Diamond-anvil cells

Diamond-anvil cells can generate very high pressure by compressing materials between two gem-quality diamond anvils. The diamond anvils are fixed and supported in a piston-cylinder assemblage. Screws drive the piston and cylinder together. Diamond is the strongest among known materials and is transparent to the wide ranges of electromagnetic radiations, such as X-rays, infrared, and visible light. The beam is directed into the sample generally through apertures along pressure loading axis. This enables us to carry out in situ diffraction or spectroscopic measurements at extreme pressure-temperature conditions (1,000-2,000,000 bar and 300-4000 K). Several different types of the diamond-anvil cells are available in the Mineral physics lab including the symmetric-type and the membrane-type cells.

Laser heating system

Samples in the diamond cells can be heated over 5000 K using a laser heating system. Temperature can be estimated by fitting thermal radiation spectrum to the Planck's gray body equation. The laser-heating system at the Mineral Physics lab consists of a Nd:YLF laser (1053 nm, 45 W, TEM00, diode-pump, Photonics Industry), an imaging spectrometer (Acton Research) with a electrically cooled CCD detector, and other optical components. The imaging spectrometer allows for the measurements of the thermal radiation for several different spots in a sample simultaneously.

X-ray diffraction and spectroscopy

Recent developments in synchrotron facilities (e.g., Advanced Photon Source at Argonne National Laboratory, National Synchrotron Light Source at Brookhaven National Laboratory, Cornell High Energy Synchrotron Source at Cornell University, Advanced Light Source at Lawrence-Berkeley National Laboratory, and Stanford Synchrotron Radiation Laboratory at Stanford University) enable us to perform measurements for samples directly at high pressure and high temperature. Synchrotron radiation provides extremely bright X-ray beams for various techniques encompassing diffraction and spectroscopy. Diffraction using highly collimated intense synchrotron X-ray beam and two dimensional detectors opens new opportunities to study (1) multi-phase systems, (2) material strength at high pressure and temperature, (3) phase diagram, and (4) equations of state of materials. X-ray spectroscopy techniques also become available for high-pressure research and enable to measure many important physical properties of materials. We fully take advantage of these exciting developments in the large user facilities.

Raman spectroscopy

The Raman spectroscopy systems at MIT are designed to study (1) thermodynamic properties and (2) phase transitions of materials at high pressures and high temperatures. Raman spectroscopy is a complementary tool to the X-ray diffraction technique to study crystal structures: Raman spectroscopy is particularly sensitive to the short-range ordering, whereas X-ray diffraction is sensitive to the long-range ordering. Raman spectroscopy is very powerful to study volatiles (e.g., H2O and CO2) which have extremely small scattering cross sections for X-rays. For silicates and oxides, combination of X-ray diffraction and Raman spectroscopy measurements enhances our ability to understand crystal structures at high pressure and high temperature.

Two different Raman systems are available at MIT high pressure lab: (1) a dispersive Raman spectroscopy and (2) a time-resolved Raman spectroscopy systems.

The dispersive Raman system consists of an Ar/Kr-mixed-ion laser (1.4 W, Coherent laser), a triple spectrometer (Acton Research), and other optical components. The Ar/Kr-ion laser provides variety of laser lines from 458 nm to 753 nm. Short wavelength laser lines are useful for high-temperature measurements because they shift Raman spectral range to lower wavelength where thermal radiation is still relatively low. This allows for measurements up to 1200 K. Long wavelength laser lines can be used for highly fluorescent materials as they excite less fluorescence. The triple spectrometer (a single spectrometer with a double monochrometer) provides extremely high resolution (both additive and subtractive modes available). Measurements at as close as 5 cm-1 from Rayleigh line is possible with this spectrometer. For weak Raman scattering, a single spectrometer in the triple spectrometer can be used for high throughput measurements combined with a notch filter. Extremely tight focus (1-2 microns) with a confocal setup achieves high spatial and depth resolution.

The nano-second gated Raman system effectively reduces the detection ofintense thermal radiation from samples at high temperatures (> 1200 K). It consists of a diode-pump frequency-doubled Nd:YLF laser (527 nm, 0.1-10 kHz rep rate, 20 ns pulse width, Photonics Industry), an intensified gated CCD detector (>20 ns gate width, Acton Research), and other optical components. This system enhances the signal-to-background ratio by more than four orders of magnitude at high temperature. In addition, this gated Raman system can effectively reject fluorescence from yellow diamond anvils.

Both dispersive and gated Raman systems can also be used to measure ruby fluorescence spectra, which is a well calibrated pressure calibrant.

Other facilities

Several instruments for sample preparation are available at MIT Mineral Physics Lab: (1) Micro-drill systems (mechanical adn EDM) to produce small holes (50-300 microns in diameter) in the indented gaskets, (2) Two high-resolution stereomicroscopes to align the diamond anvils and load samples in the diamond anvil cells, (3) Two cryogenic gas loading systems, (4) A micro heating stage (Linkam) to heat samples up to 1800 K at ambient pressure, and (5) ring heaters (Diacell) to heat samples in the diamond anvil cell up to 1200 K.