Synthesis, Crystal Structure, and Electrical Conductivity of Pd-intercalated NbSe2

Chong HUANG; Wei ZHAO; Dong WANG; Kejun BU; Sishun WANG; Fuqiang HUANG

doi:10.15541/jim20190125

Abstract

New intercalated compounds Pd_xNbSe₂ (x=0-0.17) were synthesized via solid-state reaction. They possess the parent structure of 2H-NbSe₂ and crystalize in the hexagonal space group of P6₃/mmc. The intercalated Pd occupies the octahedral position in the van der Waals gaps of 2H-NbSe₂. Unit cell parameter c increases linearly with the Pd content, while a is nearly unchanged. The lattice parameter of Pd_0.17NbSe₂ (a=b=0.34611(2) nm, c= 1.27004(11) nm) is identified by single crystal X-ray diffraction. The intercalated Pd stabilizes the crystal structure of NbSe₂ by connecting the adjacent Nb-Se layers with [PdSe₆] octahedra and leads to the enhanced thermostability in air. Temperature dependence of electric resistivity reveals that the residual resistivity ratio of Pd_xNbSe₂ monotonically decreases with addition of the intercalated Pd content. The decreased superconducting critical temperature of Pd_xNbSe₂ indicates the suppression effect of Pd intercalation on the superconductivity in the host NbSe₂.

Keywords

crystal structure PdxNbSe2 superconducting transition metal dichalcogenide

Layered transition metal dichalcogenides (TMDs) with the general chemical formula MX 2 (M represents the transition metal and X is the chalcogen) have been widely studied due to their unique physicochemical properties and diverse applications [1, 2, 3, 4, 5] . The metallic Group ⅤB TMDs (where M = V, Nb and Ta) are prized for their fascinating electronic properties, such as charge density wave (CDW), superconductivity and Mott transition [6, 7] . Among them, 2H-NbSe 2 is featured with a high superconducting critical temperature (T C) of ~7.3 K and a quasi-two-dimensional incommensurate charge density wave (ICDW) with a T CDW of ~33 K [8] . Because of the weak van der Waals (vdW) forces connected interlayers in the crystal structure, 2H-NbSe 2 can be intercalated by various guests, including atoms, ions, and molecules [9] .

Typically, the incorporation of guest metal atoms into the vdW gaps of TMDs could give rise to the crystallographic transformation and change of electronic structure in the intercalated compounds [10] . Magnetic elements (Fe, Co) inserted into the vdW gaps of NbSe 2 resulted in the formation of superlattice [11, 12] . Alkali metal intercalation was found to remove the CDW instability in NbSe 2 [13] . Recently, noble metal, such as palladium (Pd), was applied to regulate the electronic structure efficiently for the host TMDs. Our group [14] found that Pd modified the band structure of 2H-TaS 2 through Pd-S bonding to strengthen the interaction of adjacent Ta-S layers, which led to the enhanced conductivity in Pd 0.10 TaS 2 . Pd intercalation was reported to increase the eﬀective electron- phonon coupling in 2H-TaSe 2 and enhance the T C in Pd x TaSe 2 [15] . Considering that the crystal structure of 2H-NbSe 2 is identical to that of 2H-TaX 2 (X=S, Se), Pd intercalation should be applicable to 2H-NbSe 2 and tune the physical properties.

In this work, a series of new compounds PdxNbSe2 (x=0~0.17) were synthesized and the crystal structure of Pd0.17NbSe2 was determined by single X-ray diffraction method in order to investigate the modification of crystal lattice and electrical conductivity in the Pd intercalated NbSe2.

1 Experimental

1.1 Preparation of Pd_xNbSe₂

Pd x NbSe 2 crystals were prepared by solid-state reaction. Pd (99.99%), Nb (99.5%) and Se (99.99%) powders were mixed according to stoichiometric ratio, and ground. Then the mixtures were compacted into a pellet and heated in the evacuated (< 0.133 Pa) silica tube at 1173 K for 48 h. Subsequently, the as-obtained samples were reground, re-pelletized and held at 1173 K for 72 h. Then the samples were cooled down by quenching in water. High quality single crystal of Pd 0.17 NbSe 2 was obtained by keeping Pd 0.17 NbSe 2 powder with CsI (99.9%) at 1173 K for 1 d and slowly cooling down to 823 K for 3 d.

