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Quasi-High-Pressure Effects in Transition-Metal-Rich Dichalcogenide, Hf3Te2 Ho Sung Yu,†,‡ Byungki Ryu,§ Soohyung Park,∥ Jongho Park,†,‡ Yeonjin Yi,∥ Kyu Hyoung Lee,⊥ Kimoon Lee,*,# and Sung Wng Kim*,† †

Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS), Suwon 16419, Republic of Korea § Thermoelectric Conversion Research Center, Korea Electrotechnology Research Institute (KERI), Changwon 51543, Republic of Korea ∥ Institute of Physics and Applied Physics, Yonsei University, Seoul, Republic of Korea ⊥ Department of Nano Applied Engineering, Kangwon National University, Chuncheon 24341, Republic of Korea # Department of Physics, Kunsan National University, Gunsan 54150, Republic of Korea ‡

S Supporting Information *

ABSTRACT: High pressure offers an intriguing avenue for the change in physical and chemical properties of condensed matters such as superconductivity, structural phase transition, and catalytic activity. However, it is hard to achieve the highpressure phases of elements and compounds in an ambient condition by chemical pressure due to the limitation in bonding length variation. Here, we report the quasi-highpressure state of Hf cuboid confined between two-dimensional chalcogen layers in transition-metal-rich dichalcogenide of Hf3Te2, exhibiting the enhanced degree of localization and chemical potential for valence electrons compared to ambient Hf elements. The structural analysis reveals that Hf metals in Hf3 Te 2 form a local cuboid structure, in which the coordination for central Hf is identical to that of high-pressure body-centered-cubic Hf phase. Density functional theory calculations verify that the compressed cuboid in Hf3Te2 has a key role for an abnormal heat capacity beyond Dulong−Petit limit and reduced work function, resulting from the intrinsic pressure induced s−d transfer in covalent electrons between Hf−Hf bonding. This transition-metal-rich dichalcogenide can guide a route to realize a high-pressure element under ambient condition and offer an opportunity to explore a new layered two-dimensional material.

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INTRODUCTION High pressure has a significant effect on the physical and chemical properties of condensed matters due to the change of interatomic distance accompanying the substantial change of structural and quantum phases.1,2 It is commonly believed that as the pressure increases, the crystal becomes denser and the symmetry becomes higher to be more closely packed structure with increasing number of neighboring atoms,3 whereas several elements undergo low-symmetry structure at high pressure.4 In the case of lithium, it transforms from metallic of highsymmetry structure through superconducting of low-symmetry structure at 30 GPa and finally to insulating state at extremely high pressure over 100 GPa as verified by theoretical calculations.5 Although the first-principles approach can predict structures and properties of thermodynamically stable phases up to multi-terapascal pressure, experiments are generally executed under physical pressure that can be accessed up to ∼300 GPa constrained by current technology and only provides in situ characterizations.6 © XXXX American Chemical Society

In contrast to techniques applying physical pressure that include the static pressure using diamond anvil cell (DAC)7 and dynamic pressure using shock waves,8 the chemical pressure is attained by controlling bonding length via substitutional doping or alloying,9,10 allowing various ex situ characterizations at ambient conditions. However, the chemical pressure is not effective to transform the crystal structure into a high-pressure phase due to the solubility limit of foreign atoms in a defined crystal structure.11 Meanwhile, confinement effect induced from the reduced dimensionality can be another route to apply the chemical pressure comparable to physical one associated with structural transformations occurring only at very high pressures.12−14 Two-dimensional (2D) layered structure provides a good platform from these aspects. In particular, transition metal Received: July 12, 2017 Revised: October 17, 2017

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DOI: 10.1021/acs.jpcc.7b06851 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Structural characterizations of Hf3Te2. (a) X-ray diffraction pattern measured by Cu Kα radiation for Hf3Te2 polycrystalline and single crystal. Upper and middle panels show the out-of-plane 2θ-scan for polycrystalline powder and oriented single crystal, respectively. The result from in-plane 2ϕ-scan for single crystal is displayed in the lower panel. Illustrations of crystal structure for (b) Hf3Te2, (c) hcp-structured Hf at ambient pressure (AP), and (d) bcc-structured Hf at high pressure (HP) conditions. Local structure of Hf-based cuboid unit is highlighted by the red area.

