Thermodynamic Ground States of Platinum Metal Nitrides

Abstract

The thermodynamic stabilities of various phases of the nitrides of the platinum metal elements are systematically studied using density functional theory. It is shown that for the nitrides of Rh, Pd, Ir and Pt two new crystal structures, in which the metal ions occupy simple tetragonal lattice sites, have lower formation enthalpies at ambient conditions than any previously proposed structures. The region of stability with respect to those structures extends to 17 GPa for PtN2. Calculations show that the PtN2 simple tetragonal structures at this pressure are thermodynamically stable also with respect to phase separation. The fact that the local density and generalized gradient approximations predict different values of the absolute formation enthalpies as well different relative stabilities between simple tetragonal and the pyrite or marcasite structures are further discussed. all these compounds have been shown to be at least metastable at ambient conditions. The resulting crystal structures have been investigated by several groups both experimentally and theoretically [2][3][4][5][6][7][8], and by now consensus has been reached concerning the observed crystal structures and stoichiometry (one metal atom for every nitrogen dimer). PtN 2 and PdN 2 are formed in the pyrite crystal structure, a cubic phase with the metal atoms occupying fcc sites. The nitrogen dimers are centered around the fcc octahedral interstitial sites, oriented in all the four possible 111 directions, such that all the nearest neighbor dimers make an angle of 70.53• with each other. A rotation of dimers such that two pairs point in the [111] and [111] direction, respectively, results in the marcasite structure, the predicted ground state phase of RuN 2 , RhN 2 and OsN 2 [7]. A small lattice distortion of marcasite then yields the monoclinic baddeleyite (CoSb 2 ) structure found in IrN 2 [5,7].The low-pressure phase diagrams of these systems are still not well understood. In particular their degree of thermodynamic stability has only been addressed briefly [3], where it was proposed that the Pt-N compound is only metastable at low pressures. In this work, we address this issue by calculating the formation enthalpies of all the PM nitrides as a function of pressure. We also report the discovery of two new low-energy crystal structures that at low pressures are thermodynamically more stable than any of the crystal phases that have up to now been synthesized experimentally or calculated from theory. To our knowledge, these new crystalline phases have not previously been observed in any other compound.

A novel layer-structured PtN2: First-principles calculations

“…Comparing to pure Pt (ρ = 21.46 g/cm 3 ), the density of these studied structures reduced significantly owing to the presence of light element N. Among these structures, the pyrite structure has the largest density, while the density of the two layer structures SH and newly uncovered layer structure is less than 50 % of that for pure Pt. Concerning the enthalpies of the studied structures, the ST structure has the lowest enthalpy (ΔH = -0.9622 eV), which is consistent with [13]. The pyrite structure has the second lowest enthalpy (ΔH = -0.7335 eV) and marcasite, CoSb 2 , and LS have almost equal enthalpies (ΔH = -0.5921, -0.5918, and -0.5977 eV, respectively).…”

“…The SH structure contains peculiar double N＝N bonded N 2 units (the bond length is 1.179 Å), which was also uncovered for MN 2 (M = Os, Ir, Ru, and Rh) compounds [34]. The LS together with the ST structure proposed in [13] can be considered as layer structures, as their interlayer distance is 2.902 Å and 2.840 Å, respectively. It is necessary to emphasize that the ST structure has also been reproduced by the present setting of evolutionary simulations.…”

“…An independent theoretical work by Wessel and Dronskowski [12] reached the same conclusion and emphasized that PtN 2 is incorrectly formulated as "nitride" despite the fact that by definition nitride phase should contain isolated nitrogen and it must be called as platinum pernitride based on the presence of nitrogen dimmers. In 2008, a simple tetragonal structure (P4/mbm) was predicted to be more thermodynamically stable than the pyrite structure PtN 2 at low pressures and indicated that it can be obtained from marcasite or pyrite structures through martensitic transformation [13]. Yildiz et al [14] performed a systematic study of the ground state properties of the zinc-blende, rock-salt, tetragonal, cuprite, fluorite, and pyrite phases of platinum nitride.…”

Predicting new superhard phases

“…However, the true crystal structure of cBC 2 N still remains debatable. For the compounds formed by heavy transition metals and light elements, a variety of structure types within the substitutional methodology were considered to design new superhard materials, including the zinc-blende, rock salt, pyrite, rutile, fluorite, WC, CaCl 2 , MoC 2 , and CoSb 2 types [61][62][63][64][65][66][67][68][69][70][71][72]. The theoretical results demonstrated that these compounds possess ultrahigh bulk modulus (ultraincompressibility).…”

Structure and mechanical properties of tantalum mononitride under high pressure: A first-principles study

“…[1][2][3] Especially, 5d-TM dinitrides (PtN 2 , IrN 2 , and OsN 2 ) have been studied widely and synthesized under high pressure and temperature, showing very high bulk modulus and high shear modulus. [4][5][6][7][8][9] However, the drawback of these TMN 2 is their extreme high-pressure condition for synthesizing them. Moreover, theoretical results show that experimental synthesized of 5d-TMN 2 are metastable.…”