1.2 Characterization

Single crystal data collections of Pd0.17NbSe2 was conducted on a Bruker D8 QUEST diffractometer equipped with Mo Kα radiation at room temperature. The crystal structure determination and refinement were performed with the APEX3 program. The crystal structure of Pd0.17NbSe2 was drawn by using the VESTA program[16]. The morphology and the composition of the Pd0.17NbSe2 were investigated by a scanning electron microscope (SEM, JSM6510) coupled with energy dispersive X-ray spectroscopy (EDXS, Oxford Instruments). The micro-structure of Pd0.17NbSe2 was uncovered by a high-resolution transmission electron microscope (HRTEM, JEM-2100F) and the selected area electron diffraction (SAED). The valence analysis of the Pd0.17NbSe2 was obtained from X-ray photoelectron spectroscope (XPS) carried out on the RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (hv=51253.6 eV). The binding energy in XPS analysis was corrected by referencing C 1s peak at 284.6 eV. Powder X-ray diffraction (PXRD) data of these PdxNbSe2 samples were collected by using a Bruker D8QUEST diffractometer equipped with Cu Kα radiation (λ=0.15405 nm). Thermogravimetric analysis (TG) and differential thermal analysis (DTA) were carried out on a NETZSCH STA449C thermal analyzer for investigating the thermal stability of Pd0.17NbSe2 and NbSe2 in air. Resistivity of the as-obtained PdxNbSe2 at different temperatures was executed on a Physical Properties Measurement System (PPMS, Quantum Design). A four-probe method was adopted for measurements of the resistance. More specifically, the powders were pressed into a disk. Silver paste and copper wire acted as the contact electrode and conduct wire, respectively. Normalized resistivity (ρ/ρ300 K) versus temperature curves were obtained via dividing the measured resistivity (ρ) by the resistivity value (ρ300 K) at room temperature.

2 Results and discussion

The crystal structure of Pd0.17NbSe2 identified by single crystal X-ray diffraction method is shown in Fig. 1(a-b), where the gray, blue, orange spheres represent Pd, Nb, and Se atoms, respectively. The crystal data and structure refinement of Pd0.17NbSe2 are given in Table 1. The fractional atomic coordinates and equivalent isotropic displacement parameters are summarized in Table S1. The atomic displacement parameters and the geometric pa rameters are shown in Table S2-S3. The space group of Pd0.17NbSe2 is determined to be P63/mmc with lattice parameters of a=0.34611(2) nm, c=1.27004(11) nm. Pd0.17NbSe2 contains one independent Nb site (2b), one independent Se site (4f) and one independent Pd site (2a). Pd0.17NbSe2 consists of Nb-Se layer and Pd-Se layer, which are stacked alternately along c axis. Each Nb atom is coordinated by 6 Se atoms which formed a [NbSe6] triangular prism (Fig. 1(c)). The length of Nb-Se bond in [NbSe6] triangular prism is 0.26006(4) nm which is comparable to 0.25941(5) nm in NbSe2[17]. These [NbSe6] triangular prisms are connected by edge-sharing to form the Nb-Se layer.

Figure 1.Crystal structure of Pd_0.17NbSe₂ along (a) the bc-plane and (b) the ab-plane, (c) [NbSe₆] triangular prism in Pd_0.17NbSe₂, (d) [PdSe₆] octahedron in Pd_0.17NbSe₂

Atom	Wyck. Site	x	y	z	U_iso* or U_eq/nm²	Occ.
Nb	2b	0	1	1/4	0.000075(2)	1
Se	4f	1/3	2/3	0.38104(5)	0.0000736(17)	1
Pd	2a	0	1	1/2	0.000059(11)	0.171(3)

Table 1.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters of Pd_0.17NbSe₂

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Each Pd atom is coordinated by 6 Se atoms to form [PdSe 6] octahedron (Fig. 1 (d)). The average length of Pd-Se bond in [PdSe 6] octahedron is 0.25051(4) nm which is comparable to 0.248602(0) nm of PdSe 2 [18] . These [PdSe 6] octahedra partially fill in the vdW gaps of NbSe 2, where the occupation of Pd sites is 17%.

The Pd 0.17 NbSe 2 plate with the size about 5 μm was observed by SEM (Fig. 2 (a)). The Pd atoms are in a homogenous dispersion in Pd 0.17 NbSe 2, which is confirmed by the elemental mapping analysis of Pd 0.17 NbSe 2 . HRTEM image of Pd 0.17 NbSe 2 (Fig. 2 (b)) reveals that the lattice fringes with a spacing of 0.301 nm are assigned to (101) plane and (11¯1) plane between which the angle is 60°. This result is also verified by the corresponding SAED.