Starting materials were prepared in an Ar-filled glovebox. To prevent the tellurium evaporation during the arc-melting process, we melted the mixture under high-purity argon atmosphere (Ar, 99.999%). To obtain a homogeneous sample, we turned the ingot over several times and repeated the melting process. From the arc-melting and solidification process, large grain (few millimeters) single crystal sample was grown with a 1 mm thickness on the top of the polycrystalline Hf3Te2 ingot. To remove the orientation and get a homogeneous stoichiometric polycrystalline sample, we pulverized the ingot down to 45 μm. Then the spark plasma sintering (SPS) process was conducted to get a high-quality polycrystalline Hf3Te2. All samples were handled and stored in the Ar-filled glovebox to prevent the surface oxidation. Electrical Measurements. Electrical properties of large grain single-crystal Hf3Te2 were examined by using the stamp method.24 The sample was cleaved by 3M Scotch tape and pressed onto the lithographically patterned Ti/Au electrodes on a Si/SiO2 wafer. Polycrystalline Hf3Te2 was polished up to 400 μm thickness to improve the signal-to-noise ratio. Then we made a Hall bar pattern onto the polycrystalline Hf3Te2 by an Au ion coater. Silver paste was employed to contact between the sample and external wire. Electrical and thermal measurement for both single crystal and polycrystalline sample were performed in the physical property measurement system (PPMS, Quantum Design). Since the surface of the Hf3Te2 was sensitive to air, all handling of sample preparations including polishing and contacts was performed in an Ar-filled glovebox. Photoelectron Measurements. UPS measurements were performed with a PHOIBOS 150 electron spectrometer (SPECS) at room temperature. The He I (hυ = 21.22 eV) discharge lamp was used as a photon source. The spectrometer was calibrated with respect to the Fermi edge of a clean Au sample, and the total system broadening (the half-width of Fermi step) was 90 meV. A sample bias of −10 V was applied to obtain the true cutoff of the secondary electron emission. The Shirley-type background was removed from the valence region to show the spectral features clearly. The electronic structure of Hf3Te2 was measured after vacuum cleaving in the

dichalcogenides (TMDs) with TmCh2 combination (Tm: transition metal; Ch: chalcogen) have been intensively studied because they show the various exotic ground states including superconductivity15,16 or Weyl semimetallicity17 associated with diverse structural polymorphs. In most 2D Tm−Ch binary systems, Tm coordinates with six Ch elements having trigonal prismatic or octahedral substructure.18 The distances between each Tm element in these coordination are typically 15−25% greater than the bond lengths reported in elemental transition metal solids. Because such a long Tm−Tm distance limits the energetic and spatial overlap of the d orbitals of Tm elements, it is regarded that there are many possibilities to reach the unrevealed states which are often predicted and explored by extremely high pressure.19 On the other hands, Tm-rich chalcogenide with layered structure is prominent due to its exceptional coordination in Tm−Tm elements.20 The coordination number (CN) and short bond length in the Tm-rich system is rather comparable to those from the elemental Tm in extreme conditions;21 hence, it intrigues us that a layer structured Tm metal-rich chalcogenide may induce distinct properties expected in the high-pressure phase of Tm elements. Herein, we report that the layered Tm-rich chalcogenide of Hf3Te2 showing quasi-high-pressure properties for Hf element in an ambient condition. Through the structural and electrical characterizations, it is revealed that Hf3Te2 exhibits metallic conductivity with a high electron concentration of 1.5 × 1022 cm−3, mainly due to the Hf cuboid compressed by Te layers. Enhanced Sommerfeld coefficient with exceeded heat capacity beyond the Dulong−Petit limit and lowered work-function value of 3.7 eV than those of elemental Hf support the increment in both electronic density of states and chemical potential of Hf in Hf3Te2 relative to Hf metal at ambient pressure. Density functional theory (DFT) calculation reveals that an electron transfer from 6s to 5d occurs as it results in the shifts of band energy to higher level, while charges are spatially localized between Hf−Hf bonding as predicted from the pressurized Hf elements.

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EXPERIMENTAL SECTION Synthesis. The Hf3Te2 compound was prepared by the arcmelting process with stoichiometric mixtures of Hf and HfTe2. B

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The Journal of Physical Chemistry C ultrahigh-vacuum chamber (3.0 × 10−10 Torr) to prevent unwanted contamination and oxidation on the surface. Electronic Structure Calculations. First-principles electronic structure calculations were performed within DFT. We use the plane-wave basis set, the projector augmented wave (PAW) pseudopotential, and the generalized gradient approximation (GGA) exchange correlation energy functional parametrized by Perdew−Burke−Ernzerhof (PBE), which are implemented in the Vienna Ab initio Simulation Program (VASP). The vacuum levels of Hf3Te2 and Hf are derived from the surface calculations and the reference potential method.

