Synthesis and Microdiffraction at Extreme Pressures and Temperatures

1. Diamond Anvil Cell and Gasket Preparation

1. Select a pair of diamond anvils with conical design¹² and matching culet size. The conical anvil design is chosen for the wide angular x-ray windows it provides, allowing to collect relatively high resolution x-ray microdiffraction data. The culet (flat or beveled tip of a diamond anvil) is selected according to the maximum target pressure. The diameter of the flat part of the culet ranges from about 1-0.07 mm for target pressures from 10 to more than 200 GPa, respectively.
2. Position a diamond anvil in the matching conical housing of a tungsten carbide seat and place the two in a mounting jig. Make sure that the bottom cone of the diamond anvil is firmly sitting in the tungsten carbide seat by rotating the anvil with tweezers.
3. Apply load (few kg) on the diamond. No gap should remain between the diamond and the seat. Apply a small amount of glue around the anvil outer circumference.
4. Cure for 1 hr at 120 °C. Repeat the operation for the second diamond and seat.
5. Insert the pair of seats with glued diamonds in the two parts of a diamond anvil cell. Bring the diamond tips (culet) in close proximity (about 30 µm distance).
6. Using a stereoscopic microscope and inspecting the anvils sideways, align the flat tips of the diamond anvils (culets) laterally using the DAC lateral set screws (Figure 1a).
7. Gently, bring the diamond tips in contact. Use the interference fringes (Figure 1b), visible in axial stereoscopic view, to monitor the anvils relative tilt.
8. Rock the top anvil using the DAC top screws. Fringes becoming wider and "moving out of the culet" (Figure 1c) indicate that the diamonds relative tilt is decreasing. In axial view, adjust the horizontal alignment (Figure 1d) using the DAC lateral set screws. Aligned diamonds show no interference fringes and overlapping edges (Figure 1e). Clean the diamonds thoroughly using acetone.
9. Position a small metal foil, acting as gasket, between the tips of two aligned diamonds and attach the foil to the DAC body with waxy material. Suggested gasket materials include high yield strength metals such as rhenium, stainless steel or tungsten. Typical foil size is about 5 mm x 5 mm square with a thickness of 0.15-0.25 mm. The gasket should have fine texture and be free of cracks.
10. While monitoring the total thickness of the DAC with a micrometer, turn the DAC bolts evenly and in small steps producing an indentation of a thickness that varies with the culet size in the range of about 80-20 µm (Figure 2a).
11. Drill a hole in the center of the indentation of diameter between 1/2 and 1/3 of the culet size using a laser drilling system (Figure 2b) or an EDM machine¹³. The gasket-hole should be centered in the indentation and be perpendicular to it.
12. Carefully clean the gasket using a needle and ultrasonic bath for about 5 min in ethanol.
13. Clean the diamond tips carefully using sandpaper first and then acetone dipped wipes. Reposition the gasket firmly, using waxy material, on one anvil tip.

2. Sample and Gas Loading

1. The ruby fluorescence spectral shift^{14, 15, 16} and the gold compressibility¹⁷ are used as secondary pressure gauges. Using a fine tip tungsten needle, handpick one or two ruby spheres (~10 µm in diameter) and place them on the walls of the gasket-hole, repeat the operation for a small amount of fine gold powder (Figure 2c). The pressure standards positioned on the gasket should not come in contact with the sample to avoid unwanted reactions and parasitic scattering effects. Perform all sample loading operations without the aid of bonding materials, small grains adhere weakly to the needle, the gasket or the diamond by electrostatic forces.
2. Using an electronic balance, weigh the pure reagents, mix thoroughly appropriate proportions using a mortar.
3. Place a small amount of the mixture between the tips of two aligned diamonds of a second DAC. Press the sample squeezing the cell by hand. The powder will be compacted in a foil about 10 µm thick. Due to the small size of the sample, if the starting material is a powder mixture, as in the case of Fe₄O₅, it is difficult to load exact proportions, it is therefore essential that the starting powders are very fine (below 1 µm) and very well mixed.
4. Carefully, with the fine tip tungsten needle, break the foil of compacted powder into pieces. Select a flake about 40 µm large. Note: The size of the loaded sample should be smaller than the size of the sample chamber at the target pressure. The sample chamber shrinks according to the assembly design and materials, and the compressibility of the materials filling the sample chamber, which is very high for the Ne and He media we use. A sample chamber starting 40 µm thick and 120 µm large loaded with pressurized Ne (see below) shrinks to about 15 µm thickness and 70 µm diameter at 20 GPa.
5. Transfer the flake on the center of the diamond culet in the first DAC. Reassemble the two parts of the DAC (Figure 2c).
6. Besides the absorbance of the particular laser wavelength used, the sample heating efficiency crucially depends on the sample preparation. In particular it is important to load a sample with uniform thickness that should be well insulated from diamonds and gasket. It is crucial to ensure that the sample does not adhere to the thermal conductive diamonds, which would dissipate the heat. Few grains of loose powder that almost inevitably end up around the pressed flake are usually sufficient to keep the sample lifted from the anvils. The pressure-transmitting medium will then provide a thermal insulating layer.
7. Fill the sample chamber with a pressure-transmitting medium. Recommended media include pre-pressurized (1.7 kbar) Ne or He as these two inert media provide quasi-hydrostatic conditions to very high pressure in addition to thermal insulation. Perform the gas loading using the COMPRES/GSECARS gas loading system, described in detail elsewhere¹⁸.

