Ab Initio Thermodynamics Research Articles

Epitaxial Core-Shell Oxide Nanoparticles: First-Principles Evidence for Increased Activity and Stability of Rutile Catalysts for Acidic Oxygen Evolution.

Due to their high activity and favorable stability in acidic electrolytes, Ir and Ru oxides are primary catalysts for the oxygen evolution reaction (OER) in proton‐exchange membrane (PEM) electrolyzers. For a future large‐scale application, core‐shell nanoparticles are an appealing route to minimize the demand for these precious oxides. Here, we employ first‐principles density‐functional theory (DFT) and ab initio thermodynamics to assess the feasibility of encapsulating a cheap rutile‐structured TiO2 core with coherent, monolayer‐thin IrO2 or RuO2 films. Resulting from a strong directional dependence of adhesion and strain, a wetting tendency is only obtained for some low‐index facets under typical gas‐phase synthesis conditions. Thermodynamic stability in particular of lattice‐matched RuO2 films is instead indicated for more oxidizing conditions. Intriguingly, the calculations also predict an enhanced activity and stability of such epitaxial RuO2/TiO2 core‐shell particles under OER operation.

An equilibrium ab initio atomistic thermodynamics investigation of hydrogen adsorption on the low index iridium surfaces and the morphology of iridium nanoparticles

The ab initio atomistic thermodynamics technique was used to calculate the surface free energy diagrams. These diagrams were obtained for hydrogen over the unreconstructed low-index iridium surfaces. Results indicated that the Ir(110) surface acquired the highest hydrogen intake among the low-index surfaces with a coverage of up to 2.33 ML. However, high-pressure hydrogen conditions were essential to maintain the stability of this phase. The Ir(100) surface required −0.81 eV of hydrogen chemical potential to produce a stable H/Ir phase with a 1.0 ML coverage. This potential represented the lowest value among the low-index surfaces. Phase diagram calculations of H/Ir(111) illustrated that the 0.75 ML phase has the highest stability at low to moderate hydrogen pressures. On the other hand, the 1.25 ML-H/Ir(111) phase became the most stable under high-pressure conditions. Finally, Wulff construction was implemented to predict the shapes of iridium nanoparticles in a hydrogen environment. Predictions indicated a truncated-octahedron shape for the nanoparticles irrespective of the value of the hydrogen chemical potential.

Atomistic prediction on the composition- and configuration-dependent bandgap of Ga(As,Sb) using cluster expansion and ab initio thermodynamics

The composition- and configuration-dependent bandgaps of pseudobinary Ga(As,Sb) are examined by the cluster expansion method and statistical thermodynamics based on density functional theory. The bandgaps and energetic stability of 330,000 configurations in the entire composition range show a consistent inverse relationship, in that a configuration with lower energy has a higher bandgap for a given composition. This inverse relation can be deduced from the opposite signs of effective cluster interaction coefficients for bandgap and energy, and can be quantified by the correlations of properties with short-range order parameters. The bandgap of GaAs0.5Sb0.5 varies from 0.02 to 0.93 eV depending on the atomic configuration, which suggests another tremendous chance to tune the bandgap by the configuration control. The average bandgap of a certain composition, calculated by the ab initio thermodynamics, decreases with increasing temperature. The calculated average bandgap shows feasible agreement with the experimental bandgap, reproducing the bandgap bowing.

Layered Pt-Co alloys: Bulk, surface and nanoparticle analysis, based on DFT

Pt-based catalysts are commonly used as cathode materials in various types of fuel cells. In this work we apply DFT calculations combined with cluster expansion method to elucidate the most stable structures of Pt-Co bulk alloy across the full range of Pt (or Co) concentrations. Our calculations reveal high stability of alloy configurations with alternating layers in (001) crystallographic planes of face centered cubic (fcc) structure exclusively occupied either by Pt or Co atoms. In particular, we find that Pt1Co1, Pt2Co1 and Pt3Co1 layered structures (periodically repeated 1–3 Pt layers, separated by single Co layers) have exceptionally high stability with Pt2Co1 possessing the lowest energy of formation among all analysed alloy configurations. Employing ab initio thermodynamics approach, we discuss stability of three types of surface terminations (Miller indices (100), (110) and (111)) for these three layered structures and also for highly symmetric fcc Pt3Co1 configuration, previously identified experimentally. Applying the Wulff construction, we obtain the nanoparticle shapes as functions of chemical potentials of constituent atoms (Pt or Co), allowing for design of computational models of Pt-Co nanoparticles and subsequent analysis of their chemical properties.

