Synthesis and Microdiffraction at Extreme Pressures and Temperatures

The overall goal of this procedure is to perform synthesis at high pressure and temperature, and to perform synchrotron micro diffraction characterization of the high pressure phases. This is accomplished by first loading a sample at target pressure in a wide opening diamond anvil cell. The second step is to heat the sample using a double-sided laser heating system.

Meanwhile, the temperature is monitored by exploiting the emitted black body radiation. The final step is to collect diffraction data, adjusting the data collection strategy to the sample grain size, single crystal, multiple crystals and powder. Ultimately, this allows a robust structural analysis of high pressure phases and a comprehensive description of complex heterogeneous samples.

The main advantage of this technique over the existing methods like resistive heating, is that it can reach to much higher temperatures on the high pressure conditions in a diamond iron cell. This method can help answer key questions in electro geophysics and geochemistry fields, such as understanding the phases, composing planetary interiors, the properties and planetary dynamics. These high pressure and temperature synthesis experiments are performed at the advanced photon source of Argonne National Laboratory, either at the geo soil and Viro Center for Advanced Radiation sources, 13 IDD beam line, or here at the high pressure collaborative access team, 16 IDB beam line.

The typical energy of the x-rays at these beam lines is 30 kilo electron volts, and the beam size at the focal spot is about five micrometers by five micrometers. Full width at half maximum. Start with a sample loaded to target pressure in a diamond anvil cell mount.

The high pressure device in the water cooled copper holder. Then mount this in the sample stage. The laser beam used for heating and the x-ray beam used for diffraction are preed to the sample position on the rotation axis of the sample stage.

Now, exit and close the experimental station. Always follow laboratory safety procedures. The heating and diffraction systems are operated remotely.

First use x-ray absorption profiles of the sample measured with a photo diode to precisely position the sample on the rotation axis of the sample stage. Next, move the upstream and downstream laser heating optical components into the x-ray beam path. Focus the optics for clear sample viewing.

Then adjust the heating mirrors to compensate for the tilting of the diamond anvils. Turn on the 1063 nanometer infrared lasers and slowly increase the laser power delivered to the sample when the sample starts to glow. Collect the thermal radiation spectrum using an imaging spectrograph.

Assume black body behavior and fit the observed spectrum to the plank radiation function. To determine the sample temperature, adjust the power of the upper and lower lasers to achieve the target temperature as measured on both sides of the sample while heating the sample. Collect a fraction patterns to monitor the disappearance of the initial material characteristic peaks, and the appearance of the new phase peaks as the temperature of the sample increases.

Next, heat the sample as uniformly as possible by moving it in the plain perpendicular to the x-ray beam in steps of a few micrometers. Once this is done, turn off the lasers and move the optical components out of the x-ray path. Once synthesis has been completed, start the x-ray diffraction data collection.

Acquire a set of still diffraction images in a 2D grid distribution that covers the sample area with the step size of about five micrometers. The approximate x-ray beam size. Use the diffraction images to select a set of locations best suited for single crystal and powder diffraction analysis Images showing few spots are likely generated by one or a few crystals and might provide good single crystal diffraction patterns.

Patterns showing smooth or spotty buy rings are suitable for powder diffraction analysis. Translate the sample stage to place the first single crystal location in the x-ray path and collect a wide angle diffraction image. In addition, collect a set of rotation diffraction patterns with an angular step size of one degree or less.

Repeat this for each single crystal location. Next, move the sample. So a site identified for powder diffraction is in the x-ray beam.

Collect rotation diffraction images with a large angular step size, for example, for a 70 degree total opening. Seven images with a 10 degree step size repeat for each powder diffraction location. These powder diffraction patterns are obtained after a mixture of hematite and iron was heated at 15 giga pascals, and 1700 kelvin.

The pressure transmitting medium is neon. The high pressure phase of FE 4 0 5 is expected to be formed, but the results vary with location. This data refers to a marginal portion of the sample where the synthesis of FE 4 0 5 did not occur.

Instead, wite FEO was formed. The locally high iron to oxygen ratio is likely caused by a slightly higher local iron to hematite ratio or a thermally induced chemical gradient. At this location, FE 4 0 5 was formed.

Its peaks are in red. In addition, Dubai rings of fine-grained, non crystallized hematite are seen. This suggests the reaction is incomplete, most likely due to nonhomogeneous heating.

The final site is representative of a location where the reaction was complete and nearly pure FE 4 0 5 was formed. Here is the single crystal diffraction pattern that led to the discovery of EPI 4 0 5. It is indexed using the G-S-E-A-D-A and RSV software.

A three dimensional view of the peak locations in the reciprocal space, along with the FE 4 0 5 unit cell is shown using the RSV software. Like powder diffraction data, the single crystal diffraction data suffer from low resolution and intensity reliability limitations when measured in a diamond anil cell. Nonetheless, the three dimensional nature of the data allows for more robust interpretation.

Once mastered, this technique can be done in less than an hour Following this procedure. Other methods like microanalysis of recovered samples and first principle calculations may be performed. This will support the interpretation of structural data and understand the stability and properties of high pressure phases.