Structural and thermogravimetric studies of alkali metal amides and imides

Thesis

This work presents an in-depth study of the crystal structures and hydrogen sorption potential of the Li - N - H and Li - Na - N - H systems. The structures of the materials have been studied using X-ray and neutron diffraction, Raman spectroscopy and inelastic neutron scattering. The behavior of the materials during heating was studied using variable temperature X-ray diffraction, intelligent gravimetric analysis in conjunction with neutron diffraction, intelligent gravimetric analysis combined with mass spectrometry and differential scanning calorimetry.

The role of cation disorder in the Li - N - H (D) system has been explored, indicating that crystallographic ordering of the Li⁺ ions within lithium amide and lithium imide significantly affects the hydrogen sorption properties of the materials. Order-disorder transitions were observed both during hydrogen desorption from ordered LiNH₂ and during deuterium adsorption on ordered Li₂ND. Such transitions were not observed in disordered samples of the materials. The intrinsic disorder and the stoichiometry of Li - N - H(D) materials was shown to depend strongly on the techniques used during their synthesis.

Studies regarding the synthesis, crystal chemistry and decomposition properties of the mixed Li / Na amides are presented. Two distinct mixed Li / Na amides of formulae Li₃Na(NH₂)₄ and LiNa₂(NH₂)₃ were observed in the LiNH₂ / NaNH₂ phase space. Na was also seen to be soluble in LiNH₂, forming sodium-doped LiNH₂ . Li₃Na(NH₂)₄ and Na-doped LiNH₂ were found to exhibit significant cation non-stoichiometry, whereas LiNa₂(NH₂)₄ was shown to exist as a line phase material.

Thermogravimetric and calorimetric studies of the mixed Li / Na amides suggested that these materials decompose primarily with loss of H₂.

Authors: Lowton, R

• Publication date: 1999
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: Oxford University, UK
• Language: English
• Keywords: alkali metal imidies; scanning calorimetry; alkali metal amides; gravimetric analysis; neutron diffraction; x-ray diffraction
• Subjects: Inorganic chemistry