Tiny cages hold big promise. Understanding the chemical reactions that can create tiny molecular cages that hold other “guest” molecules—structures called clathrates—is key to creating a new generation of electronic devices and possible energy materials. Timothy Strobel and team are the first to use high-pressure synthesis to create a stable clathrate of sodium (Na) and silicon (Si)—the least understood system of the so-called group 14 clathrates. Strobel also created a new clathrate of hydrogen sulfide (H2S) and molecular hydrogen (H2). Both findings open the door for major advances in materials science.
Until now, scientists created most silicon clathrates by heating a chemical precursor in a vacuum. This process, however, is not ideal for controlling growing conditions and keeping the cages stable. Synthesis under high pressure provides a reliable means to control growth of certain other materials (e.g., diamond and cubic boron nitride), so the Strobel team decided to pursue that approach for clathrates. They subjected mixtures of Na-Si to various pressures and temperature regimes and found a type of clathrate, Na8Si46, that formed at pressures ranging from 20,000 to 60,000 times atmospheric pressure (2 to 6 gigapascals, GPa) and temperatures of 1160 to 1520°F (900 to 1100 K). When the pressure was increased to 79,000 atmospheres (8 GPa), a new clathrate structure NaSi6 formed. The latter material behaves like a metal and has never been seen before.
In addition to these experiments, the researchers performed calculations to predict how the materials would behave. Calculations and experiments revealed that sodium clathrates are thermodynamically stable at high pressure. The consistency suggests that scientists can use theoretical calculations to predict new synthesis routes for other compounds.
Strobel and colleagues also discovered that a clathrate formed from hydrogen sulfide and molecular hydrogen (H2S + H2) exhibited different behaviors under different pressure conditions. At pressures of 35,000 atmospheres (3.5 GPa), the compound crystallized into a stable “guest/host” structure held together by weak attractive forces (van der Waals forces). At 170,000 atmospheres (17 GPa), the hydrogen bonding increased between neighboring H2S molecules. This type of bonding alone, however, was not sufficient for stability; when the hydrogen molecules were removed from the cage, the H2S molecule framework collapsed. The research showed that novel cooperative interactions arising from molecular packing are necessary to hold such structures together.