Cheiron School 2010

BL10XU: High Pressure Research
The beamline BL10XU is fully dedicated for X-ray diffraction measurements at highpressure and low/high temperature using a diamond anvil cell (DAC) (Figure 1) [1]. The
high-resolution monochromatic angle-dispersive X-ray diffraction patterns allow us to theaccurate determination of equations of state, precise determination of phase relation, structurerefinement by Rietveld analysis, and charge density distribution analysis in crystals submitted toextreme pressures. Pressure can induce dramatic changes in physical and chemical properties ofmaterials through volume compaction or decrease in inter-atomic distance, providing
significantly difference from common properties known at ambient pressure. To have a better
understanding of high-pressure study using a combination of synchrotron radiation and a DAC
technique through this BL practice course, in situ high-pressure and high-temperature X-ray
diffraction experiments will be carried out, including a calibration of energy, sample loading
into a DAC, X-ray diffraction measurements, and data analysis.

Figure1. Schematic layout of beamline BL10XU [1].
References:[1] Ohishi et al. (2008) High Press. Res. 28, 163–173.

— Glossary —Diamond anvil cell

Diamond anvil cell is one of high-pressure
generation techniques by static compression. The
sample is placed in a pressure chamber createdbetween the flat parallel faces (culets) of two opposed
diamond anvils and the hole penetrating a hardened
metal foil (= gasket) (Figure 2). Pressure is applied byforcing the diamonds together. Diamond is the premieranvil material because of hardest substance andtransparency to electromagnetic radiation over a wide
spectral range from the infrared to hard X-rays.

Figure 2. Diamond-anvil cell technique.

– Practice –The mineral olivine is one of the common and most abundant minerals on Earth.
Transparent olivine is sometimes used as a gemstone called peridot. This is a magnesium
silicate with a composition of (Mg,Fe)2SiO4, a continuous solid solution between forsterite
Mg2SiO4 and fayalite Fe2SiO4. Olivines have high-pressure polymorphs, which is important tounderstanding the nature of the Earth’s upper mantle.
In the practice, synthesize and identify high-pressure phases on olivine (Mg,Fe)2SiO4, oneof the primary minerals in upper mantle, using a laser-heated diamond anvil cell and in-situ
synchrotron radiation X-ray diffraction.

1) Energy calibration
Angle dispersive X-ray diffraction (XRD) requires a monochromatic X-ray beam. The
accurate determination of the wavelength for incident X-ray beam and of the distance between
sample and detector of imaging plate (IP) allow to determine precise crystal structures and theunit-cell parameters of materials. In this practice, we use photon energy tuned to 30 keV(0.4133 Å) for the XRD measurement. The calibration of the X-ray wavelength and the distanceis based on the standard material of cerium oxide (celia, CeO2).
1.
Obtain XRD images of CeO2 at two different IP distances.
2.
Convert X-ray images to conventional one-dimensional X-ray pattern.
3.
Calculate the wavelength and the distance between sample and detector accurately.

2) Sample loading into a DAC
We use a symmetric cell suitable for double-sided laser heating. In DACs, a sample isplaced in a chamber hole drilled in a rhenium foil between the culets of two opposed diamondanvils.

3) High-pressure generation and pressure measurement
After sample loading into a gasket, pressure is generated with forcing the two opposing
anvils. For a symmetric cell, controlled tightening of force generation through screws forces
together the two plates that support the two anvils.
Pressure in the DAC is determined by ruby pressure scale, established by measuring
simultaneously the shift of the ruby R1 luminescence line and the specific volume of metals. Onmonitoring the shift of R1 line, increase to desired pressure above 25 GPa.

4) Laser-heating for high-temperature generation and in-situ XRD data collection
We carry out double-sided laser heating with Nd:YAG laser (1064 nm) to synthesize
high-pressure phases on the (Mg,Fe)2SiO4 system. To study stable phases at high pressure andhigh temperature, collect in-situ XRD images.

5) Data analysis and phase identification

The basic principle of the X-ray diffraction is the Bragg's law: λ = 2dsinθ, where λ is thewavelength of the X-ray, d the lattice spacing, θ the angle of the incident beam and thediffracting lattice plane. The diffraction peaks are observed when the d-spacings satisfied theBragg's law.
1.
Convert the X-ray image to conventional one-dimensional X-ray pattern.
2.
Calculate d-spacing for each peak from 2θ value using the Bragg's law.
3.
Index peaks and identify high-pressure phases.