New insights into how high pressures transform hydrogen into a liquid metal have been gleaned by crushing samples of deuterium using intense laser pulses. The research was done by an international team of physicists, who say that their study of the hydrogen isotope clears-up some discrepancies in results from previous experiments.
Hydrogen sits atop the alkali metals in the periodic table so it is not surprising that hydrogen could have a metallic phase. However, the element forms diatomic molecules over a wide range of temperatures and pressures and this makes it an insulator under normal conditions.
For nearly a century, increasingly powerful calculations have predicted that under very high pressures, hydrogen’s molecular bonds will break and the material will become an atomic solid – or liquid at higher temperatures. Under these conditions, hydrogen is expected to be a metal.
Confusing results
The required pressures are extremely high – in the 100s of gigapascals – and are very difficult to achieve in the laboratory. While several groups have reported evidence for liquid metallic hydrogen, the results have been confusing.
Now, Peter Celliers at the Lawrence Livermore National Laboratory LLNL) in California, Alexander Goncharov of the Carnegie Institution for Science in Washington, DC and an international team have done a series of experiments to try to gain more insight into how liquid deuterium becomes metallic at high pressures and high temperatures.
Deuterium is an isotope of hydrogen that contains a neutron in addition to a proton – essentially doubling its mass. This leads to a significant “isotopic shift” that is expected to cause the insulator-metal transition to occur under slightly different conditions compared with hydrogen. Doing experiments with hydrogen and well as deuterium provides an important test of any theory that claims to describe hydrogen at extreme pressures.
Copper piston
The team began a measurement by condensing a thin layer of liquid deuterium between two solid plates. One plate is a copper “piston”, which is mounted on a hollow capsule called a “hohlraum” and the other plate is a transparent window. When intense pulses from LLNL’s National Ignition Facility laser strike the holhraum, it blows apart and forces the piston plate against the window.
The team made optical measurements of pressure, reflectivity and other properties of the deuterium over a timescale of about 35 ns. The team then calculated the temperature of the liquid using theoretical assumptions – this is necessary because measuring temperature is extremely difficult in such “dynamic” measurements.
During the compression process, a series of pressure waves travel through the deuterium. This ramps-up the pressure of the deuterium to about 600 GPa and the temperature to nearly 2000 K.
On reflection
At low pressures, the liquid deuterium is transparent to light – which means that it is an insulator. As the pressure rose to about 150 GPa, the liquid absorbed light and became opaque. Then, at about 200 GPa the liquid began to reflect light as would be expected if it was making a transition from and insulator to a metal.
The researchers say that their observations provide valuable information to physicists who are trying to develop accurate computer simulations of the properties of hydrogen at extreme pressures and temperatures. This is of great interest to physicists studying the interiors of gas-giant planets such as Jupiter, which are believed to contain liquid metallic hydrogen.
LLNL’s Marius Millot says, “These results are a true experimental tour de force and are particularly important because they provide a very stringent test on the different varieties of numerical simulations that one can use to predict the properties of planetary constituents at high pressure — necessary to model the internal structure and evolutionary processes of Jupiter and Saturn.”
Phase diagram
According to Isaac Silvera of Harvard University, the ultimate goal of this and other experiments is to map-out the phase diagram of hydrogen to determine the boundary between the insulator and metal phases as a function of pressure and temperature. This has proven difficult to do using dynamic experiments, which rely on calculated temperatures.
Indeed, these calculations may be the origin of discrepancies between different dynamic experiments – and could also explain discrepancies between some dynamic and static measurements that are apparent at lower pressures.
Show us your metal
Static measurements are made by squeezing hydrogen in diamond anvil cells, where it can be studied for long period of time and its temperature measured accurately. The problem with static measurements, however, is that they cannot reach the same high pressures as the dynamic experiments.
Silvera and colleagues have just performed a series of static studies of dense fluid deuterium (and hydrogen) and his results are in some agreement with the LLNL work. This suggests that the temperature calculations done by Celliers and colleagues are robust. However, Silvera does say that he disagrees with how the LLNL team interprets temperature plateaus in their data as being related to the onset of the light-absorbing phase, rather than the onset of the metallic phase.
The LLNL study is described in Science and Silvera’s recent work is described on arXiv.
