Structural, elastic, vibrational, thermophysical properties and pressure-induced phase transitions of ThN2, Th2N3, and Th3N4: An ab initio investigation

Abstract

The structural, electronic, elastic, lattice dynamical properties and pressure-induced phase transitions in ThN2, Th2N3, and Th3N4 have been investigated through density functional theory based electronic band structure calculations. Our theoretical calculations on ThN2 reveal the monoclinic structure (C2/m space group) at 0 GPa instead of the previously reported cubic (Fm3¯m spatial crystal symmetry) phase [K. O. Obodo and N. Chetty, J. Nucl. Mater. 440, 229 (2013)]. More refined calculations on enthalpy of formation reveal that this ground state C2/m phase of ThN2 transforms to an orthorhombic structure (Pnma symmetry) at a pressure of ∼7 GPa. In agreement with experimental observations, we predict the La2O3-type trigonal structure (P3¯ml symmetry) in Th2N3 at ambient conditions, which is further predicted to transform to an initial monoclinic structure again at ∼62 GPa. Our theoretical results also agree with the experiment regarding the rhombohedral structure (R3¯m symmetry) of Th3N4 revealed at 0 GPa, which, at ∼37 GPa, is predicted to transform to an another rhombohedral structure with reduced space group symmetry of R3¯. The predicted structural phases are further substantiated with the mechanical and dynamical stability criteria in the pressure regime of their structural stability. Furthermore, the electronic band structure calculations at zero pressure suggest that with limited density of states above Fermi energy, ThN2 and Th2N3 exhibit semi-metallic characteristics, whereas a bandgap of ∼1.44 eV in Th3N4 makes it a semiconductor. The semiconducting nature of Th3N4 ceases at a transition pressure of ∼62 GPa.