
- Journal of Inorganic Materials
- Vol. 35, Issue 4, 505 (2020)
Abstract
Layered transition metal dichalcogenides (TMDs) with the general chemical formula MX2 (M represents the transition metal and X is the chalcogen) have been widely studied due to their unique physicochemical properties and diverse applications[
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[
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 PdxNbSe2
PdxNbSe2 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 Pd0.17NbSe2 was obtained by keeping Pd0.17NbSe2 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[
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[
Figure 1.Crystal structure of Pd0.17NbSe2 along (a) the
Atom | Wyck. Site | |||||
---|---|---|---|---|---|---|
Nb | 0 | 1 | 1/4 | 0.000075(2) | 1 | |
Se | 1/3 | 2/3 | 0.38104(5) | 0.0000736(17) | 1 | |
Pd | 0 | 1 | 1/2 | 0.000059(11) | 0.171(3) |
Table 1.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters of Pd0.17NbSe2
Each Pd atom is coordinated by 6 Se atoms to form [PdSe6] octahedron (Fig. 1(d)). The average length of Pd-Se bond in [PdSe6] octahedron is 0.25051(4) nm which is comparable to 0.248602(0) nm of PdSe2[
The Pd0.17NbSe2 plate with the size about 5 μm was observed by SEM (Fig. 2(a)). The Pd atoms are in a homogenous dispersion in Pd0.17NbSe2, which is confirmed by the elemental mapping analysis of Pd0.17NbSe2. HRTEM image of Pd0.17NbSe2 (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 Pd0.17NbSe2 and the corresponding elemental mapping analysis, and (b) HRTEM image of Pd0.17NbSe2 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 Pd0.17NbSe2. As displayed in Fig. 3(a), the Pd 3d region is the only difference between Pd0.17NbSe2 and NbSe2. The Pd 3d region of Pd0.17NbSe2 shows two peaks, which locate at the binding energy of 341.95 eV (3d3/2) and 336.70 eV (3d5/2) (Fig. 3(b)). The valance state of Pd in Pd0.17NbSe2 is identified as +2 according to these two peaks[
Figure 3.XPS results of Pd0.17NbSe2 and NbSe2(a) Survey spectra, (b) Pd 3d spectrum of Pd0.17NbSe2, (c) Se 3d spectrum, and (d) Nb 3d spectrum
The intercalated amounts of Pd in NbSe2 could be variable, resulting in the formation of a series of PdxNbSe2. The powder XRD patterns of PdxNbSe2 are displayed in Fig. 4(a), with the pristine NbSe2 as reference. The peaks of Pd0.17NbSe2 are well matched to the simulated one obtained from single crystal data, which suggests a high degree of phase purity. The NbSe2 still maintains its space group (P63/mmc) after Pd intercalation. The (004) peak gradually shifts to a lower angle compared with 2H-NbSe2. 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 NbSe2 (Fig. 4(b)).
Figure 4.(a) Powder XRD patterns of Pd
The influence of Pd intercalation on the thermostability of the samples was investigated. As clearly seen in Fig. 5(a), the weight of NbSe2 begins to increase slightly at 559 K due to the formation of Nb2Se4O13[
Figure 5.(a) TG and (b) DTA curves of Pd0.17NbSe2 (blue) and NbSe2 (red)
The electrical conductivity of PdxNbSe2 was measured by PPMS. The resistivity of PdxNbSe2 increases with the rising temperature (Fig. S1) exhibiting metallic behavior. Moreover, the residual resistivity ratio (RRR) [(resistivity at 300 K)/(resistivity just above TC)] for the Pd0.17NbSe2 is ~1.09, extremely lower than NbSe2 (~7.67) (Fig. 6(a)).
Figure 6.(a) Temperature dependence of the RRR (
The poor RRR in Pd0.17NbSe2 indicates that the intercalated Pd may be an electronically disruptive dopant in NbSe2, which is similar to the copper (Cu) in CuxNbSe2 and the gallium (Ga) in GaxNbSe2[
3 Conclusions
In summary, we introduced noble metal Pd into the vdW gaps of NbSe2, and synthesized a series of new intercalated compounds PdxNbSe2. The Pd0.17NbSe2 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 NbSe2 by connecting the adjacent Nb-Se layers with [PdSe6] octahedra leading to the enhanced thermostability in air. PdxNbSe2 remains the metallic character, which is verified by the resistivity measurements. In addition, the incorporation of Pd decreases the TC of NbSe2, implying that Pd is negative for the superconductivity in NbSe2.
Supporting information:
Nb | 0.000074(3) | 0.000074(3) | 0.000075(4) | 0.0000372(13) | 0 | 0 |
S | 0.000064(2) | 0.000064(2) | 0.000094(3) | 0.0000318(10) | 0 | 0 |
Pd | 0.000062(13) | 0.000062(13) | 0.000052(19) | 0.000031(7) | 0 | 0 |
Table 2.
