The study of matter under extreme pressures has long captivated physicists, chemists, and materials scientists. Diamond anvil cells (DACs), capable of generating pressures exceeding 1 terapascal (TPa), have become indispensable tools in this quest. These devices compress tiny samples between two diamond anvils, subjecting them to pressures comparable to those found in planetary cores or even neutron stars.
The journey began with Percy Bridgman's pioneering work in the early 20th century, achieving pressures of a few gigapascals (GPa). Today's advanced DACs routinely reach 300-400 GPa, with some specialized setups pushing into the petapascal (PPa, 1015 Pa) range. This represents an increase of five orders of magnitude in just a century.
Modern DACs employ precisely aligned single-crystal diamond anvils with culet diameters as small as 10-50 micrometers. The diamonds' exceptional hardness and transparency to most spectroscopic probes make them ideal for these experiments. Pressure is typically measured using ruby fluorescence or X-ray diffraction standards.
As pressure increases, electron orbitals distort dramatically, leading to profound changes in atomic and molecular behavior. At petapascal pressures, even fundamental assumptions about chemical bonding must be reconsidered.
Density functional theory (DFT) calculations predict numerous exotic phases at ultrahigh pressures, but experimental verification remains challenging. The pressure gradient across the sample, stress anisotropy, and kinetic barriers to phase transitions all complicate interpretation.
| Material | Predicted Phase Transition (GPa) | Experimental Verification (GPa) |
|---|---|---|
| Hydrogen | 400-500 (metallic phase) | ~400 (partial evidence) |
| SiO2 | 270 (post-stishovite phases) | ~250 (observed) |
| Fe | 150-200 (hcp to double-hcp) | Confirmed at ~150 |
Modern high-pressure facilities combine DACs with synchrotron X-ray sources and optical spectroscopy to probe samples under extreme conditions. Key techniques include:
At petapascal pressures, traditional pressure markers may themselves undergo phase transitions or become opaque. Researchers are developing new standards based on:
Next-generation facilities aim to combine petapascal pressures with other extreme conditions:
As experiments push deeper into the petapascal regime, theorists face new challenges:
Petapascal research informs diverse fields:
Behind every pressure measurement lies hours of meticulous work:
At these extreme conditions, conventional physics gives way to exotic phenomena:
Remaining obstacles in petapascal research include:
Under petapascal pressures, elements behave in ways that defy conventional wisdom. Oxygen forms metallic polymers, sodium becomes transparent, and hydrogen may transform into a room-temperature superconductor. These transformations aren't merely academic curiosities—they represent fundamental tests of our understanding of quantum mechanics and condensed matter physics.
First predicted in 1935 by Wigner and Huntington, metallic hydrogen remains the "holy grail" of high-pressure physics. Recent experiments at pressures approaching 500 GPa have shown promising signs of this phase, but definitive proof remains elusive. The challenges are immense:
Laboratory studies at petapascal pressures directly inform our understanding of astrophysical objects:
| Planetary Body | Core Pressure Range | Relevant Experiments |
|---|---|---|
| Earth | 300-400 GPa | Fe alloy phase diagrams |
| Jupiter | 300-700 GPa | Hydrogen phase transitions |
| Neutron star crust | >106 GPa | Nuclear pasta analogs |