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RESULTS AND DISCUSSION Figure 1a shows X-ray diffraction (XRD) patterns for polycrystalline powder and single crystalline flake of Hf3Te2. From the analysis of the Rietveld refinement of the powder XRD pattern, it is revealed that Hf metals form a cuboid structure between Te atomic layers with high c/a ratio of 4.85 and large interlayer distance of 3.785 Å (Detailed structural parameters are shown in Table S1 of the Supporting Information, ref 16.) As illustrated in Figure 1b, the Hf3Te2 is layered structure consisting of Hf cuboid (region marked by the red box) sandwiched between two Te atomic layers as agreed well with previous results.20 The characteristic feature of tetragonal layered structure is demonstrated by the exclusive reflection peaks from (00l) planes for out-of-plane (middle panel) and periodic reflections with 4-fold symmetry for azimuthal in-plane ϕ-scans (bottom panel) observed for the single crystalline sample. It indicates that the crystal surface is well oriented to the (002) plane where the van der Waals gap is located. It is noted that the coordination of Tm (Hf) in Hf3Te2 is different from that of typical TMDs with TmCh2 composition. In Hf3Te2, two types of coordination for Hf are observed while Tm coordinates with six Ch elements in TmCh2: one bound only to Hf and the other bound to both Hf and Te as denoted as Hf(1) and Hf(2) in Figure 1b, respectively. The most noticeable feature is an exceptional CN of 8 for Hf(1) with shorter nearest-neighbor distance (3.1226 Å) than that of elemental Hf (3.1325 Å) with hexagonal close-packed structure (hcp-Hf) as shown in Figure 1c. Abdon et al. also noted such an exceptional bonding nature between Hf(1) and Hf(2) confirmed by structural refinement from single crystalline Hf3Te2 and crystal orbital overlap population (COOP) calculation.20 Because the CN of 8 is only observed from body-centered-cubic structured Hf (bcc-Hf, cuboid region marked by the red box in Figure 1d) that has been attained under high pressure of ∼65 GPa,21 it is expected that the physical and chemical properties of Hf3Te2 might be rather comparable with those from compressed bcc-Hf than hcp-Hf at ambient conditions. Further, the Hf metal cuboid in Hf3Te2 is not three-dimensionally coordinated but confined between 2D Te atomic layers, resulting in a slightly rectangular form with ∼7% longer bonding length in ab-plane under compression along c-axis. The electrical properties of the Hf3Te2 were examined by measuring the temperature (T) dependence of resistivity (ρ), carrier density (ne), and Hall mobility (μH) for polycrystalline pellet and single crystalline flake as displayed in Figure 2. Both Hf3Te2 samples exhibit positive T dependence of ρ, verifying metallic transport nature with quadratic T dependence up to 20 K (Figure 2a), which implies that the electron−phonon interaction is much stronger than the electron−electron

Figure 2. Electron transport properties of Hf3Te2. Temperature (T) dependence of (a) resistivity (ρ) and (b) carrier concentration (ne). The inset in (a) illustrates the schematic of sample preparation to measure the in-plane electrical properties of single crystalline Hf3Te2. Hall mobilities (μH) for a polycrystalline sample and single crystal are shown in the inset of (b).

interaction except for the low-T region.22 From the Hall-effect measurement, it is revealed that the majority carriers of both samples are electrons, and the ne values are similar as ∼1.5 × 1022 cm−3 at 300 K, indicating that the large ne value of Hf3Te2 is the intrinsic nature of the crystal22,23 (Figure 2b). The obtained ne value is much higher than that from half-metallic layer structured HfTe2 compound (∼1021 cm−3 at 300 K). Furthermore, the T dependence of μH for both samples shows a typical behavior of 2D materials (the inset of Figure 2b), which shows a constant value at the low-temperature region,24 while the in-plane μH value for Hf3Te2 single crystal is relatively low compared to that (∼30 cm−3 V−1 s−1 at 300 K) of layer structured HfTe2.25 This suggests that the electronic transport properties for Hf3Te2 are mainly dominated from the contribution of Hf cuboid structure confined between atomic Te layers, which led to the increased density of states (DOS) around Fermi level (EF) associated with the reduced Hf−Hf distance with enhanced 5d-orbital overlapping.26 Thus, it is speculated that the conduction carriers are more localized around Hf cuboid of Hf3Te2 compared to Hf atomic layer of HfTe2, resulting in the relatively low μH value and low-T region for electron−electron interaction on the contrary to the typical 2D materials. Heat capacity (Cp) measurement verifies the localization nature of conduction carriers in Hf cuboid for Hf3Te2. The most noticeable feature is Cp value at high T region (T ≫ ΘD) where Cp exceeds beyond the classical Dulong−Petit limit (3NkB indicated by red dashed line where N is the number of atoms and kB is Boltzmann’s constant) as shown in Figure 3a. The abnormal excess Cp is a well-known characteristic feature of electropositive metals such as alkali and alkaline earth metals27 because the chemical potential (μ) of valence electrons that is relevant to the electron affinity of materials is closely related to the internal total energy of a system. From the linear relationship of CpT−1 versus T2 as shown in the inset of Figure 3a, the lattice (βT3) and electronic contribution (γT) to total Cp are calculated as confirming that the Hf3Te2 is the system C

