High temperature PVT relationships and primary pressure scales

Chang Sheng Zha and William A. Bassett,

Cornell High-Energy Synchrotron Source,
Wilson Lab and Department of Earth and Atmospheric Sciences,
Cornell University, Ithaca, NY 14853

Principle:

To create a primary pressure scale, we need to conduct both volume and sound velocity measurements on a sample at the same PT conditions. This can be done either by switching the sample between two measuring devices while keeping the PT conditions constant, or by making measurements separately but returning the data to the same PT conditions during data reduction.

For example, Brillouin scattering can be used in conjunction with x-ray diffraction at a synchrotron source. The first approach requires building the Brillouin scattering system in the x-ray hutch at a synchrotron source. The second approach could be accomplished if we can find a suitable optical pressure sensor that operates at high temperature. This sensor could serve as a bridge connecting the x-ray diffraction and Brillouin scattering experiments and make it possible to combine the data at corresponding PT conditions.

Experimental Techniques:

1. Pressure generation: Large Volume Press (LVP) ----- Powder sample, Solid pressure medium (30GPa). Diamond Anvil Cell (DAC) ------ Powder or single-crystal sample, Solid or gas pressure medium (300GPa).
2. Temperature generation: Laser Heating: DAC (5000 K) External Resistive Heating: DAC (1400 K). Internal Resistive Heating: DAC and LVP (3000 K).
3. Volume measurement: Polycrystalline x-ray diffraction: DAC and LVP. Single crystal x-ray diffraction: DAC
4. Sound velocity measurement: Ultrasonic interferometry: DAC and LVP. Brillouin scattering: DAC. Inelastic x-ray scattering: DAC.

Proposed Experiment

Our proposed experiment consists of separately conducted Brillouin spectroscopy and x-ray diffraction measurements using an optical sensor to tie them together. Previous experiments show that possible candidates for the optical sensor might be nitrogen or cubic boron nitride (c-BN). Both have simple, strong PT-sensitive Raman signals measurable at high PT conditions. There are several potentially suitable materials for the pressure calibraint itself, e.g., MgO, CaF₂, or NaF. The choice from among these will depend on suitable symmetry, Debye temperature, and melting curve. Let us assume the choice of c-BN as the optical sensor, and MgO as the target material to serve as pressure calibrant.

The experimental procedure would be:

• X-ray diffraction:- Polycrystalline MgO, polycrystalline Au, and a single crystal fragment of c-BN are placed in a DAC. Resistive heating, either internal or external, is used to heat the sample. X-ray diffraction is used to measure the densities of MgO and Au at various PT conditions. Raman spectra of c-BN at the various PT conditions are recorded. The choice of pressure medium depends on the temperature and pressure range of the experiment. The pressure can be temporarily estimated from Au EOS, the PVT relationship for MgO can be temporarily created from Au EOS.
• Brillouin scattering:- An MgO single crystal and a c-BN fragment are loaded into a DAC. Resistive heating, either internal or external, is used to heat the sample. Brillouin spectra are collected on the MgO, Raman spectra on the c-BN. Once again, the choice of pressure medium depends on the temperature and pressure range of the experiment.
• Combining:- The bulk modulus of MgO measured from Brillouin scattering and the density of MgO measured from x-ray diffraction at certain PT conditions can be used to determine an independent PVT relationship suitable as a primary pressure scale. The Au scale that was used only temporarily as a pressure marker can be abandoned, or our measurements can be used to back check and correct it for future use as a calibrant. In addition the Raman signals of c-BN can also serve as a pressure calibrant in optical experiment.