Title: Phase transformation B1 to B2 in TiC, TiN, ZrC and ZrN under pressure
Author(s):

	V.I. Ivashchenko (Institute of Problems of Material Science of the National Academy of Sciences of Ukraine, 3 Krzhyzhanovsky St., 03142 Kyiv, Ukraine ) ,
	P.E.A. Turchi (Lawrence Livermore National Laboratory (L-352), P.O. Box 808, Livermore, CA 94551, USA) ,
	V.I. Shevchenko (Institute of Problems of Material Science of the National Academy of Sciences of Ukraine, 3 Krzhyzhanovsky St., 03142 Kyiv, Ukraine )

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.

Key words: titanium and zirconium carbides and nitrides, first-principles calculations, phase transformation, electronic and phonon structures
PACS: 64.70.K-, 63.20.-e, 71.15.Nc, 71.20.-b

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