Relativistic Effects in Platinum Nanocluster Catalysis: A Statistical Ensemble-Based Analysis.

Nanoclusters are materials of paramount catalytic importance. Among various unique properties featured by nanoclusters, a pronounced relativistic effect can be a decisive parameter in governing their catalytic activity. A concise study delineating the role of relativistic effects in nanocluster catalysis is carried by investigating the oxygen reduction reaction (ORR) activity of a Pt7 subnanometer cluster. Global optimization analysis shows the critical role of spin-orbit coupling (SOC) in regulating the relative stability between structural isomers of the cluster. An overall improved ORR adsorption energetics and differently scaled adsorption-induced structural changes are identified with SOC compared to a non-SOC scenario. Ab initio atomistic thermodynamics analysis predicted nearly identical phase diagrams with significant structural differences for high coverage oxygenated clusters under realistic conditions. Though inclusion of SOC does not bring about drastic changes in the overall catalytic activity of the cluster, it is having a crucial role in governing the rate-determining step, transition-state configuration, and energetics of elementary reaction pathways. Furthermore, a statistical ensemble-based approach illustrates the strong contribution of low-energy local minimum structural isomers to the total ORR activity, which is significantly scaled up along the activity improving direction within the SOC framework. The study provides critical insights toward the importance of relativistic effects in determining various catalytic activity relevant features of nanoclusters.

Theoretical insight into the interaction between hydrogen and Hägg carbide (χ-Fe5C2) surfaces

In heterogeneous catalysis, the interaction between adsorbates and substrate surface plays an important role on surface structure evolution and even the reaction mechanism. Herein, unified DFT calculation and ab initio atomistic thermodynamics were adopted to explore the interaction between hydrogen and multiple Fe5C2 surfaces. The computation indicates that the dissociation of hydrogen molecule is both thermodynamically and kinetically favorable on Fe5C2 surfaces. The adsorption energy of hydrogen atoms is related to the summed H bond valance (SBVH), a higher SBVH corresponding to a more stable adsorption strength. Based on calculated surface free energy with different hydrogen coverage, the equilibrium crystalline morphology evolution of Fe5C2 under dynamic conditions was constructed with Wulff theorem. C-rich facets expose more area in Wulff construction with decreasing hydrogen chemical potential. The generation of carbon vacancy through reducing to methane has also been investigated, and the reduction of (111) and (11-1) facets is more facile than other facets. We anticipate this work could complement some knowledge to help understanding the surface structure and performance of iron carbide catalysts under experimental conditions.

From a bistable adsorbate to a switchable interface: tetrachloropyrazine on Pt(111).

Virtually all organic (opto)electronic devices rely on organic/inorganic interfaces with specific properties. These properties are, in turn, inextricably linked to the interface structure. Therefore, a change in structure can introduce a shift in function. If this change is reversible, it would allow constructing a switchable interface. We accomplish this with tetrachloropyrazine on Pt(111), which exhibits a double-well potential with a chemisorbed and a physisorbed minimum. These minima have significantly different adsorption geometries allowing the formation of switchable interface structures. Importantly, these structures facilitate different work function changes and coherent fractions (as would be obtained from X-ray standing wave measurements), which are ideal properties to read out the interface state. We perform surface structure search using a modified version of the SAMPLE approach and account for thermodynamic conditions using ab initio thermodynamics. This allows investigating millions of commensurate as well as higher-order commensurate interface structures. We identify three different classes of structures exhibiting different work function changes and coherent fractions. Using temperature and pressure as handles, we demonstrate the possibility of reversible switching between those different classes, creating a dynamic interface for potential applications in organic electronics.

Ga and Zn increase the oxygen affinity of Cu-based catalysts for the CO x hydrogenation according to ab initio atomistic thermodynamics.

