Abstract
Phase stability of various phases of MX (M = Ti, Zr; X = C, N) at equilibrium and under pressure is examined based on first-principles calculations of the electronic and phonon structures. The results reveal that all B1 (NaCl-type) MX structures undergo a phase transition to the B2-structures under high pressure in agreement with the previous total-energy calculations. The B1-MX structures are dynamically stable under very high pressure (210-570 GPa). The pressure-induced B2 (CsCl-type) MC phases are dynamically unstable even at high pressures, and TiN and ZrN are found to crystallize with the B2-structure only at pressures above 55 GPa. The first-order B1-to-B2 phase transition in these nitrides is not related to the softening of phonon modes, and the dynamical instability of B2-MX is associated with a high density of states at the Fermi level.
Highlights
Transition metal compounds (TMC) form a class of materials with the NaCl-type crystal structure (B1) in a wide composition range, and exhibit extremely high melting points, hardness, and metallic conductivity [1, 2]
Phase stability of various phases of MX (M = Ti, Zr; X = C, N) at equilibrium and under pressure was examined based on the first-principles calculations of the electronic and phonon structures
It follows from the total energy calculations that all B1-MX structures undergo a phase transformation to the B2-structures under high pressure in agreement with the previous total-energy calculations
Summary
Transition metal compounds (TMC) form a class of materials with the NaCl-type crystal structure (B1) in a wide composition range, and exhibit extremely high melting points, hardness, and metallic conductivity [1, 2]. These materials are exploited under extreme conditions (high temperature, high pressure, etc.), and they are widely used as the main layers in ultra-hard nano-composite coatings [3]. It is important to understand the peculiarities of their behavior under extreme conditions In this investigation we focus on titanium and zirconium carbides and nitrides (MX, M = Ti, Zr; X = C, N) that crystallize with the NaCl-type (B1) lattice since their high-pressure stability is not very well known. Note that a description of these phases can be found in reference [6]
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