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Figure 3. Thermodynamic and photoemission measurements for Hf3Te2. (a) Heat capacity (Cp) as a function of temperature for Hf3Te2. The red dashed line indicates the classical Dulong−Petit limit of 3NkB. The inset shows the CT−1 versus T2 plot to obtain Debye T and Sommerfeld coefficient (γ). (b) Secondary electron emission from fractured pellet surface of polycrystalline Hf3Te2 free from surface contamination. Work function value is determined from linear extrapolation as marked by red dashed line. The inset shows the valence band spectra with nonzero states at Fermi edge (EF). Combining with DFT results (see Figure S2), characteristic peaks for each orbital contribution are also labeled with arrows.

with itinerant electrons like a normal metal.22 The intercept and slope of the plot give the values of Sommerfeld coefficient (γ) and Debye T (ΘD) as 6.92 mJ mol−1 K−2 and 211 K, respectively. Because the γ value is directly related to DOS at EF[Nγ(0)] in γ = (1/3)π2k2BNγ(0), the γ value for electronic Cp of Hf3Te2 even larger than that of Hf element (2.16 mJ mol−1 K−2)28 suggests that the higher DOS around EF of Hf3Te2 originates from the enhanced localized states of Hf 5d-orbitals as forming a compressed Hf cuboid structure.29 Photoemission experiments demonstrate the effect of the enhanced localized states for Hf 5d-orbitals on electrical and thermal properties of Hf3Te2. As measured in the valence band spectrum (the inset of Figure 3b), the location of EF verifies the metallic nature as consistent with the electrical transport results (refer to Figure 1a). The spectrum shows the peaks of Hf 5d-states (indicated by arrows) mainly contribute the EF as confirmed by the DFT calculations (see Figure S1 in the Supporting Information). From the cutoff of secondary electron emission (Figure 3b), the work function value is determined as 3.7 eV, which is 0.2 eV lower than that of an ambient pressure hcp-Hf phase.30 This result strongly indicates that the enhanced μ of valence electrons from Hf 5d-orbitals for Hf3Te2 makes the system to be more electropositive than hcp-Hf metal.31 Finally, we performed the first-principles calculations based on DFT to understand the details of physical properties originating from Hf cuboid structure in Hf3Te2. Figure 4a shows the electronic band structure of Hf3Te2, exhibiting no energy gap, which is consistent with the metallic nature ensured from the electronic transport and photoemission measurements. From the partial charge density map near EF as shown in Figure 4b, it is verified that most electrons near EF are twodimensionally distributed in Hf cuboid between Te atomic layers. Markedly, the charge localization between Hf(1) and Hf(2) shows a distinct nonspherical behavior, indicating the increase of covalency in the bonding between Hf(1) and Hf(2) atoms.32 Since such a large degree of covalency is unusual character in the transition metals, but expected in the compressed system associated with a structural transition into

Figure 4. Electronic structures of Hf3Te2 and its related Hf phases. (a) Calculated band dispersion of Hf3Te2. (b) Partial charge density map of (100) surface (left) and (200) surface (right) for low-energy electrons near Fermi level (EF) within 0.5 eV. (c) Projected density of states (PDOS) for ambient pressure (AP)-Hf and high pressure (HP)Hf and central Hf element in Hf3Te2 compound. Decomposed (d) sorbital and (e) d-orbital composition from PDOS for Hf element. (f) Comparison of the band energy position relative to vacuum level for EF, center of d-band (Cd), and bottom of s-band (Bs). The inset schematic represents the evolution of electronic structure resulting from the s−d transfer. (g) d-orbital occupation for each Hf element up to EF. D

