We previously developed MgO, Au, and Pt pressure scales at high temperatures, without relying on any pressure scale. The MgO scale was obtained by MD simulations using semi-empirical potential models derived from the observed data of MgO at 0 GPa, including the volume thermal expansion, the temperature dependence of bulk modulus, and the elastic constants and their temperature and pressure derivatives. On the other hand, the Au and Pt scales were both derived using the Mie-Grüneisen-Debye type treatment based on the combined use of the observed data from shock compression, volume thermal expansion at 0 GPa, and temperature dependence of bulk modulus at 0 GPa from ultrasonic measurements. Based on simultaneous volume measurements of Au and MgO, Pt and MgO, and Au and Pt, previously reported using synchrotron X-ray diffractions at high temperatures and high pressures, it is found that our MgO, Au, and Pt scales are mutually very consistent over wide temperature and pressure ranges.
Uncertainties in pressure-scales have been one of the most important problems in the high-pressure science and technology. In order to overcome this problem, a method called Scale-Free Unified Analysis (SFUA) was developed to analyze P-V-T equations of state (EOS) of materials available as primary pressure-scales in high-pressure and high-temperature experiments. Here we introduce the concept and analytical procedures of SFUA, and the EOS models determined by applying the method for MgO, Au, and Pt. We will show the great advantages of SFUA to obtain most probable EOS models, and also point out some future prospects to completely dissolve the problem.
Through shock-compression experiments, pressure-density relation of condensed matter can be directly and precisely determined. We have established the high-time-resolution streak photographic system using a long-pulsed dye laser equipped with both the powder gun and two-stage light gas gun. By using them accuracy of the measurement is much improved. We started the Hugoniot-measurement experiments on pressure-scale materials such as Au and MgO up to >200 GPa for the purpose of improvement of the pressure scales. We are now measuring the Hugoniots of Au and MgO as well as those of some metals (Al, Cu, Ta, W, etc.) to determine the Hugoniot data free from the Los Alamos data. In this article, the backgrounds and problems of the various pressure scales are reviewed, and our approach to establish pressure scales by means of the shock-compression at Kumamoto University is described.
The equation of state of gold is widely used as an x-ray pressure standard. The determination of volume by powder x-ray diffraction is, however, severely affected by the presence of uniaxial stress in the sample at high pressures. If a crystal lattice deforms under uniaxial stress, the measured d-spacings deviate from the hydrostatic values, depending on the geometry of the x-ray diffraction experiment. We can analyze the stress state of a cubic crystal with the use of the gamma plot. In the case of gold pressurized with helium, we found that the uniaxial stress differs by experiment, specifically how large the gasket hole deforms under high pressure. This could be the major reason for the discrepancy of the measured lattice parameter of gold. Some general remarks on the solid pressure media are also given.
In this article, room-temperature pressure scales of MgO, Au, Pt, NaCl(B2), and Ar are compared using reported volume-volume measurements up to 198 GPa with laser-annealing method. MgO: Speziale et al. 2001, Au: Hirose et al. 2008, Pt: Holmes et al. 1989, NaCl(B2): Sata et al. 2002 (MgO calibration, mBM3), and Ar: Jephcoat 1998 are consistent each other. Simultaneous volume measurements of Au and Pt using He medium are not consistent with the laser-annealed data.
Recently, atomic-scale materials modeling based on first-principles quantum mechanics plays an important role in the Earth science. However, it is known that this kind of modeling has a significant uncertainty for prediction of physical properties of materials at extreme conditions. In contrast, conventional high-pressure experiments have an unsolvable disadvantage for “pressure scale”. We outline the uncertainty in the first-principles calculations and show how such disadvantage can be improved using high-pressure experimental data. After a summary of accuracy for new method combining the first-principles calculations and static compression experiments, we then discuss a matter of argument that the investigation of equations of sate for materials used as pressure calibrant in high-pressure experiments.
An accurate pressure scale is essential for conducting high-pressure experiments. Simultaneous measurements of elasticity and density enable us to construct a primary pressure scale, which can independently determine the pressure, but this has been so far limited to low pressures and at ambient temperature due to the experimental difficulties. Recent development of Brillouin scattering and synchrotron X-ray diffraction techniques have proven to be highly suitable for exploring the sound velocities and lattice parameters under high-pressure and high temperature conditions. Here I describe the strategy, challenges and current status toward an establishment of primary pressure scale applicable to Mbar pressure using both Brillouin scattering and synchrotron X-ray diffraction techniques.
Simultaneous elastic wave velocity and in situ synchrotron X-ray measurements enable us to determine absolute pressure self-consistently without using other pressure scales. Here I show a case study for MgO up to a volume reduction V/V0 of ∼0.9 (up to 23.6 GPa in our P-V-T relation of MgO) and at temperatures between 300 and 1650 K using a multi-anvil apparatus. The resultant adiabatic modulus and unit-cell volume led to the first self-consistent P-V-T relation of MgO, which reproduces the existing pressure-scale-independent data set. This paper focuses on the analytical procedure to determine self-consistent P-V-T relation from elastic wave velocity and unit-cell volume data.
Angle dispersive powder X-ray diffraction experiments using a flat imaging plate (IP) are one of the most popular methods in high-pressure material science. In order to support such experiments, we developed two software, IPAnalyzer and PDIndexer. IPAnalyzer can convert a two-dimensional Debye-ring pattern to one-dimensional (Bragg-Brentano) geometry. IPAnalyzer can also calibrate experimental parameters (wave length, camera length, and so on) automatically. PDIndexer can display the converted pattern(s) and diffraction peaks calculated for any crystals.