  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.
