As a class of silicide compounds, metal silicides, such as TaSi₂, TiSi₂, WSi₂, Mg₂Si, CrSi₂ and Ta₅Si₃, have been extensively investigated due to their low resistivity, superior thermal stability and excellent oxidation resistance^[1-4].These silicides have been employed as coating materials for energy and aerospace^[5-6], low resistance contacts and interconnects in large scale integrated circuits^[7], gate materials for CMOS microelectronic devices^[8], strain-sensitive materials for strain gauges^[5]and thermoelectric materials for stable transducers^[9].Generally, the structural stability and electrical transport properties of materials are more attractive for the semiconductor industry, materials science and physics.In addition to temperature, pressure is an alternative approach to studying structural stability and physicochemical properties of materials, as well as to synthesizing new compounds and exploring novel physical mechanisms^[10].Recently, Li et al.^[11] found that the hexagonal C40 structure of TaSi₂ is stable in a wide pressure range (0-50 GPa).Electronic devices made of metal silicides, like TaSi₂, may become available under stress or pressure if electrical transport properties permit.

The intrinsic electrical properties of metal silicides have long been studied at different temperatures.For TaSi₂, superconductivity has been found below the critical temperatures of 0.35 and 4.4 K for bulk single crystals and a thin-film on sapphire as substrate, respectively^[12-13].The electrical resistivity of the TaSi₂ single crystal along the 〈0001〉 crystallographic direction was less than along the 〈1010〉 direction in the range of 4.2-1100 K and the anisotropy of electrical resistivity at room temperature was almost 100%^[14].The current-voltage characteristics of the TaSi₂/n-Si junction were also investigated at different temperatures and the junction exhibited Schottky behavior^[15].However, the electrical transport properties of metal silicides under high pressure have rarely been reported.

In this work, we used TaSi₂ as an example metal silicide to study electrical stability at high pressure and room temperature.The structural stability of TaSi₂ powder was examined by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy under pressures up to 20 GPa.The electrical stability is reflected by the electrical resistances of the sample under different pressures, which were measured using an in situ high-pressure four-probe method.Finally, the electronic energy band structure of TaSi₂ at 0 GPa and high pressure was calculated to explain the experimental results.

1. Experimental Section

1.1 In situ High-Pressure Characterizations

A Mao-Bell type symmetric diamond anvil cell (DAC) with 400 μm-diameter anvil culets was used to generate pressure.T301 stainless steel with a thickness of 250 μm was pre-indented to a thickness of ~50 μm to serve as gaskets.A 150 μm diameter hole was laser drilled (λ=1 064 nm) at the center of the pre-indentation to serve as the sample chamber.Then, TaSi₂ powder (Alfa Aesar, 99.9%) was loaded into the chamber, and silicone oil was used as a pressure-transmitting medium.A ruby ball preplaced at the center of the culet was employed to determine the pressure by the ruby fluorescence method^[16].

The in situ high-pressure angle-dispersive X-ray diffraction (AD-XRD) experiments were performed at the BL15U1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF) using a wavelength of 0.061 99 nm.The sample to detector distance was 170.5 mm.The obtained two-dimensional image data were converted to one-dimensional XRD patterns by Fit2D-WAXD software^[17].Rietveld refinements were conducted using the General Structure Analysis System (GSAS) with the user interface EXPGUI package^[18-19].The in situ high-pressure Raman scattering spectra were collected by a Raman spectrometer with an excitation laser (λ=532 nm, Renishaw 1000).

1.2 In situ High-Pressure Resistance Measurements

The pressure-dependent electrical resistance was measured using a four-probe method in a symmetric DAC with 500 μm-culet sized anvils.To create an insulated environment, a ~500 μm diameter hole was drilled in the pre-indented gasket, then the wall of the hole and indentation were fully coated with c-BN and the other gasket surface was covered with epoxy (AB gel).After that, four platinum electrodes (25 μm in thickness) were arranged on one side of the gasket to connect the copper leads and the sample in the chamber.There was no pressure transmitting medium inside the chamber.The resistance measurements were carried out on the as-fabricated microcircuit using electronic equipments (Keithley 6221 current source, 2182A nanovoltmeter, and 7001 switch system), and the pressure was determined by the ruby fluorescence.The resistivity was calculated according to the Van der Pauw method^[20].

