A Diffusionless Transformation Path Relating Th3P4 and Spinel Structure:   Opportunities to Synthesize Ceramic Materials at High Pressures
  This thesis investigates a transformation path between the Th3P4-type and the spinel-type structures of hafnium (IV) and titanium (IV) nitride, M3N4 (M = Hf, Ti) with computational methods. For both compounds, the Th3P4-type was synthesized experimentally at high-pressure conditions and was quenched to ambient conditions. Computations reveal that while at high pressure Th3P4-type structures are favored by enthalpy, at low pressure the Th3P4-type is only meta-stable. The spinel-type is energetically more favorable at zero pressure than the Th3P4- type in both systems. However, even the spinel-type is surpassed by another ground state modification for both Hf3N4 and Ti3N4. The research presented in this thesis then addresses (i) thermal stability of the Th3P4 polymorphs and (ii) the conditions necessary to synthesize spinel-type structures of both compounds. The foundation for the study is a simple structural relation between Th3P4-type and the spinel-type structures: while the Th3P4-type is based on a body-centered cubic arrangement of anions with cations in interstitial sites, the spinel-type is best described by a face-centered cubic arrangement of anions with cations in interstitial sites. With anions following the Bain path of the bcc-fcc transformation, and cations undergoing a minor shuffle distortion, the Th3P4 to spinel transformation parallels the martensitic transformation known in many metal systems -- only with additional cations in the structure. 
Arindom Goswami
cond-mat.mtrl-sci