The structure of calcium-ammonia solutions by neutron diffraction

Area of Science:

• Materials Science
• Physical Chemistry
• Solution Chemistry

Background:

• Understanding the microscopic structure of metallic solutions is crucial for explaining their unique properties.
• Calcium-ammonia solutions exhibit phase separation, transitioning between metallic and non-metallic states.

Purpose of the Study:

• To elucidate the microscopic structures of calcium-ammonia solutions using neutron diffraction.
• To investigate the intermediate-range ordering and phase separation phenomena.
• To determine the solvation shell of calcium ions and the extent of hydrogen bonding.

Main Methods:

• Neutron diffraction at 230 K to measure total structure factors.
• Isotopic labeling (*N by 15N) with difference analysis for solvent structure investigation.
• Classical Monte Carlo computer simulations for detailed analysis.

Main Results:

• An intense diffraction prepeak was observed in metallic solutions, intensifying and shifting with increasing metal concentration (4 to 10 mole percent).
• This prepeak indicates well-developed intermediate-range ordering among solvated cations, correlating with phase separation.
• Calcium ions are solvated by approximately 6-7 ammonia molecules with a Ca-N distance of ~2.45 Å.
• Evidence of hydrogen bonding was found within the ammonia solvent molecules.

Conclusions:

• The observed diffraction prepeak is a microstructural signature of phase separation in calcium-ammonia solutions.
• Neutron diffraction and simulations provide detailed insight into calcium solvation and hydrogen bonding.
• Solvation and hydrogen bonding persist even in concentrated metallic solutions.