The direct hydrogenation of CO or CO2 to methanol, a highly vivid research area in the context of sustainable development, is typically carried out with Cu-based catalysts. Specific elements (so-called promoters) improve the catalytic performance of these systems under a broad range of reaction conditions (from pure CO to pure CO2). Some of these promoters, such as Ga and Zn, can alloy with Cu and their role remains a matter of debate. In that context, we used periodic DFT calculations on slab models and ab initio thermodynamics to evaluate both metal alloying and surface formation by considering multiple surface facets, different promoter concentrations and spatial distributions as well as adsorption of several species (O*, H*, CO* and ) for different gas phase compositions. Both Ga and Zn form an fcc-alloy with Cu due to the stronger interaction of the promoters with Cu than with themselves. While the Cu-Ga-alloy is more stable than the Cu-Zn-alloy at low promoter concentrations (<25%), further increasing the promoter concentration reverses this trend, due to the unfavoured Ga-Ga-interactions. Under CO2 hydrogenation conditions, a substantial amount of O* can adsorb onto the alloy surfaces, resulting in partial dealloying and oxidation of the promoters. Therefore, the CO2 hydrogenation conditions are actually rather oxidising for both Ga and Zn despite the large amount of H2 present in the feedstock. Thus, the growth of a GaO x /ZnO x overlayer is thermodynamically preferred under reaction conditions, enhancing CO2 adsorption, and this effect is more pronounced for the Cu-Ga-system than for the Cu-Zn-system. In contrast, under CO hydrogenation conditions, fully reduced and alloyed surfaces partially covered with H* and CO* are expected, with mixed CO/CO2 hydrogenation conditions resulting in a mixture of reduced and oxidised states. This shows that the active atmosphere tunes the preferred state of the catalyst, influencing the catalytic activity and stability, indicating that the still widespread image of a static catalyst under reaction conditions is insufficient to understand the complex interplay of processes taking place on a catalyst surface under reaction conditions, and that dynamic effects must be considered.

Global optimization of extraframework ensembles in zeolites: structural analysis of extraframework aluminum species in MOR and MFI zeolites.

Metal-modified zeolites are versatile catalytic materials with a wide range of industrial applications. Their catalytic behaviour is determined by the nature of externally introduced cationic species, i.e., its geometry, chemical composition, and location within the zeolite pores. Superior catalyst designs can be unlocked by understanding the confinement effect and spatial limitations of the zeolite framework and its influence on the geometry and location of such cationic active sites. In this study, we employ the genetic algorithm (GA) global optimization method to investigate extraframework aluminum species and their structural variations in different zeolite matrices. We focus on extraframework aluminum (EFAl) as a model system because it greatly influences the product selectivity and catalytic stability in several zeolite catalyzed processes. Specifically, the GA was used to investigate the configurational possibilities of EFAl within the mordenite (MOR) and ZSM-5 frameworks. The xTB semi-empirical method within the GA was employed for an automated sampling of the EFAl-zeolite space. Furthermore, geometry refinement at the density functional theory (DFT) level of theory allowed us to improve the most stable configurations obtained from the GA and elaborate on the limitations of the xTB method. A subsequent ab initio thermodynamics analysis (aiTA) was chosen to predict the most favourable EFAl structure(s) under the catalytically relevant operando conditions.

Implicit Solvation Methods for Catalysis at Electrified Interfaces.

Implicit solvation is an effective, highly coarse-grained approach in atomic-scale simulations to account for a surrounding liquid electrolyte on the level of a continuous polarizable medium. Originating in molecular chemistry with finite solutes, implicit solvation techniques are now increasingly used in the context of first-principles modeling of electrochemistry and electrocatalysis at extended (often metallic) electrodes. The prevalent ansatz to model the latter electrodes and the reactive surface chemistry at them through slabs in periodic boundary condition supercells brings its specific challenges. Foremost this concerns the difficulty of describing the entire double layer forming at the electrified solid–liquid interface (SLI) within supercell sizes tractable by commonly employed density functional theory (DFT). We review liquid solvation methodology from this specific application angle, highlighting in particular its use in the widespread ab initio thermodynamics approach to surface catalysis. Notably, implicit solvation can be employed to mimic a polarization of the electrode’s electronic density under the applied potential and the concomitant capacitive charging of the entire double layer beyond the limitations of the employed DFT supercell. Most critical for continuing advances of this effective methodology for the SLI context is the lack of pertinent (experimental or high-level theoretical) reference data needed for parametrization.

