Although the multi-anvil press is a reliable high-pressure-temperature apparatus, the conventional multi-anvil press has two disadvantages. First, we cannot directly see the sample under high pressure and high temperature. We do not know what changes are occurring with the sample. Second, we do not know the sample pressure. In conventional multi-anvil experiments, the sample pressure is calibrated against the press load by detecting several known high-pressure phase transitions. However, the sample pressure varies in a complicated and often unpredictable way by heating.
In situ X-ray observation solves the above problems. In this method, highly brilliant, directional, and penetrative X-rays from synchrotron radiation reach the sample through the gasket and pressure medium. The X-ray signals generated by reacting with the sample are detected out of the press to provide real-time information about the sample. X-ray diffraction provides information about the sample's phase transition and chemical reaction. Particularly, the change in diffraction intensities of existing phases indicates the direction of the reaction. The radiograph by transmitted X-rays provides information about the sample geometry and dimension. By placing a pressure standard material next to the sample, we can estimate the sample pressure from the change in the unit cell volume of the pressure standard material together with the temperature measured by a thermocouple.
Energy-dispersive X-ray diffraction
In situ X-ray observation is a modern and relatively routine experimental technique for multi-anvil workers. Although there are many multi-anvil beamlines in synchrotron radiation facilities worldwide, we preferably use the following two facilities:
We adopt the energy-dispersive X-ray diffraction (EDXD), where the incident beam is high-energy white X-rays, and diffracted X-rays are collected using a solid-state detector (SSD) at a fixed diffraction angle. The main advantage of EDXD is high counts against the strong absorption by the gaskets, pressure medium, and sample itself.
Figure 1. The concept of the energy-dispersive X-ray diffraction with a multi-anvil press.
We adopt in situ X-ray observation exclusively for determining phase relations although many other workers conduct rock and mineral deformation and viscosity measurement. Below, we explain our advanced experimental technique for phase-relation studies.
High-pressure cell assembly
Figure 2. Schematic cross-sections of the cell assembly. Two sintered samples and a sintered MgO pressure marker (P.M.) were set at the center of the cell assembly. Fo100 and Fo70 denote the samples with bulk compositions of Mg2SiO4 and (Mg0.7Fe0.3)2SiO4, respectively, for example.
Figure 3. Radiographic images of the sample part. (a) before compression, (b) after compression to 7 MN.
Precise pressure determination
We usually adopt MgO as a pressure calibrant for the following reasons. (1) no phase occurs transitions in the Earth's interior. (2) The melting temperature is high, 1800℃ even at ambient pressure. (3) The crystal structure is simple (B1). Figure 4 shows a typical diffraction pattern of the MgO pressure calibrant. Except for very tiny diamond peaks, the diffraction pattern consists of MgO peaks only. Since we can read more than seven peaks, (111), (200), (220), (311), (222), (400), (331), (420), and (422), the precision in pressure determination is very high: 0.05 GPa at 25 GPa. This high precision enables us to determine phase relations reliably and in detail.
Figure 4. A typical diffraction of MgO pressure calibrant. The right is the expansion of the red rectangle in the left figure. The peaks up to (422) can clearly be observed.
Challenge for determining phase relations
There are several challenges in determining phase boundaries by combining multi-anvil experiments with in situ X-ray observation. Let us consider the phase boundary of two high-pressure phases.
Solution for accurate determination of phase relations
To overcome the above challenges, we adopt the following strategy.
Figure 5. A schematic drawing explaining the novel strategy to determine a phase boundary accurately and precisely. The pressure spontaneously and gradually decreases while keeping the temperature and press load constant. In the HP- and LP-phase stability fields, HP- and LP-phases increase with time, respectively, enabling to bracket the phase boundary. The numbers in the circles indicate the steps in the novel strategy explained above.
Bridgmanite + periclase (Brg + Pc) decreases and ringwoodite (Rw) increases from 23.33 (3) GPa to 23.29(3) GPa at 2038 K. ⇒ ringwoodite stability
Bridgmanite (Brg) decreases and akimotoite (Ak) increases from 22.22(4) GPa to 22.24(7) GPa at 1448 K. ⇒ akimotoite stability
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