Figure 2.(a) SEM images of Pd_0.17NbSe₂ and the corresponding elemental mapping analysis, and (b) HRTEM image of Pd_0.17NbSe₂ along [101¯] zone axis with inset showing the corresponding SAED pattern

XPS data was obtained to confirm the valence state variation of the elements in Pd 0.17 NbSe 2 . As displayed in Fig. 3 (a), the Pd 3d region is the only difference between Pd 0.17 NbSe 2 and NbSe 2 . The Pd 3d region of Pd 0.17 NbSe 2 shows two peaks, which locate at the binding energy of 341.95 eV (3d 3/2) and 336.70 eV (3d 5/2) (Fig. 3 (b)). The valance state of Pd in Pd 0.17 NbSe 2 is identified as +2 according to these two peaks [14] . There are two peaks locating at 55.27 (Se 3d 3/2) and 54.50 eV (Se 3d 5/2) in the Se 3d region of Pd 0.17 NbSe 2, similar to those in the Se 3d region of NbSe 2 (Se 3d 3/2 at 55.25 eV and Se 3d 5/2 at 54.49 eV) (Fig. 3 (c)). Therefore, the valance state of Se in Pd 0.17 NbSe 2 is considered as -2. The Nb 3d region shows a mixture of oxidation states because of the slightly oxidation of the samples (Fig. 3 (d)) [19] . The peaks locating at 206.93 and 204.20 eV are attributed to the Nb-Se bonding in Pd 0.17 NbSe 2 . In comparison with these two peaks in pristine NbSe 2 (207.01 and 204.25 eV), there is a slight redshift in Pd 0.17 NbSe 2, implying the partial reduction of Nb as a result of Pd intercalation [14] .

Figure 3.XPS results of Pd_0.17NbSe₂ and NbSe₂(a) Survey spectra, (b) Pd 3d spectrum of Pd_0.17NbSe₂, (c) Se 3d spectrum, and (d) Nb 3d spectrum

The intercalated amounts of Pd in NbSe 2 could be variable, resulting in the formation of a series of Pd x NbSe 2 . The powder XRD patterns of Pd x NbSe 2 are displayed in Fig. 4 (a), with the pristine NbSe 2 as reference. The peaks of Pd 0.17 NbSe 2 are well matched to the simulated one obtained from single crystal data, which suggests a high degree of phase purity. The NbSe 2 still maintains its space group (P6 3 /mmc) after Pd intercalation. The (004) peak gradually shifts to a lower angle compared with 2H-NbSe 2 . Furthermore, the lattice parameter a undergoes a negligible change. In a sharp contrast, lattice parameter c increases remarkably because of Pd intercalation enlarging the interlayer space of NbSe 2 (Fig. 4 (b)).

Figure 4.(a) Powder XRD patterns of Pd_xNbSe₂ (x=0, 0.05, 0.10, 0.15, 0.17), (b) composition dependence of the lattice parameters a and c for Pd_xNbSe₂ (0≤x≤0.17)

The influence of Pd intercalation on the thermostability of the samples was investigated. As clearly seen in Fig. 5 (a), the weight of NbSe 2 begins to increase slightly at 559 K due to the formation of Nb 2 Se 4 O 13 [20] . Subsequently, TG curve of NbSe 2 suffers a dramatic decrease because of the complete oxidation of NbSe 2 to Nb 2 O 5 . However, the process of mass increase could not be found in Pd 0.17 NbSe 2, suggesting that the intercalated Pd enhances the thermostability of NbSe 2 with a higher oxidizing temperature. According to DTA curves (Fig. 5 (b)), the oxidizing temperature of Pd 0.17 NbSe 2 is 608 K, higher than NbSe 2 (544 K). The enhanced thermostability in air could stem from the intercalated Pd which stabilizes the crystal structure of NbSe 2 by connecting the adjacent Nb-Se layers [11, 21 - 22] .

Figure 5.(a) TG and (b) DTA curves of Pd_0.17NbSe₂ (blue) and NbSe₂ (red)

The electrical conductivity of Pd x NbSe 2 was measured by PPMS. The resistivity of Pd x NbSe 2 increases with the rising temperature (Fig. S1) exhibiting metallic behavior. Moreover, the residual resistivity ratio (RRR) [(resistivity at 300 K)/(resistivity just above T C)] for the Pd 0.17 NbSe 2 is ~1.09, extremely lower than NbSe 2 (~7.67) (Fig. 6 (a)).

