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This thesis investigates a transformation path between the Th3P4-type and thespinel-type structures of hafnium (IV) and titanium (IV) nitride, M3N4 (M = Hf,Ti) with computational methods. For both compounds, the Th3P4-type wassynthesized experimentally at high-pressure conditions and was quenched toambient conditions. Computations reveal that while at high pressure Th3P4-typestructures are favored by enthalpy, at low pressure the Th3P4-type is onlymeta-stable. The spinel-type is energetically more favorable at zero pressurethan the Th3P4- type in both systems. However, even the spinel-type issurpassed by another ground state modification for both Hf3N4 and Ti3N4. Theresearch presented in this thesis then addresses (i) thermal stability of theTh3P4 polymorphs and (ii) the conditions necessary to synthesize spinel-typestructures of both compounds. The foundation for the study is a simplestructural relation between Th3P4-type and the spinel-type structures: whilethe Th3P4-type is based on a body-centered cubic arrangement of anions withcations in interstitial sites, the spinel-type is best described by aface-centered cubic arrangement of anions with cations in interstitial sites.With anions following the Bain path of the bcc-fcc transformation, and cationsundergoing a minor shuffle distortion, the Th3P4 to spinel transformationparallels the martensitic transformation known in many metal systems -- onlywith additional cations in the structure.

A Diffusionless Transformation Path Relating Th3P4 and Spinel Structure: Opportunities to Synthesize Ceramic Materials at High Pressures