Surface half metallicity and thermodynamic stability of 001-plane Ti2XSi (X=Mn, Co) Heusler alloys (HAs): A DFT approach

We report the surface stability and surface half-metallicity of Ti2XSi (X=Mn/Co) [001] slab with natural TiSi and TiX(X=Mn/Co) terminals from the first principles calculation. We started our calculation from the bulk optimization followed by the electronic and magnetic properties by adopting GGA exchange correlation for treating all electrons interaction. We have also analyzed the surface stability by calculating the surface energies as a function of constituents chemical potentials within the framework of ab-initio thermodynamics. Within the allowed chemical potentials range, TiSi(Ti2MnSi) terminated surface found to be the most energetically favorable thin film while the TiX terminated surfaces show strong molecular attraction. The bulk half metallicity is preserved in TiSi(Ti2MnSi) terminated surface with 100% spin polarization while the other terminal surfaces are metallic. The results of atomic site partial magnetic moments in the surface states along with their corresponding values in the bulk structure are also presented.

Thermodynamics and Kinetics of Molecular Hydrogen Adsorption and Dissociation on MXenes: Relevance to Heterogeneously Catalyzed Hydrogenation Reactions

The interaction of molecular hydrogen with a series of 28 two-dimensional (2D) carbides and nitrides, known as MXenes, has been studied by means of periodic density functional calculations. This study shows that trends in atomic and molecular adsorption energies can be rationalized in terms of the electrostatic potential above the surface site and the Bader charge on the surface metal atoms. For all systems, molecular hydrogen is found to dissociate with almost negligible barriers, meaning that at low temperature the MXene surface will be passivated by adsorbed atomic hydrogen. The conditions at which the MXene surface is partly covered and, hence, able to participate in hydrogenation reactions are investigated by means of ab initio thermodynamics and phase diagrams derived from microkinetic simulations. The first provide the equilibrium conditions for a given H coverage on the MXene of interest, whereas the second provides the conditions at which a given configuration is reachable at the working conditions. For fast enough processes, both approaches necessarily lead to the same result, but this may differ when high energy barriers are involved, as it the case here for the H adatoms recombination step. With this suite, we show that Fe2C, W2N, and Mo2C are promising hydrogenation catalysts. This work serves as a first step toward the rational design and implementation of MXene-based hydrogenation catalysts.

Ab initio thermodynamics reveals the nanocomposite structure of ferrihydrite

Ferrihydrite is a poorly crystalline iron oxyhydroxide nanomineral that serves a critical role as the most bioavailable form of ferric iron for living systems. However, its atomic structure and composition remain unclear due in part to ambiguities in interpretation of X-ray scattering results. Prevailing models so far have not considered the prospect that at the level of individual nanoparticles multiple X-ray indistinguishable phases could coexist. Using ab initio thermodynamics we show that ferrihydrite is likely a nanocomposite of distinct structure types whose distribution depends on particle size, temperature, and hydration. Nanoparticles of two contrasting single-phase ferrihydrite models of Michel and Manceau are here shown to be thermodynamically equivalent across a wide range of temperature and pressure conditions despite differences in their structural water content. Higher temperature and water pressure favor the formation of the former, while lower temperature and water pressure favor the latter. For aqueous suspensions at ambient conditions, their coexistence is maximal for particle sizes up to 12 nm. The predictions inform and help resolve different observations in various experiments.

Decrypting the photocatalytic bacterial inactivation of hierarchical flower-like Bi2WO6 microspheres induced by surface properties: Experimental studies and ab initio calculations

Bi2WO6 is considered an effective photocatalyst, even under the visible part of the solar spectrum, and recent advances in the modification of its structure leave promise for harnessing its enhanced effectiveness. Our experimental E. coli disinfection results have shown that the flower-like morphology brought considerable enhancement to the overall performance over the nanoparticle form. To clarify the photocatalytic mechanism of Bi2WO6, a combination of experimental and computational methods has been employed to investigate the surface properties of Bi2WO6 nanosheets self-assembly flower-like architecture. Although experimental evidence has determined the band edge positions of the as-synthesized samples, so as to address the photocatalytic properties, the mechanistic basis of this concept remains unclear. The calculation results demonstrated here deepen our understanding and indicate the potential of surface configuration to considerably alter the electronic structure and related photocatalytic properties of Bi2WO6 nanosheets. Firstly, we consider a range of surface slab models of Bi2WO6 (010) facet and calculate their surface Gibbs free energies. Having determined that the bi-termination is energetically more favorable by ab initio atomistic thermodynamics, hydrogen passivated termination was proved to be most stable. Through the analysis of the electronic band structure within the DFT-1/2 scheme and work function of the most stable termination, excellent agreement of experimental and theoretical predictions provided a meaningful understanding on the kinetic dependence of photocatalytic bacterial inactivation.