Figure 6.(a) Temperature dependence of the RRR (ρ/ρ_{300 K}) for Pd_xNbSe₂ (0≤x≤0.17) with inset showing enlarged temperature regionof the superconducting transition, (b) composition dependence of T_C

The poor RRR in Pd 0.17 NbSe 2 indicates that the intercalated Pd may be an electronically disruptive dopant in NbSe 2, which is similar to the copper (Cu) in Cu x NbSe 2 and the gallium (Ga) in Ga x NbSe 2 [8, 23] . All of the Pd x NbSe 2 samples exhibit a sharp decrease at low temperature region from 8 K to 2K, indicating that the superconductivity occurs in these samples. Fig. 6 (b) shows that the T C decreases with a higher intercalated amount of Pd (7.4 K for NbSe 2 and 2.7 K for Pd 0.17 NbSe 2). Eventually, the zero resistivity cannot be observed at 2 K in Pd 0.17 NbSe 2 . Therefore, it declares that the intercalated Pd has a negative effect on the superconductivity in NbSe 2 . Similar phenomena are also found in Cu x NbSe 2, Ga x NbSe 2, Fe x NbSe 2 and Al x NbSe 2 [8, 23 - 24] . The reason for this might be that Pd intercalation disrupts the coherence of the CDW, and suppresses the pairing channel which contributes to the higher T C in NbSe 2 [8] .

3 Conclusions

In summary, we introduced noble metal Pd into the vdW gaps of NbSe 2, and synthesized a series of new intercalated compounds Pd x NbSe 2 . The Pd 0.17 NbSe 2 crystalizes in hexagonal structure with cell parameter a = 0.34611(2) nm, c =1.27004(11) nm. The intercalated Pd stabilizes the crystal structure of NbSe 2 by connecting the adjacent Nb-Se layers with [PdSe 6] octahedra leading to the enhanced thermostability in air. Pd x NbSe 2 remains the metallic character, which is verified by the resistivity measurements. In addition, the incorporation of Pd decreases the T C of NbSe 2, implying that Pd is negative for the superconductivity in NbSe 2 .

Supporting information:

	U₁₁/nm²	U₂₂/nm²	U₃₃/nm²	U₁₂/nm²
Nb	0.000074(3)	0.000074(3)	0.000075(4)	0.0000372(13)
S	0.000064(2)	0.000064(2)	0.000094(3)	0.0000318(10)
Pd	0.000062(13)	0.000062(13)	0.000052(19)	0.000031(7)

Table 2.

