c), Table 1, and Table S4. As shown in Table S5, the Nb−Se bond distance of Phase IV is comparable to that of Phase I. The Se−Nb−Se bond angle α of Phase I is a little bit larger than that in Phase IV, and the Se− Nb−Se bond angle β of Phase I is smaller than that in Phase IV. Both the Nb−Se bond distance and Se−Nb−Se bond angles are the main factors that decide the lattice parameter a of Phase IV larger than that of Phase I. Moreover, the Rb

Figure 2. (a) The schematic of Se−Nb−Se bond angles α and β in the NbSe6 trigonal prism. (b) The evolution of lattice parameters a and c for RbxNbSe2 (0.3 ≤ x ≤ 0.5) (Phase IV) as a function of x. 7566

DOI: 10.1021/acs.inorgchem.9b00862 Inorg. Chem. 2019, 58, 7564−7570

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Figure 3. Representations of the crystal structures for (a) 2H−RbxNbSe2 (Phase I), (b) 6R-Rb0.2NbSe2 (Phase II), (c) 6H−Rb0.2NbSe2 (Phase III), and (d) 2H−Rb0.4NbSe2 (Phase IV). (e) The phase diagram for RbxNbSe2 for 0 ≤ x ≤ 0.5.

the composition range 0 ≤ x ≤ 1. Na- and K-intercalated NbSe2 compounds do undergo phase evolution due to a change in the stacking sequence of the NbSe2 layers.29,30 Similarly, AxTiS2 (where A = Na, K, Rb, and Cs) all show discrete phases. It has been speculated that if the ionic size of intercalant is larger than the vacant octahedral site in the vdW layer of TMDs, the S or Se layers will break. The energy associated with this demands a minimum alkali concentration and the presence of a two-phase region at low alkali concentration, often leading to the formation of multistage compounds.44 The structures with second and fourth staging have been commonly observed in the intercalated TMDs; however, the structures with odd staging except one staging only exist in the hydrated phase and the electrochemically intercalated sample such as LixNbSe2.43,44 The 6H phase of

The absence of SC for RbxNbSe2 (0.2 ≤ x ≤ 0.5) may be ascribed to the decreasing hole carrier density with the increase of Rb content. The isothermal magnetization at 3 and 300 K and the temperature-dependent magnetic susceptibility from 3 to 300 K for Phases II, III, and IV all show paramagnetic features (see Figures S10, S11, and S12). A more detailed characterization and analysis of the physical properties of RbxNbSe2 (0.2 ≤ x ≤ 0.5) will come in a future work.

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DISCUSSION

With increasing Rb content, the phases observed in Rbintercalated NbSe2 differ in terms of the staging of Rb intercalation and the stacking sequence of the NbSe2 layers. These phases are discrete, in contrast to LixNbSe228 and LixTiS244 which have a continuous intercalation structure over 7567

DOI: 10.1021/acs.inorgchem.9b00862 Inorg. Chem. 2019, 58, 7564−7570

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Figure 4. Electronic resistance and Hall carrier number for RbxNbSe2 (0 ≤ x ≤ 0.4). (a) The temperature dependence of normalized resistance for RbxNbSe2 (0 ≤ x ≤ 0.4). (b) The temperature dependence of Hall carrier number nH for RbxNbSe2 (Phase I) (x = 0), Rb0.2NbSe2 (Phase II), and Rb0.4NbSe2 (Phase IV).

Rb-intercalated NbSe2 stands out as the first third staging prepared by solid-state reaction for the intercalated TMDs, which enriches the structure of intercalated TMDs. To the best of our knowledge, this work represents the first appearance of 6R and 6H phases in metal-intercalated 2H-NbSe2 via a solidstate reaction mechanism. Once Rb intercalates into NbSe2, it leads to a gliding of the adjacent NbSe2 slabs. Such a gliding does not need much energy as the lamellae are bound only by a weak vdW interaction. The Nb−Se bond does not break upon the Rb intercalation, persisting as those in the host 2H-NbSe2 structure. Meanwhile, the glided NbSe2 layers create trigonal prismatic sites for Rb atoms bonded by six Se atoms through Coulombic interaction, different from the octahedral coordination of Li atoms in LixNbSe2 (0 ≤ x ≤ 1). The intercalated Na and K atoms usually occupy either trigonal prismatic sites (e.g., Na0.5NbSe230 and K0.67NbSe2) or octahedral sites (e.g., NaNbSe2 and KNbSe2).29 The coordination of Rb is generally trigonal prismatic in intercalated MS2 (M = Ti, Zr, Hf, Mo, and W), except for the octahedral coordination in intercalated NbS2.46 In generally, the trigonal prismatic structure is stabilized by the larger cations, whereas the octahedral one is favored with higher alkali content. The ionic size of Rb is large enough, the ratio r(Rb+)/r(Se2−) ≈ 0.9, to keep the Se atoms in two NbSe2 slabs adjacent to the Rb layers with the same (x, y) positions and support the trigonal prismatic coordination with a formation energy lower than that of the octahedral one.47

metallic electronic transport behavior. The polymorph of Rbintercalated NbSe2 provides an opportunity to further study the relationship between the crystal structure and properties in TMDs.

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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.9b00862. Crystallographic parameters, PXRD patterns, firstprinciples calculations, magnetic properties (PDF) Accession Codes

CCDC 1906740, 1906743, and 1906850 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]. uk, 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]. ORCID

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Gang Wang: 0000-0001-9110-3942

CONCLUSIONS We systematically investigated the structural, magnetic, and electronic properties of RbxNbSe2 (0 ≤ x ≤ 0.5), which shows the phase transition from a 2H phase, via a 6R phase, to a second 2H phase. The structural evolution in RbxNbSe2 is different from the cases in Li, Na, and K intercalated NbSe2 where either a continuous structure exists (Li) or different stacking sequences are employed to accommodate the cation intercalates. Annealing the 6R phase at a relatively low temperature (750 K) results in the formation of a 6H phase. Both the 6R and 6H phases are, to our knowledge, the first of their kind to be shown in metal-intercalated 2H-NbSe2. All phases with a composition between 0.2 ≤ x ≤ 0.5 show

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.W. would like to thank Dr. Q. S. Lin of Ames Laboratory for useful discussions. This work was partially supported by the National Natural Science Foundation of China (51572291, 51832010, and 51532010), the National Key Research and Development Program of China (2017YFA0302902), the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (QYZDJ-SSW-SLH013), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB07000000). 7568

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DOI: 10.1021/acs.inorgchem.9b00862 Inorg. Chem. 2019, 58, 7564−7570