Nature and identity of the active site via structure-dependent microkinetic modeling: An application to WGS and reverse WGS reactions on Rh

We propose a methodology to perform a structure-dependent microkinetic analysis of a catalytic process. The methodology makes it possible to unveil the nature and identity of the active site in a self-consistent manner. The morphology of heterogeneous catalyst nanoparticles as a function of the gas chemical potential is determined using ab initio thermodynamics and Wulff-Kaishew construction methods. The reaction rates are calculated by integrating a microkinetic model which describes the catalytic activity of the crystal facets exposed by the catalyst under reaction conditions. The method is applied for the analysis of the direct and reverse water-gas shift (WGS) reacting systems on a 4% Rh/α-Al2O3 kinetic experiments from the literature. Our findings make it possible to rationalize that far from equilibrium the two different reacting systems not only follow different reaction pathways in agreement with the experimental evidence but also show that the dominant active sites are different for WGS and reverse WGS. Indeed, the WGS reaction occurs mainly on the Rh(111) facet, whereas reverse WGS proceeds on the active sites of Rh(100). As a whole, this methodology makes it possible a concomitant description of the nature of the catalyst material in reaction conditions and of its catalytic consequences in terms of reactivity. As such, it paves the way towards the use of first-principles methods for the interpretation of the experimental evidence in terms of structure-activity relationships.

Modelling the Atomic Layer Deposition of Cobalt and Ruthenium on NHx-Terminated Metal Surfaces

Atomic layer deposition (ALD) is widely used in microelectronics and semiconductor industry to deposit very thin films as part of device fabrication in nano- or subnano-dimensions. The key advantages of ALD are the conformality and precise thickness control at the atomic scale, which are difficult for physical or traditional chemical vapor deposition methods. Cobalt (Co) and Ruthenium (Ru) are used as a seed layers for metallization of interconnects. They are also potential materials for the electrode in dynamic random-access memory (DRAM) capacitors and metal-oxide-semiconductor field-effect transistors (MOSFETs). Plasma-enhanced ALD (PE-ALD) is used for low-temperature thin film growth by alternating exposures of metal precursors and plasma reactants. During the PE-ALD growth of metals, N-plasma, for example, NH3 or a mixture of N2 and H2, has been applied to avoid surface metal oxidation. The PE-ALD of Ru and Co has been experimentally investigated using metal precursors such as RuCp2, Ru(EtCp)2, CoCp2, and CoCp(CO)2. However, the reaction mechanism is not clear and theoretical studies on the reaction mechanism is entirely lacking.In this presentation, we study the PE-ALD growth of Co and Ru by first principle calculations. The (001) surface of both metals, with a hexagonal structure, is the most stable and the (100) surface with a zigzag structure is less stable but has high reactivity. These two surfaces allow the study of the influence of the surface facet. The surface saturation coverage was studied by considering individual adsorption and co-adsorption of NH and NH2 to terminate both surfaces. On the (001) surface, the zero K saturation coverage for NH is 1ML on Ru and 6/9ML on Co. For NH2, the saturation coverages are 6/9ML on Co and Ru surfaces. On the (100) surface, the saturation coverages are 2ML for NH on Ru and Co surfaces, 1.33 ML for NH2 on Ru, and 1ML for NH2 on Co surface. The higher saturation coverage on (100) surface is attributed to the unique trench structure, which provides more available surface sites than that of (001) surface. We also consider co-adsorption of NH and NH2 on (001) and (100) surfaces. The results are then analyzed with ab initio thermodynamics by calculating the Gibbs free energy. Both the ultra-high vacuum (UHV) condition and standard ALD operating condition are used to elucidate the effect of pressure and temperature on the termination of metal surfaces.The adsorption and reactions of metal precursors (CoCp2 and RuCp2) on NHx terminated metal surfaces were investigated with the inclusion of van der Waals corrections. Two possible adsorption structures, namely the precursor lying parallel and perpendicular to the NHx-terminated surfaces, were considered at zero K and ALD conditions. Possible reactions include: precursor adsorption, first hydrogen transfer, first Cp ligand dissociation, first Cp ligand desorption, second hydrogen transfer, and second Cp ligand desorption. The barrier for proton transfer was calculated using climbing image nudged elastic band (CI-NEB) method. Our results show that (100) surface has higher activity than (001) surface. In addition, the Cp ligand elimination of CoCp2 has lower barrier than RuCp2, regardless of surface facet. After the metal precursor pulse, the surface is terminated with MCp fragment or M atom depending on surface facet, where M is Ru or Co. The surface N atom and H atom can be eliminated during the following plasma step by forming NH3, N2 or H2. After a full cycle, the surface is an NHx-terminated metal surface and ready for the next cycle. This work will be important to reveal the mechanism and feasibility of atomic layer deposition of metals using N-plasma.