Atomic displacement parameters of Pd0.17NbSe2
Bond | Distance/nm | Bond | Distance/nm |
---|---|---|---|
Nb1—Se2i | 0.26006(4) | Se2—Pd3viii | 0.25051(4) |
Nb1—Se2ii | 0.26006(4) | Se2—Nb1viii | 0.26006(4) |
Nb1—Se2iii | 0.26006(4) | Se2—Nb1vii | 0.26006(4) |
Nb1—Se2iv | 0.26006(4) | Pd3—Se2ix | 0.25051(4) |
Nb1—Se2 | 0.26006(4) | Pd3—Se2iv | 0.25051(4) |
Nb1—Se2v | 0.26006(4) | Pd3—Se2x | 0.25051(4) |
Nb1—Pd3vi | 0.31751(3) | Pd3—Se2i | 0.25051(4) |
Nb1—Pd3 | 0.31751(3) | Pd3—Se2xi | 0.25051(4) |
Se2—Pd3vii | 0.25051(4) | Pd3—Nb1xi | 0.31751(3) |
Se2—Pd3 | 0.25051(4) | ||
Bond | Angle/(°) | Bond | Angle/(°) |
Se2i—Nb1—Se2ii | 134.813 (8) | Pd3viii—Se2—Nb1 | 133.822 (3) |
Se2i—Nb1—Se2iii | 79.58 (2) | Nb1viii—Se2—Nb1 | 83.434 (16) |
Se2ii—Nb1—Se2iii | 83.434 (16) | Pd3vii—Se2—Nb1vii | 76.881 (6) |
Se2i—Nb1—Se2iv | 83.434 (16) | Pd3—Se2—Nb1vii | 133.822 (3) |
Se2ii—Nb1—Se2iv | 79.58 (2) | Pd3viii—Se2—Nb1vii | 133.822 (3) |
Se2iii—Nb1—Se2iv | 134.813 (7) | Nb1viii—Se2—Nb1vii | 83.434 (16) |
Se2i—Nb1—Se2 | 83.433 (16) | Nb1—Se2—Nb1vii | 83.434 (16) |
Se2ii—Nb1—Se2 | 134.812 (7) | Se2—Pd3—Se2ix | 92.612 (17) |
Se2iii—Nb1—Se2 | 134.812 (8) | Se2—Pd3—Se2iv | 87.388 (17) |
Se2iv—Nb1—Se2 | 83.433 (16) | Se2ix—Pd3—Se2iv | 180.0 |
Se2i—Nb1—Se2v | 134.812 (8) | Se2—Pd3—Se2x | 92.612 (17) |
Se2ii—Nb1—Se2v | 83.434 (16) | Se2ix—Pd3—Se2x | 87.388 (17) |
Se2iii—Nb1—Se2v | 83.434 (16) | Se2iv—Pd3—Se2x | 92.612 (17) |
Bond | Angle/(°) | Bond | Angle/(°) |
Se2iv—Nb1—Se2v | 134.812 (8) | Se2—Pd3—Se2i | 87.388 (17) |
Se2—Nb1—Se2v | 79.58 (2) | Se2ix—Pd3—Se2i | 92.612 (17) |
Se2i—Nb1—Pd3vi | 129.790 (11) | Se2iv—Pd3—Se2i | 87.388 (17) |
Se2ii—Nb1—Pd3vi | 50.210 (11) | Se2x—Pd3—Se2i | 180.0 |
Se2iii—Nb1—Pd3vi | 50.210 (11) | Se2—Pd3—Se2xi | 180.0 |
Se2iv—Nb1—Pd3vi | 129.790 (11) | Se2ix—Pd3—Se2xi | 87.388 (17) |
Se2—Nb1—Pd3vi | 129.790 (11) | Se2iv—Pd3—Se2xi | 92.612 (17) |
Se2v—Nb1—Pd3vi | 50.210 (11) | Se2x—Pd3—Se2xi | 87.388 (17) |
Se2i—Nb1—Pd3 | 50.210 (11) | Se2i—Pd3—Se2xi | 92.612 (17) |
Se2ii—Nb1—Pd3 | 129.790 (11) | Se2—Pd3—Nb1 | 52.909 (12) |
Se2iii—Nb1—Pd3 | 129.790 (11) | Se2ix—Pd3—Nb1 | 127.092 (12) |
Se2iv—Nb1—Pd3 | 50.210 (11) | Se2iv—Pd3—Nb1 | 52.908 (12) |
Se2—Nb1—Pd3 | 50.210 (11) | Se2x—Pd3—Nb1 | 127.092 (12) |
Se2v—Nb1—Pd3 | 129.790 (11) | Se2i—Pd3—Nb1 | 52.908 (12) |
Pd3vi—Nb1—Pd3 | 180.0 | Se2xi—Pd3—Nb1 | 127.091 (12) |
Pd3vii—Se2—Pd3 | 87.388 (17) | Se2—Pd3—Nb1xi | 127.091 (12) |
Pd3vii—Se2—Pd3viii | 87.388 (17) | Se2ix—Pd3—Nb1xi | 52.908 (12) |
Pd3—Se2—Pd3viii | 87.388 (17) | Se2iv—Pd3—Nb1xi | 127.092 (12) |
Pd3vii—Se2—Nb1viii | 133.822 (2) | Se2x—Pd3—Nb1xi | 52.908 (12) |
Pd3—Se2—Nb1viii | 133.822 (3) | Se2i—Pd3—Nb1xi | 127.092 (12) |
Pd3viii—Se2—Nb1viii | 76.881 (6) | Se2xi—Pd3—Nb1xi | 52.909 (12) |
Pd3vii—Se2—Nb1 | 133.822 (2) | Nb1—Pd3—Nb1xi | 180.0 |
Pd3—Se2—Nb1 | 76.881 (6) |
Table 3.
Geometric parameters for Pd0.17NbSe2
Table Infomation Is Not EnableFigure S1.Temperature dependence of the resistivity for Pd
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