During the development of widely accepted pressure scales for ambient temperature conditions, we may still be permitted the luxury of looking ahead to similar developments for static high temperature-pressure conditions.

In large volume press applications with resistance heaters, it is typical to have conditions ranging from 0-4 GPa (gas vessel and piston-cylinder conditions) to 4-25 GPa (Kawai type device with carbide anvils) and more recently up to 1 Mbar using sintered diamond anvils.  In these situations it is routine to have highly static temperature conditions (within a few degrees Celsius) up to over 2000 K for hours or days, with temperature measured using a thermocouple.  Externally heated diamond-anvil cells (DACs) can hold static temperatures up to about 1000 K for similar periods of time and also use thermocouples as the normal method of temperature measurement.  Quasi-static temperature conditions in laser-heated diamond-anvil cells can be maintained for twenty minutes with small (10 micron) spot sizes and greater fluctuations, but conditions of over 2000 K and pressures routinely up to and over a megabar can be accessed.  All of these capabilities drive the need for accurate and precise methods of pressure measurement.

With x-ray access, the method of measurement of lattice parameters of standards is the most routine.  The different pressure standards are in various amounts of disagreement when they are measured side by side, and this presents one challenge to a standardization – it is usually not evident which standard if any is closer to “correct” than the others.  Measurement of primary pressure standards through simultaneous diffraction and velocity measurements is possible and a very few such measurements have been performed, not enough to resolve the ambiguity in these standards.

With x-ray access only being available at beam lines, a number of pressure measurement methods are used in conventional laboratories and should be included in a standardization.  These include phase boundaries that provide “fixed points” for each temperature, that can be solid-solid or melting boundaries.  They can be detected by sample recovery for the solid-solid transformations or by differential thermal analysis for the melting.  Melting measurements have the advantage of being valid for a range of pressures while solid-solid transformations are valid only at specific pressures and thus many such transformations are needed to provide wide pressure coverage.

An alternative is pressure measurement using a saturated solid solution (barometry).  Fixed-point comparisons combined with realistic thermodynamic treatments can be used to develop barometric standards that cover a range of pressures and temperatures. Our laboratory has been developing various standards, such as rutile saturation in SiO₂-GeO₂ and TiO₂-GeO₂, and other laboratories have been working on similar projects.  The combination of all of these can provide a wide range of P-T coverage where pressure can be estimated for each experiment after the samples are recovered.