New Metal Nitride Compounds: Can they be Synthesized at High-Pressures?
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1040-Q09-23
New Metal Nitride Compounds: Can they be Synthesized at High-Pressures? Peter Kroll Chemistry and Biochemsitry, University of Texas at Arlington, 700 Planetarium Pl, Arlington, TX, 76019 ABSTRACT We apply our procedure of including nitrogen fugacity into thermochemical calculations to compute phase diagrams in the rhenium-nitrogen and ruthenium-nitrogen systems. The combination of first-principle and thermochemical calculations let us predict the sequential nitridation of Re at high-pressure/high-temperature conditions. At 3000 K, Re will react with nitrogen at about 32 GPa yielding ReN. Formation of ReN2 with CoSb2-type structure is predicted for pressures exceeding 50 GPa at this temperature. The recently proposed marcasitetype RuN2 will be attainable at 3000 K at pressures above 30 GPa from a mixture of Ru and RuN2. INTRODUCTION In our continuing interest in computing phase diagrams of new nitride compounds we recently introduced a scheme to explicitly include the fugacity of nitrogen in thermo-chemical calculations at high-pressure/high-temperature conditions [1]. We studied the syntheses of new modifications of Hf3N4 and Zr3N4 [2], proposed new compounds of Ta3N5 and WN2 attainable in HP/HT experiments [1], contributed to the exploration of the Pt-N phase diagram [3], and gave an account of the possible decomposition of C3N4 into carbon and nitrogen [4]. Following the recent synthesis and characterization of a new PtN2 compound, a pernitride exhibiting N2-dumbells [5,6], several studies focused on the possibility of such structures among the transition metals including Os, Ir, and Ru [7]. For some transition metals, however, even a mononitride MN is unknown. Rhenium nitride can only be synthesized with up to about 50% nitrogen content [8]. A Ru mononitride with a proposed NaCl-type structure was only recently synthesized [9]. In this contribution we investigate the nitrogen-rich side of the phase systems Re-N and Ru-N. We will apply our procedure of including nitrogen fugacity into the derivation of a phase diagram using thermochemical calculations. THEORY For the computation of structure and energy we employ density functional theory as implemented in the Vienna Ab-initio Simulation Package (VASP) [10-13]. We prefer the generalized-gradient approximation (GGA) for treating exchange and correlation of the electrons, since GGA describes more accurately enthalpy differences between structures with different coordination environments of atoms. We use 500 eV for the expansion of the wave function into a plane-wave basis set and make use of the projector-augmented plane wave m
ethod (PAW). The Monkhorst-Pack scheme is used for the sampling of the Brillouinzone. All structures are optimized so that individual residual forces are below 10-3 eV/Å. Overall we find that the computed energies are converged to about 1 meV per atom with respect to our choice of basis set, k-point grids, and atomic relaxations. The entropy contributions to the free enthalpy are estimated for the reactant nitrogen only. For a d
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