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High pressure synthesis of rhenium nitride pernitride ReN$_2$ with crystal structure unusual for transition metal dinitrides and high values of hardness and bulk modulus attracted significant attention to this system. We investigate the thermodynamic and electronic properties of the P2$_1$/c phase of ReN$_2$ and compare them with two other polytypes, C2/m and P4/mbm phases suggested in the literature. Our calculations of the formation enthalpy at zero temperature show that the former phase is the most stable of the three up to pressure p=170 GPa, followed by the stabilization of the P4/mbm phase at higher pressure. The theoretical prediction is confirmed by diamond anvil cell synthesis of the P4/mbm ReN$_2$ at $\approx$175 GPa. Considering the effects of finite temperature in the quasi-harmonic approximation at p=100 GPa we demonstrate that the P2$_1$/c phase has the lowest free energy of formation at least up to 1000 K. Our analysis of the pressure dependence of the electronic structure of the rhenium nitride pernitride shows a presence of two electronic topological transitions around 18 GPa, when the Fermi surface changes its topology due to an appearance of a new electron pocket at the high-symmetry Y$_2$ point of the Brillouin zone while the disruption of a neck takes place slightly off from the $\Gamma$-A line., Comment: 15 pages, 7 figures, 3 tables

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN_{4}. A triclinic phase of beryllium tetranitride tr-BeN_{4} was synthesized from elements at ∼85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN_{4} layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN_{4} layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN_{4} layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.

Ab initio-calculated ideal strength and toughness describe the upper limits for mechanical properties attainable in real systems and can, therefore, be used in selection criteria for materials design. We employ density-functional ab initio molecular dynamics (AIMD) to investigate the mechanical properties of defect-free rocksalt-structure (B1) TiN and $\text{B}1\mathrm{T}{\mathrm{i}}_{1\ensuremath{-}x}\mathrm{A}{\mathrm{l}}_{x}\mathrm{N}\phantom{\rule{4pt}{0ex}}(x=0.25$, 0.5, 0.75) solid solutions subject to [001], [110], and [111] tensile deformation at room temperature. We determine the alloys' ideal strength and toughness, elastic responses, and ability to plastically deform up to fracture as a function of the Al content. Overall, TiN exhibits greater ideal moduli of resilience and tensile strengths than (Ti,Al)N solid solutions. Nevertheless, AIMD modeling shows that, irrespective of the strain direction, the binary compound systematically fractures by brittle cleavage at its yield point. The simulations also indicate that $\mathrm{T}{\mathrm{i}}_{0.5}\mathrm{A}{\mathrm{l}}_{0.5}\mathrm{N}$ and $\mathrm{T}{\mathrm{i}}_{0.25}\mathrm{A}{\mathrm{l}}_{0.75}\mathrm{N}$ solid solutions are inherently more resistant to fracture and possess much greater toughness than TiN due to the activation of local structural transformations (primarily of B1 \ensuremath{\rightarrow} wurtzite type) beyond the elastic-response regime. In sharp contrast, (Ti,Al)N alloys with 25% Al exhibit similar brittleness as TiN. The results of this work are examples of the limitations of elasticity-based criteria for prediction of strength, brittleness, ductility, and toughness in materials able to undergo phase transitions with loading. Comparing present and previous findings, we suggest a general principle for design of hard ceramic solid solutions that are thermodynamically inclined to dissipate extreme mechanical stresses via transformation toughening mechanisms.

In this work, we discuss the mixing thermodynamics of cubic (B1) Ti1-xAlxN/TiN(001) multilayers. We show that interfacial effects suppress the mixing enthalpy compared to bulk Ti1-xAlxN. The strong ...

The potential of quarternary wurtzite TMx/2Mx/2Al1-xN (TM = Ti, Zr, Hf; M = Mg, Ca, Zn) alloys for piezoelectric applications is investigated using first-principles calculations. All considered alloys show increased piezoelectric response compared to pure AlN, and competing with the best ternary system proven to date: ScAlN. (Zr, Hf)(x/2)(Mg, Ca)(x/2)Al1-xN alloys are particularly promising. Calculations reveal positive mixing enthalpies indicative for phase separating systems; their values are smaller compared to related nitride alloys, which still can be grown as metastable thin films. The wurtzite phase of the alloys is lowest in energy at least up to x = 0.5 and for Tix/2Znx/2Al1-xN in the full composition range. Moreover, calculations reveal that wurtzite TM0.5Zn0.5N (TM = Ti, Zr, Hf) are piezoelectric alloys with d(33,f) = 19.95, 29.89, and 24.65 pC/N respectively, up to six times that of AlN. Finally, we discuss the physical origin behind the increased piezoelectric response and show that the energy difference between tetrahedrally coordinated zinc-blende (B3) and the layered hexagonal (B-k) phases of the TM0.5M0.5N alloy can be used as a descriptor in a high-throughput search for complex wurtzite alloys with high piezoelectric response. Funding Agencies|Knut and Alice Wallenberg Foundation Scholar Grant; Swedish Research Council (VR) Linkoping Linnaeus Initiative LiLi-NFM [2008-6572]; Swedish Government Strategic Research Area Grant in Materials Science on Advanced Functional Materials [MatLiU 2009-00971]; Swedish Research Council [621-2011-4417, 330-2014-6336]; Swedish Foundation for Strategic Research (SSF) Programme SRL [10-0026]; Swedish Research Council (VR) [621-2011-4426]

