The B-R (Brillouin/Raman) instrument is a high pressure diamond anvil cell designed especially for spectroscopy and X-ray diffraction experiments. The cell is capable of achieving pressures to several hundred kilobars or several hundred thousand atmospheres, depending upon the size of the diamond anvils and the pressure bearing surfaces (culets). This diamond cell has a four post design, allowing a precise fit to the circular halves of the cell. The instrument is machined from hardened 440c steel. Pressure is developed by tightening four Allen cap screws (two left hand, two right hand) in sequence, compressing Belleville spring loaded washers.
Crystallographers use the instrument for high pressure single crystal X-ray diffraction studies. Its compact size is suitable for mounting on a goniometer head. The accessible space is defined by a ~8O degree cone, depending on seat and diamond size. Beryllium seats are used for X-ray while light scattering techniques use hardened steel discs with cone angle openings.
Brillouin scattering spectroscopy measures Doppler-shifted light that is inelastically scattered by acoustic phonons. A beam of light from a laser is introduced into a sample and the light scattering by the sample is analyzed for its frequency. The Doppler effect describes how the frequency of light is affected by the relative motion between the light source and the observer.
When the incident light is scattered by the sample, there are Doppler shifts for the scattered light. The phonon velocities can be calculated by the frequency shifts, providing information about the elastic moduli of the sample. The measurement of elastic constants as a function of pressure can provide valuable information for understanding atomic forces under deep Earth conditions.
Raman scattering with this diamond cell involves measuring vibrational spectroscopy of matter under pressure excited by a laser beam or other light sources. Vibrational modes in crystals are known as phonons. Raman scattering is the measurement of light scattering by optical phonons. From pressure induced shifts in phonon energies, a researcher can gain information on interatomic bonding forces, high pressure phase transitions, and structural instabilities of minerals composing the Earth’s interior.
The diamond cell can be assembled with Type I anvils, Type I anvils screened for Raman fluorescence, or Type IIa anvils for use with IR radiation. The diamond anvils are cut with 0.6 mm culets, 70% tables and 16 pavilion facets. Smaller culets or beveled culets can be provided.
Inconel or stainless steel (T301) gasket material and ruby hips for pressure calibration are included with the cell. Beryllium diamond mounts are used for high pressure X-ray experiments to pressures of approximately 200 kilobars.
The Brillouin-Raman diamond anvil cell can be used to study physical properties of liquid or solid samples under measurable pressure conditions. The cone angle openings of this instrument will allow Brillouin or Raman scattering measurements at variable angles up to 50 degree central line. If beryllium diamond mounts are used, X-ray diffraction data can be taken.
Store the instrument with a piece of cardboard between the two diamond anvils to prevent abrasions. Anvils should be inspected before and after each use.
CLEANING THE ANVILS
The anvils can be cleaned by wiping acetone or alcohol across the working surfaces using tissue paper twisted into a pointed tip. Do not let the solvent come into contact with the epoxy as it will eventually loosen the bond holding the diamond to its mount. The working surfaces can also be cleaned by scraping with a wooden or plastic toothpick. The undersides of the diamond anvils are cleaned by scrubbing with a round toothpick.
HIGH PRESSURE STUDIES:
Clean the two diamond anvils as instructed.
A pre-indented gasket is recommended prior to making the sample chamber hole. Either T301 full-hard stainless steel or Inconel 718, supplied by High Pressure Diamond Optics, Inc., can be the choice material of the gasket. T301 full-hard stainless steel is a much harder metal with Ultimate Tensile Strength greater than 180,000 psi and will require a special drilling device to make holes; Inconel 718 with Ultimate Tensile Strength greater than 120,000 psi can be alternative to T301 stainless steel and drilled with a traditional drill press. Make alignment markings on the gasket and the diamond cell with wax or nail polish for future reference when positioning the gasket. Indent the gasket to half its thickness, then drill a hole as close to the center of the indentation as possible. Remove any rough edges (burrs) around the hole with a rat-tail file or a micro drill bit. If a pre-drilled gasket is used, position the hole as close to the center of the culet as possible. Then indent the gasket and clean any rough edges with #78, #79 or #80 drill bits.
Cut soft samples into small pieces (e.g. 50 microns die). Harder samples may be polished with fine grit sandpaper and reduced by using a sharpened needle.
Position the gasket with two #5-40 gasket screws, supplied by H.P.D.O., Inc., or support the gasket on the lower anvil with balls of paraffin, noting previous alignment markings.
Level the lower support and load the sample, pressure calibrant (e.g. ruby chips) and pressure transmitting medium (e.g. liquid argon) under magnification1. Several attempts may be needed when performing this step. After completing this step, proceed to step #6 immediately.
Carefully assemble the instrument with hollows aligned. Insert and turn the two left-hand Allen cap screws with red markings aligned until firm. Then insert and turn two right-hand Allen cap screws until firm. Tighten the four Allen cap screws alternately in very small increments of a few degrees per turn until a moderate amount of pressure is applied to the gasket. This procedure is needed to ensure the pressure transmitting medium has been successfully loaded.
Observe the chamber hole. Under pressure, the hole will change shape slightly. However, if the hole is significantly deformed, the instrument may have lost the pressure medium. If medium loss is uncertain, apply more pressure by tightening the four Allen screws and observing the sample hole for any changes. Begin the process from step #1 if the pressure medium has been lost. Otherwise, proceed to step #8.
Ruby fluorescence can be used to calibrate the in-situ pressure. The frequency shifts of the R1 and R2 lines determine the pressure by using the following formula:
Where Δλ and λ0 are the wavelength (in nm) change under pressure and the wavelength at ambient pressure respectively. Lower case letter b is a parameter, would be 5 or 7.665 corresponds to non-hydrostatic or quasi-hydrostatic pressure respectively. Light sources using an argon ion laser from 488 nm and 514.5 nm lines are commonly used in this technique.
Record the current pressure.
The pressures can be repeatedly altered by tightening or loosening the four Allen cap screws in an alternate sequence. The chamber hole may shift as the pressure changes. Turn the Allen screw(s) to move the chamber hole toward the opposite direction if the hole is off center to the culet. Notice that turning the screws will alter the pressure. Follow this step closely to avoid sample failure.
Calibrate the pressure according to step #8, then continue experiments.
Disassemble the cell by loosening the screws sequentially in small increments after finishing experiments. Store the diamond cell as instructed.
1 Pressure calibrants such as ruby chips should be distributed evenly around the sample for monitoring hydrostatic pressure conditions.