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high CN unit, it is worth to investigate the detailed electronic states of Hf(1) (the central Hf element in cuboid unit) in Hf3Te2 in comparison with ambient pressure hcp-Hf and high pressure bcc-Hf phases. To compare the electron configurations of Hf(1) of Hf3Te2 with those from other Hf phases, we plot the projected DOS (PDOS) for Hf atom as shown in Figures 4c−e. The total PDOS for Hf in Hf3Te2 exhibits more localized nature composed of discrete and narrow bandwidths than those for hcp-Hf (AP) and bcc-Hf (HP) with continuous and broad ones (Figure 4c). From the fact that the superior DOS value near EF for Hf3Te2 to that for hcp-Hf is attributed to d-band electrons rather than s-band ones, it is concluded that the s−d transfer occurs due to the formation of quasi-high-pressure Hf cuboid in Hf3Te2 as predicted in the high-pressure transition metals.4 The band energy calculation demonstrates the quasi-highpressure effect inducing s−d transfer in Hf atoms for Hf3Te2. As shown in Figure 4f, the bottom of Hf s-band (Bs) for Hf3Te2 significantly raises relative to those for hcp- and bcc-Hf. Accompanied by the increase in d-band DOS at EF, it demonstrates the electron transfer from s- to d-state in Hf3Te2 due to the repulsion force between Hf 6s orbitals.33 Since such an electron transfer inevitably causes the increase of μ which is equivalent to EF, it is concluded that the reduced work function for Hf3Te2 originates from the high-pressurized Hf cuboid confined between Te atomic layers. (Note that the difference between calculated EF for Hf3Te2 and hcp-Hf is 0.413 eV, which is similar to the experimental value of 0.2 eV as obtained from Figure 3b.) While Bs for Hf in Hf3Te2 is much higher than even that from high-pressurized bcc-Hf, the EF level cannot exceed EF of bcc-Hf. From the Bader charge analysis, Hf in Hf3Te2 has more positive charges (+0.64e, where e is an electron charge) compared to hcp- and bcc-Hf phases (Table S2). This implies that the charge transfer does not fully occurs in Hf atoms but also takes place from Hf to Te bands as suppressing the sufficient d-band energy and occupation in Hf3Te2 as depicted in Figures 4f and 4g, respectively. As a result, the degree of charge localization in Hf cuboid of Hf3Te2 can be regarded as the quasi-high-pressurized state between ambient pressure hcp-Hf and high pressure bcc-Hf.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b06851. Structural information on Hf3 Te 2; powder X-ray diffraction (PXRD) analysis; Rietveld refinement; total and projected DOS; Barder charge analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.L.). *E-mail: [email protected] (S.W.K.). ORCID

Yeonjin Yi: 0000-0003-4944-8319 Kimoon Lee: 0000-0002-5989-5633 Sung Wng Kim: 0000-0002-4802-5421 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Creative Materials Discovery Program (2015M3D1A1070639), by Basic Science Research Program (NRF-2016R1D1A3B03933785) through the National Research Foundation of Korea (NRF) funding, and by IBS-R011-D1. B. Ryu acknowledges the Korea Electrotechnology Research Institute (KERI) Primary research program through the National Research Council of Science & Technology (NST) funded by the Ministry of Science, ICT and Future Planning (MSIP) (No. 17-12-N0101-38, Development of design tools of thermoelectric and energy materials).

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CONCLUSIONS

In summary, we investigated the quasi-high-pressure effect in Tm-rich dichalcogenide, Hf3Te2, and examined the physical properties combined with DFT calculations. As constructing compressed 2D metallic cuboid between Te anions, the central Hf atom in Hf3Te2 undergoes into quasi-pressurized state, having an identical CN to high-pressure bcc-Hf. The enhanced γ with excess Cp beyond the Dulong−Petit limit as well as reduced work function confirm that valence electrons of Hf cuboid in Hf3Te2 are transferred into localized d-band as raising its own potential. Theoretical calculations strongly support that the localized carriers with high density at the upshifted EF result from the s−d electron transfer in central Hf atoms as expected in transition metals under high pressure. We anticipate that transition-metal-rich chalcogenides can intrigue a new route to reach the high-pressure phase by intrinsic chemical pressure at ambient condition and to understand rich physical and chemical properties of high pressure. E

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