SPring-8, the large synchrotron radiation facility

Release Date

27 Mar, 2014

Okayama University
University of Hyogo
RIKEN
Japan Synchrotron Radiation Research Institute

<<Background>>
Since the discovery of the transition from perovskite (Pv) to post perovskite (pPv) in MgSiO₃, pPv-MgSiO₃ has been considered to be a major component of the D'' layer just above the core–mantle boundary. Seismological observations of the D'' layer are difficult to interpret as they suggest both non-uniform response at the layer boundary and non-uniform anisotropy inside the layer. With this background, elastic anisotropy, or single-crystal elasticity, of pPv-MgSiO₃ has been the focus of theoretical calculations. However, no experimental data are available on the single-crystal elasticity of pPv-MgSiO₃, because it is unstable at ambient pressure.
Cmcm-CaIrO₃, or pPv-CaIrO₃, has been frequently selected as a representative analog of pPv-MgSiO₃. According to the phase diagram, pPv-CaIrO₃ is the stable phase at ambient conditions and Pv-CaIrO₃ is the high-temperature and low-pressure phase. Here we report the first experimental data on single crystal elasticity of Pv and pPv structures in a homogeneous chemical composition of CaIrO₃.
Figure 1 shows single crystals of Pv-CaIrO₃ and pPv-CaIrO₃ used in the present study. The Pv-CaIrO₃ sample was synthesized at 2 GPa and 1450°C for 15 h in a piston cylinder apparatus at Okayama University. We produced many grains with size of a few tens of microns, although most of them turned out to have multiple domains. We examined more than 20 grains using a four-circle X-ray diffractometer at Yamaguchi University and selected a quasi-single domain crystal with size of ~20 µm. Its lattice constants were determined as a = 5.3527(1), b = 5.5969(5), and c =7.6804(6) Å, which yields density (ρ) of 8091 kg/m³. The pPv-CaIrO₃ sample was synthesized by slow cooling from 1000°C in CaCl₂ flux¹⁵. It had a pine-needle shape with length of a few hundred microns and width of a few tens of microns. Its lattice constant and densities were respectively a = 3.145(2), b = 9.861(6), and c = 7.297(5) Å, and ρ = 8211 kg/m³, which are consistent with literature values.

<<Method and results>>
Measurement of IXS at SPring-8^*1 BL35XU. The small size and black color of the present samples limit the techniques that can be used to investigate the elastic properties. In particular, the crystal sizes are too small to employ GHz ultrasonic methods or resonant ultrasound spectroscopy, which both need crystals larger than 100 µm. In addition, the crystal opacity makes it difficult to apply Brillouin scattering. Therefore, IXS with an X-ray beam diameter of ~70 µm was employed. Each specimen was attached to glass fibers (~100 µm in diameter) using manicure resin and mounted on the four-circle goniometer installed at BL35XU of SPring-8. The incident X-ray beam energy was 17.794 and 21.747 keV for Pbnm-CaIrO₃ and Cmcm-CaIrO₃ measurements, respectively.
The IXS technique allows us to determine the elastic properties of a material by measuring the energies of acoustic longitudinal and shear phonon modes. When this information is combined with the precise reciprocal space position (momentum transfer) for each measurement, we can determine the sound velocity. If measurements are made in many different directions, the velocity information can be combined with the density, allowing all the elastic constants of a material to be determined. In the present work, the two-dimensional, 4 × 3, analyzer array of BL35XU is especially advantageous as it allows access to 12 different directions of momentum transfer simultaneously.
Figure 3 summarizes the present idea for seismic anisotropy and LPO structure in the D'' layer. The VTI of the vertical c axis and that of the vertical b axis can be interpreted as transformation LPO and deformation LPO, respectively. It is noted that isotropic pPv-MgSiO₃ is a reasonable interpretation for the seismic feature beneath the Atlantic Ocean; an alternative interpretation may be the VTI of the vertical a axis because of less lateral polarization anisotropy than in other cases. The difference between beneath the Pacific and Atlantic may be attributed to differences in temperature, chemical composition, and strain rate. The proposed LPO behavior for pPv-MgSiO₃ obtained from the present mineral physics perspective is consistent with seismic observation.

<<Glossary>>
1 SPring-8
SPring-8 is a synchrotron radiation facility that provides the world’s highest-brilliance synchrotron radiation. It is owned by RIKEN and located in Harima Science Park City, Hyogo Prefecture, Japan. JASRI is responsible for the operation, management, and promotion of the use of SPring-8. The name “SPring-8” is derived from “Super Photon ring-8 GeV”. When the direction of the electron beams accelerated to nearly the speed of light is changed by magnets, electromagnetic waves are emitted in the tangential direction; these waves are synchrotron radiation. When the electron beam has a higher energy and the change in the traveling direction is large, synchrotron radiation contains shorter-wavelength lights such as X-rays. In particular, the following three facilities are known as the third-generation large synchrotron facilities: SPring-8 in Japan, the Advanced Photon Source (APS) in the USA, and the European Synchrotron Radiation Facility (ESRF) in France. Because the ring at SPring-8 enables an electron acceleration energy of 8 giga-electronvolts to be generated, synchrotron radiation in a wide range of wavelengths can be obtained including far-infrared light, visible light, vacuum ultraviolet light, soft X-rays, and hard X-rays. SPring-8 is used by researchers both in Japan and overseas for joint research in various fields such as materials science, earth science, life science, environmental science, and industrial applications.