Atomic displacement parameters of Pd_0.17NbSe₂

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Bond	Distance/nm	Bond	Distance/nm
Nb1—Se2ⁱ	0.26006(4)	Se2—Pd3^viii	0.25051(4)
Nb1—Se2ⁱⁱ	0.26006(4)	Se2—Nb1^viii	0.26006(4)
Nb1—Se2ⁱⁱⁱ	0.26006(4)	Se2—Nb1^vii	0.26006(4)
Nb1—Se2^iv	0.26006(4)	Pd3—Se2^ix	0.25051(4)
Nb1—Se2	0.26006(4)	Pd3—Se2^iv	0.25051(4)
Nb1—Se2^v	0.26006(4)	Pd3—Se2^x	0.25051(4)
Nb1—Pd3^vi	0.31751(3)	Pd3—Se2ⁱ	0.25051(4)
Nb1—Pd3	0.31751(3)	Pd3—Se2^xi	0.25051(4)
Se2—Pd3^vii	0.25051(4)	Pd3—Nb1^xi	0.31751(3)
Se2—Pd3	0.25051(4)
Bond	Angle/(°)	Bond	Angle/(°)
Se2ⁱ—Nb1—Se2ⁱⁱ	134.813 (8)	Pd3^viii—Se2—Nb1	133.822 (3)
Se2ⁱ—Nb1—Se2ⁱⁱⁱ	79.58 (2)	Nb1^viii—Se2—Nb1	83.434 (16)
Se2ⁱⁱ—Nb1—Se2ⁱⁱⁱ	83.434 (16)	Pd3^vii—Se2—Nb1^vii	76.881 (6)
Se2ⁱ—Nb1—Se2^iv	83.434 (16)	Pd3—Se2—Nb1^vii	133.822 (3)
Se2ⁱⁱ—Nb1—Se2^iv	79.58 (2)	Pd3^viii—Se2—Nb1^vii	133.822 (3)
Se2ⁱⁱⁱ—Nb1—Se2^iv	134.813 (7)	Nb1^viii—Se2—Nb1^vii	83.434 (16)
Se2ⁱ—Nb1—Se2	83.433 (16)	Nb1—Se2—Nb1^vii	83.434 (16)
Se2ⁱⁱ—Nb1—Se2	134.812 (7)	Se2—Pd3—Se2^ix	92.612 (17)
Se2ⁱⁱⁱ—Nb1—Se2	134.812 (8)	Se2—Pd3—Se2^iv	87.388 (17)
Se2^iv—Nb1—Se2	83.433 (16)	Se2^ix—Pd3—Se2^iv	180.0
Se2ⁱ—Nb1—Se2^v	134.812 (8)	Se2—Pd3—Se2^x	92.612 (17)
Se2ⁱⁱ—Nb1—Se2^v	83.434 (16)	Se2^ix—Pd3—Se2^x	87.388 (17)
Se2ⁱⁱⁱ—Nb1—Se2^v	83.434 (16)	Se2^iv—Pd3—Se2^x	92.612 (17)
Bond	Angle/(°)	Bond	Angle/(°)
Se2^iv—Nb1—Se2^v	134.812 (8)	Se2—Pd3—Se2ⁱ	87.388 (17)
Se2—Nb1—Se2^v	79.58 (2)	Se2^ix—Pd3—Se2ⁱ	92.612 (17)
Se2ⁱ—Nb1—Pd3^vi	129.790 (11)	Se2^iv—Pd3—Se2ⁱ	87.388 (17)
Se2ⁱⁱ—Nb1—Pd3^vi	50.210 (11)	Se2^x—Pd3—Se2ⁱ	180.0
Se2ⁱⁱⁱ—Nb1—Pd3^vi	50.210 (11)	Se2—Pd3—Se2^xi	180.0
Se2^iv—Nb1—Pd3^vi	129.790 (11)	Se2^ix—Pd3—Se2^xi	87.388 (17)
Se2—Nb1—Pd3^vi	129.790 (11)	Se2^iv—Pd3—Se2^xi	92.612 (17)
Se2^v—Nb1—Pd3^vi	50.210 (11)	Se2^x—Pd3—Se2^xi	87.388 (17)
Se2ⁱ—Nb1—Pd3	50.210 (11)	Se2ⁱ—Pd3—Se2^xi	92.612 (17)
Se2ⁱⁱ—Nb1—Pd3	129.790 (11)	Se2—Pd3—Nb1	52.909 (12)
Se2ⁱⁱⁱ—Nb1—Pd3	129.790 (11)	Se2^ix—Pd3—Nb1	127.092 (12)
Se2^iv—Nb1—Pd3	50.210 (11)	Se2^iv—Pd3—Nb1	52.908 (12)
Se2—Nb1—Pd3	50.210 (11)	Se2^x—Pd3—Nb1	127.092 (12)
Se2^v—Nb1—Pd3	129.790 (11)	Se2ⁱ—Pd3—Nb1	52.908 (12)
Pd3^vi—Nb1—Pd3	180.0	Se2^xi—Pd3—Nb1	127.091 (12)
Pd3^vii—Se2—Pd3	87.388 (17)	Se2—Pd3—Nb1^xi	127.091 (12)
Pd3^vii—Se2—Pd3^viii	87.388 (17)	Se2^ix—Pd3—Nb1^xi	52.908 (12)
Pd3—Se2—Pd3^viii	87.388 (17)	Se2^iv—Pd3—Nb1^xi	127.092 (12)
Pd3^vii—Se2—Nb1^viii	133.822 (2)	Se2^x—Pd3—Nb1^xi	52.908 (12)
Pd3—Se2—Nb1^viii	133.822 (3)	Se2ⁱ—Pd3—Nb1^xi	127.092 (12)
Pd3^viii—Se2—Nb1^viii	76.881 (6)	Se2^xi—Pd3—Nb1^xi	52.909 (12)
Pd3^vii—Se2—Nb1	133.822 (2)	Nb1—Pd3—Nb1^xi	180.0
Pd3—Se2—Nb1	76.881 (6)

Table 3.

Geometric parameters for Pd_0.17NbSe₂

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Table Infomation Is Not Enable

Figure S1.Temperature dependence of the resistivity for Pd_xNbSe₂ (0≤x≤0.17)

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