Implementation of micro-ball nanodiamond anvils for high-pressure studies above 6 Mbar

Abstract

Since invention of the diamond anvil cell technique in the late 1950s for studying materials at extreme conditions, the maximum static pressure generated so far at room temperature was reported to be about 400 GPa. Here we show that use of micro-semi-balls made of nanodiamond as second-stage anvils in conventional diamond anvil cells drastically extends the achievable pressure range in static compression experiments to above 600 GPa. Micro-anvils (10-50 μm in diameter) of superhard nanodiamond (with a grain size below ∼50 nm) were synthesized in a large volume press using a newly developed technique. In our pilot experiments on rhenium and gold we have studied the equation of state of rhenium at pressures up to 640 GPa and demonstrated the feasibility and crucial necessity of the in situ ultra high-pressure measurements for accurate determination of material properties at extreme conditions.

Figure 1. The unit cell volume of nanocrystalline diamond (NCD) as a function of pressure.

An NCD micro-ball of ∼12 μm in diameter was compressed in a DAC in a Ne pressure transmitting medium (a schematic diagram is shown in insert in the upper right corner). Continues line is a result of fitting the experimental data (black dots; error bars are within the symbol size) up to 38 GPa with the third-order BM3 equation of state (K₃₀₀=489(5) GPa, K′=3.2(2), V₀=3.393(1) cm³ mol⁻¹). At pressures above 40 GPa the NCD seems to become more compressible, but the observed effect is simply a consequence of bridging of the micro-ball between the DAC's anvils and the development of the deviatoric stresses in it. Insert in the bottom left shows as synthesized NCD balls (10–50 μm in diameter) in a sodium chloride medium. The scale bar is 100 μm.

Figure 2. The pressure dependence of the unit cell volume of Re.

Red circles are experimental points as determined in Experiment no. 1 with the Au pressure standard; dark-red squares are experimental points as determined in Experiment no. 2 with pressure obtained using the BM4 EOS of Re established in Experiment no. 1). Continues blue line is a result of fitting experimental data using the BM4 EOS (K₃₀₀=342(6) GPa, K′=6.15(15), K″=−0.029(4), V₀=29.46(1) ų per unit cell), dashed green line is the BM3 EOS due to fitting the data collected up to 165 GPa (K₃₀₀=353(3) GPa, K′=5.80(7)). Insert in the bottom left shows an example of translucent NCD semi-balls synthesized in MgO medium and used as secondary anvils, and in the upper right is a schematic drawing of the double-stage DAC assembly. The scale bar is 10 μm. A typical diameter of NCD semi-balls is 12–20 μm, and the starting size of the sample is about 3–4 μm in diameter and about 3 μm in thickness.

Figure 3. The synchrotron powder X-ray diffraction data of a mixture of Re and Au.

(a) A part of the 2D X-ray diffraction image. (b) The GSAS plot. The data were collected at an X-ray wavelength of λ=0.3344 Å and at a pressure of 640(17) GPA. Red dots are the experimental data, the blue curve is the simulated diffraction data and the dark line is the residual difference. A mixture of Au and Re powders was compressed in a double-stage DAC using paraffin as a pressure transmitting medium. The lattice parameter of gold is a=3.303(3) Å, and those of rhenium are a=2.382(1) Å and c=3.798(2) Å. Pressure was determined according to the gold equation of state.