3. Laser Heating and High Temperature X-ray Diffraction

Heating of a sample inside a DAC is achieved by applying infrared lasers to the sample through diamond anvils. Samples are heated to high temperatures by absorbing the laser radiation. Samples that don't absorb the 1,063 nm radiation can be mixed with absorbing and chemically inert material (example: Pt black powder) for indirect laser heating. Since diamond is a superior thermal conductor, a preferred practice is to use thermal insulation layers (example: NaCl, MgO or Al₂O₃) to separate the sample from the anvil surface. Absorbing and insulating materials will, however, cause additional parasitic scattering and might react with the sample.

The online laser heating systems installed in the experimental stations 16IDB of HPCAT and of 13IDD of GSECARS^{9, 10, 11} of the APS are designed to perform heating and x-ray diffraction on samples loaded in the DAC. Typical energies of the x-ray beam are about 30 keV and the size at the focal spot is about 5×5 µm FWHM. Infrared lasers (YLF, wavelength = 1,063 nm) are used for heating samples in the DAC. A fast x-ray area detector such as MARCCD and XRD 1621 xN ESPE is utilized.

1. Secure the DAC in the water cooled copper holder. Use the offline system (a remotely controlled motorized stage and a microscope) to determine the rough sample positional coordinates. The x-ray beam and the laser beams are pre-aligned to the sample position on the rotation axis of the sample stage.
2. Mount the holder on the sample stage (Figure 3), exit and close the experimental station following the laboratory safety procedures (these are specified in the mandatory trainings required to perform experiments in the laboratory).
3. Remotely operate the laser heating and x-ray diffraction systems from inside the experimental station, equipped with computers and monitors. Position the sample on the rotation axis of the sample stage using x-ray absorption profiles of the sample measured with an x-ray photodiode.
4. Move the upstream and downstream laser heating optical components in the x-ray beam path.
5. Focus the optics for clear sample viewing and adjust heating mirrors to compensate the tilting of diamond anvils.
6. Turn on the lasers and slowly increase the laser power delivered to the sample.
7. When the sample starts to glow (Figure 4a), collect the thermal radiation spectrum using an imaging spectrograph.
8. Use the available software to fit the observed spectrum to the Plank radiation function assuming black body behavior to determine the sample temperature^{19, 20}. Adjust the laser power delivered on the two sides of the sample, together or separately, to achieve the target temperature as measured on both sides of the sample.
9. While heating the sample, collect diffraction patterns to monitor the disappearance of the starting material characteristic peaks (usually smooth Debye rings) and the appearance of the new phase peaks (usually spotty patterns). The heating time depends on the temperature and the kinetics of the particular synthesis performed. In case of the synthesis of Fe₄O₅ briefly described below, the synthesis is faster than this detection system (seconds).
10. Move the sample in the plane perpendicular to the x-ray beam in steps of a few micrometers, heating the sample as uniformly as possible. Turn off the lasers and move the optical components out of the x-ray beam path.

4. X-ray Diffraction Data Collection after Synthesis

If the synthesized phase is stable after temperature quenching, diffraction data may be collected after heating. The x-ray diffraction data are collected in transmission geometry (through the anvils). Still diffraction patterns and rotation diffraction patterns (ω-scan) are collected. The latter are collected as single rotation images (wide scan), sets of fine step scan images (one degree or less), or large step scans images (5-10°). The data collection strategy is adjusted according to the available beamtime, exposure time and detector readout speed. When using very fast detectors, fine step-scan data can be collected at each point of the 2D grid in less than an hour. This allows a complete and objective sampling of the reciprocal space across the whole sample.

1. Collect a set of still diffraction images in a 2D grid distribution covering the sample area with step size similar to the x-ray beam size, about 5 µm. In Figure 4b the grid of 8 x 8 = 64 images collected in a 40 µm sample is shown. Typical exposure time ranges between 3-20 sec, depending on the x-ray flux and the sample scattering power.
2. Inspect the diffraction images, select a set of locations best suited for single crystal (or multi-grain) and powder diffraction analysis. Diffraction images showing few spots are likely generated by one or few crystals that might provide good single crystal diffraction patterns. The corresponding sample locations, defined as Ay,z (with y and z being the horizontal and vertical coordinates in the plane perpendicular to the beam), are suitable for single crystal or multigrain analysis. Patterns showing smooth or, more commonly, "spotty" Debye rings, defined as location By,z, are suitable for powder diffraction analysis.
3. Using the sample stage translation motors, move to the first Ay,z location. Collect a wide angle diffraction image and a set of fine step ω-scan diffraction images (with omega step size of 1 degree or less). Use these patterns for single-crystal or multigrain data analysis.
4. Repeat for each selected Ay,z location.
5. Using the sample stage translation motors, move to By,z locations. Collect wide step ω-scan diffraction images, i.e. 7 images with 10° step size for 70° total opening. Collect rotation images for powder diffraction analysis to compensate for the low grain statistics typical of LH-DAC samples.