Recently, ScAlN alloys attracted attention for their giant piezoelectric moduli. In this study the piezoelectric response of the wurtzite group-III nitrides AlN, GaN, and InN mixed with 50 mol% of ...

We present a theoretical analysis on the applicability of Vegards linear rule in InxAl1-xN alloys in relation to strain related elastic and piezoelectric properties. We derive the elastic stiffness ...

We use metastable NaCl-structure Ti0.5Al0.5N alloys to probe effects of configurational disorder on adatom surface diffusion dynamics which control phase stability and nanostructural evolution duri ...

In this study we discuss the performance of the special quasirandom structure (SQS) method in predicting the elastic properties of B1 (rocksalt) Ti${}_{0.5}$Al${}_{0.5}$N alloy. We use a symmetry-based projection technique, which gives the closest cubic approximate of the elastic tensor and allows us to align the SQSs of different shapes and sizes for a comparison in modeling elastic tensors. We show that the derived closest cubic approximate of the elastic tensor converges faster with respect to SQS size than the elastic tensor itself. That establishes a less demanding computational strategy to achieve convergence for the elastic constants. We determine the cubic elastic constants $({C}_{ij})$ and Zener's type elastic anisotropy $(A)$ of Ti${}_{0.5}$Al${}_{0.5}$N. Optimal supercells, which capture accurately both the configurational disorder and cubic symmetry of elastic tensor, result in ${C}_{11}=447$ GPa, ${C}_{12}=158$ GPa, and ${C}_{44}=203$ GPa with 3$%$ of error and $A=1.40$ with 6$%$ of error. In addition, we establish the general importance of selecting proper SQS with symmetry arguments to reliably model elasticity of alloys. We suggest the calculation of nine elastic tensor elements: ${C}_{11}$, ${C}_{22}$, ${C}_{33}$, ${C}_{12}$, ${C}_{13}$, ${C}_{23}$, ${C}_{44}$, ${C}_{55}$, and ${C}_{66}$, to analyze the performance of SQSs and predict elastic constants of cubic alloys. The described methodology is general enough to be extended for alloys with other symmetry at arbitrary composition.

The origin of the anomalous, 400% increase of the piezoelectric coefficient in ScxAl1-xN alloys is revealed. Quantum mechanical calculations show that the effect is intrinsic. It comes from a stron ...

Mechanical and thermodynamic stability of the isoelectronic ternary inverse perovskites Sc3EN (E=B,Al,Ga,In) has been studied from first principles. We confirm stability of recently synthesized cub ...

Wehave performed ab initio calculations for the cubic inverse-perovskite Sc3EN(E=Al,Ga,In) systems to study their electronic band-structures and elastic properties.In this study, we used the accurate augmented plane waveplus local orbital method to find the equilibrium structural parametersand to compute the full elastic tensors. The obtained single-crystalelastic constants were used to quantify the stiffness of theSc-based ternary nitrides and to appraise their mechanical stability. Thesite-projected density of states, Fermi surfaces, and the charge-density plotshave also been used to analyze the chemical bonding betweenthe Sc6N cluster and the surrounding metallic lattice of eitherAl, Ga, or In atoms. Our calculations show that Sc3GaNhas the largest covalent Sc-N bonding-type character with the highestYoung, shear, and bulk moduli. Compared with the other twoisoelectronic systems, it also behaves as the most brittle materialwith a relatively large elastic anisotropy. Original Publication:Maurizio Mattesini, Martin Magnuson, Ferenc Tasnádi, Carina Höglund, Igor A. Abrikosov and Lars Hultman, Elastic properties and electrostructural correlations in ternary scandium-based cubic inverse perovskites: A first-principles study, 2009, Physical Review B. Condensed Matter and Materials Physics, (79), 125122.http://dx.doi.org/10.1103/PhysRevB.79.125122Copyright: American Physical Societyhttp://www.aps.org/