1.3 Theoretical Calculations of the Band Structure

The first-principle calculations of the electronic structure for hexagonal TaSi₂ under pressures of 0 and 15 GPa were performed using the CASTEP code^[21] in the Materials Studio package with the geometry optimization.The convergence tolerance in the geometry optimization was 2.0×10^-5 eV/atom.The optimization was completed when the forces were less than 0.5 eV/nm and all stress components were less than 0.1 GPa.At each pressure, a generalized gradient approximation of the Perdew-Burke-Ernzerhof (GGA-PBE) version functional of the exchange-correlation was adopted to optimize the lattice parameters for hexagonal C40 which takes the actual situation such as electron correlations into consideration.

2. Results and Discussion

2.1 Structural Stability

Here, we studied a hexagonal structure of TaSi₂ sample.The X-ray diffraction patterns of the sample under compression and decompression are shown in Fig. 1(b).Upon compression to 20.1 GPa all the Bragg peaks shifted towards high angles, revealing the shrinkage of the TaSi₂ unit cell.There was no significant variation in the XRD patterns in the number and shape of the diffraction peaks.The Bragg peaks reverted to their previous positions after decompression to ambient pressure.Fig. 1(c) shows a typical GSAS refinement of the XRD pattern at 1.0 GPa.The detailed crystallographic information for low and high-pressure phases of TaSi₂ was shown in Table 1.

Figure  1.  (a) Crystal structure of TaSi₂ in ambient conditions; (b) Synchrotron XRD patterns of TaSi₂ during compression and decompression; (c) Refinement of TaSi₂ XRD data at 1.0 GPa

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Table  1.  Rietveld refinement results of TaSi₂ under low pressure and high pressure

Pressure/GPa	Atom type	Fractional coordinates
1	Ta	(0.5, 0, 0)
1	Si	(0.16 148 66, 0.32 296 3, 0)
20	Ta	(0.5, 0.32 296 30, 0)
20	Si	(0.17 069 90, 0.34 138 9, 0)

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The good refinements confirm that the hexagonal structure of TaSi₂ is very stable without phase transitions during compression up to 20 GPa and decompression to ambient conditions.These results are consistent with a previous study^[11].From the refinements of all the diffraction patterns, the compressive behavior of the TaSi₂ lattice cell can be obtained.Fig. 2(a) shows the pressure dependence of the TaSi₂ lattice parameters and volume.We found that the a and c lattice parameters and the unit cell volume V decreased with increasing pressure.The normalized lattice parameters as a function of pressure shown in Fig. 2(b) reveal that the a axis was more compressible than the c axis and anisotropic compression increased with increasing pressure.This anisotropy of TaSi₂ under compression can be attributed to the relatively compact stacking along the 〈0001〉 direction^[11](Fig. 1(a)).Through third-order Birch-Murnaghan equation of state (EOS) fitting we can get the bulk modulus of hexagonal phase of TaSi₂ as 203(2) GPa, as shown in Fig. 2(c).In addition, the bulk modulus of TaSi₂ is too big to be easily compressed.

Figure  2.  (a) Pressure-dependent lattice parameters of TaSi₂(a₀=0.478 4 nm, c₀=0.657 0 nm); (b) Evolution of the normalized lattice parameters and volume with pressure for TaSi₂; (c) Pressure-dependent unit cell volume of TaSi₂

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Highly sensitive Raman spectroscopy was employed to obtain local structure information.Fig. 3(a) shows the Raman spectra of the sample collected during pressurization from ambient to high pressure and release to ambient pressure.From the Raman spectrum at ambient pressure, four Raman peaks can be observed at the centered frequencies of 145.0, 290.0, 355.0 and 503.8 cm^-1, denoted as A₁, A₂, A₃ and A₄, respectively.The peak A₄ could be related to the Si─Si vibration mode.During the compression-decompression cycle, these Raman modes can almost be clearly observed.Upon decompression to ambient pressure, the Raman modes A₁, A₂ and A₃ were found to revert to their original frequencies, but the Si─Si vibration mode A₄ was hysteretic.The vibrational frequencies of two obvious Raman peaks (A₃ and A₄) as a function of pressure are shown in Fig. 3(b) and exhibit blue shift under compression due to the shortening of the Si─Ta─Si and Si─Si bond.These findings indicate the local structural stability of the TaSi₂ particles under pressure, which corresponds to the structural stability analysis from XRD results.

