New Zirconium Diboride Polymorphs-First-Principles Calculations

Two new hypothetical zirconium diboride (ZrB2) polymorphs: (hP6-P6(3)/mmc-space group, no. 194) and (oP6-Pmmn-space group, no. 59), were thoroughly studied under the first-principles density functional theory calculations from the structural, mechanical and thermodynamic properties point of view. The proposed phases are thermodynamically stable (negative formation enthalpy). Studies of mechanical properties indicate that new polymorphs are less hard than the known phase (hP3-P6/mmm-space group, no. 191) and are not brittle. Analysis of phonon band structure and density of states (DOS) also show that the phonon modes have positive frequencies everywhere and the new ZrB2 phases are not only mechanically but also dynamically stable. The estimated acoustic Debye temperature, Theta(D), for the two new proposed ZrB2 phases is about 760 K. The thermodynamic properties such as internal energy, free energy, entropy and constant-volume specific heat are also presented.