First-Principles Study of the Surfaces and Equilibrium Shape of Discharge Products in Li-Air Batteries.

Li-air batteries are a promising alternative to Li-ion batteries as they theoretically provide the highest possible specific energy density. Mainly, Li2O2 (lithium peroxide) and to a lesser extent, Li2O (lithium oxide) are assumed to be the discharge products of these batteries formed with the soluble LiO2 (lithium superoxide) considered to be an intermediate product. Bulk Li2O2 is an electronic insulator, and the precipitation of this compound on the cathode is thought to be the main limiting factor in achieving high capacities in lithium-oxygen cells. For the most promising electrolytes including solvents with high donor numbers, microscopy observations frequently reveal crystallite morphologies of Li2O2 compounds, rather than uniform layers covering the electrode surface. The precise morphologies of Li2O and Li2O2 particles, and their effect and their extent of contact with the electrode, which may all affect the capacity and rechargeability, however, remain largely undetermined. Here, we address the stability of various Li2O and Li2O2 surfaces and consequently, their crystallite morphologies using density functional theory calculations and ab initio thermodynamics. In contrast to previous studies, we also consider high-index surface terminations, which exhibit surprisingly low surface energies. We carefully analyze the reasons for the stability of these high-index surfaces, which also prominently influence the equilibrium shape of the particles, at least for Li2O2, and discuss the consequences for the observed morphology of the reaction products.

Carbon Deposition and Permeation on Nickel Surfaces in Operando Conditions: A Theoretical Study

The carbon deposition and permeation on nickel surfaces were investigated from thermodynamic and kinetic aspects by using density functional theory (DFT), ab initio atomic thermodynamics, and class...

Thermodynamic Cyclic Voltammograms Based onAb InitioCalculations: Ag(111) in Halide-Containing Solutions

Cyclic voltammograms (CVs) are a central experimental tool for assessing the structure and activity of electrochemical interfaces. Based on a mean-field ansatz for the interface energetics under applied potential conditions, we here derive an ab initio thermodynamics approach to efficiently simulate thermodynamic CVs. All unknown parameters are determined from density functional theory (DFT) calculations coupled to an implicit solvent model. For the showcased CVs of Ag(111) electrodes in halide-anion-containing solutions, these simulations demonstrate the relevance of double-layer contributions to explain experimentally observed differences in peak shapes over the halide series. Only the appropriate account of interfacial charging allows us to capture the differences in equilibrium coverage and total electronic surface charge that cause the varying peak shapes. As a case in point, this analysis demonstrates that prominent features in CVs do not only derive from changes in adsorbate structure or coverage but can also be related to variations of the electrosorption valency. Such double-layer effects are proportional to adsorbate-induced changes in the work function and/or interfacial capacitance. They are thus especially pronounced for electronegative halides and other adsorbates that affect these interface properties. In addition, the analysis allows us to draw conclusions on how the possible inaccuracy of implicit solvation models can indirectly affect the accuracy of other predicted quantities such as CVs.

Defect-mediated ab initio thermodynamics of metastable γ-MoN(001).

Refractory transition metal nitrides exhibit a plethora of polymorphic expressions and chemical stoichiometries. To afford a better understanding of how defects may play a role in the structural and thermodynamics of these nitrides, using density-functional theory calculations, we investigate the influence of point and pair defects in bulk metastable γ-MoN and its (001) surface. We report favorable formation of Schottky defect pairs of neighboring Mo and N vacancies in bulk γ-MoN and apply this as a defect-mediated energy correction term to the surface energy of γ-MoN(001) within the ab initio atomistic thermodynamics approach. We also inspect the structural distortions in both bulk and surfaces of γ-MoN by using the partial radial distribution function, g(r), of Mo-N bond lengths. Large atomic displacements are found in both cases, leading to a broad spread of Mo-N bond length values when compared to their idealized bulk values. We propose that these structural and thermodynamic analyses may provide some insight into a better understanding of metastable materials and their surfaces.