Figure  3.  (a) Pressure-dependent Raman spectra of TaSi₂ at room temperature; (b) Pressure-dependent Raman peaks (A₃ and A₄) of TaSi₂ derived from the Raman spectra

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2.2 Electrical Stability

In general, decreasing the distance between the atoms and the interlayers of a crystal material under external pressure or stress can alter its electronic behavior^[22].Sometimes, this effect may adversely affect the stability of electronic devices under stress.Here, the electronic transport properties of TaSi₂ under different pressures are expressed by the electrical resistance, which was measured by a four-probe method using a fabricated microcircuit in a DAC (Fig. 4 inset).As shown in Fig. 4, the electrical resistance of the sample decreased dramatically with pressure increasing up to 5 GPa due to the gradually closer contact between the TaSi₂ particles.In the high-pressure region (5.0-16.3 GPa), the resistance trend was steady with increasing pressure and the resistance reduced by less than half.The resistance during decompression and compression was almost consistent in the high-pressure region.Therefore, the electrical transport properties of TaSi₂ are very stable in the high-pressure region during compression and decompression.

Figure  4.  The resistivity of TaSi₂ under pressure at room temperature (The inset (upper right) is a photograph of the four-probe microcircuit in the diamond anvil cell.)

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A previous study reported that the resistivity of a TaSi₂ single crystal at ambient pressure and temperature was approximately 20 μΩ·cm along the 〈0001〉 crystallographic direction and 40 μΩ·cm along the $\left\langle {10\bar 10} \right\rangle$ direction^[23].Hence, TaSi₂ exhibits metallic behavior.In our case, the pressure-dependent resistivity of the sample consisted of many particles, as shown in the Fig. 4 inset.Thus, the resistivity under pressures of 5.0 and 16.3 GPa was about 2.8 and 1.7 μΩ·cm, respectively.That is to say, the resistivity of TaSi₂ decreased by one order of magnitude at room temperature from ambient pressure to 5.0 GPa.The metallicity of TaSi₂ obviously increased with the increase of applied pressure.Therefore, it is conceivable that electronic devices made of TaSi₂ may work well under pressure and release less waste heat.

To further understand TaSi₂ electrical stability and illuminate the underlying physical mechanism of its resistivity-pressure relationship, its band structures under ambient pressure and high pressure were calculated by first-principle calculations.Fig. 5 shows the electronic band structure of TaSi₂ at ambient pressure and 15 GPa.Their topological geometries are considerably similar, i.e., the electronic energy band structure is very stable under high pressure.The difference is that the electronic energy band broadens under high pressure compared to that under ambient pressure, which is due to shortening of the lattice parameters. Moreover, the Fermi surface of TaSi₂ under ambient pressure and 15 GPa locates below the top of the valence band and the valence-band maximum crosses the conduction-band minimum, i.e., the band gap disappears.This demonstrates that TaSi₂ shows metallic behavior, which can contribute to the low-resistivity of TaSi₂ under ambient and high pressure.

Figure  5.  Calculated band structure of TaSi₂ at (a) 0 GPa and (b) 15 GPa

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3. Conclusions

In summary, we studied the crystallographic structural stability and electrical transport properties of metallic TaSi₂ under high pressure using angle-dispersive synchrotron XRD, Raman spectroscopy, and four-probe resistance measurements as well as first-principle calculations.The in situ high-pressure XRD and Raman characterizations demonstrated that the structure was stable up to 20 GPa, consistent with a previously reported result.The resistivity of TaSi₂ was steady at ~2 μΩ·cm under pressures from ambient pressure to 16.3 GPa.First-principle calculations showed that the topological geometries of the TaSi₂ electronic structure under ambient and high pressure were similar and their valence-band maximums were located over the Fermi surface, which was responsible for its electrical stability and metallic behavior.