We review results of recent combined theoretical and experimental studies of Ti_{1−}_{x}Al_{x}N, an archetypical alloy system material for hard-coating applications. Theoretical simulations of lattice parameters, mixing enthalpies, and elastic properties are presented. Calculated phase diagrams at ambient pressure, as well as at pressure of 10 GPa, show a wide miscibility gap and broad region of compositions and temperatures where the spinodal decomposition takes place. The strong dependence of the elastic properties and sound wave anisotropy on the Al-content offers detailed understanding of the spinodal decomposition and age hardening in Ti_{1−}_{x}Al_{x}N alloy films and multilayers. TiAlN/TiN multilayers can further improve the hardness and thermal stability compared to TiAlN since they offer means to influence the kinetics of the favorable spinodal decomposition and suppress the detrimental transformation to w-AlN. Here, we show that a 100 degree improvement in terms of w-AlN suppression can be achieved, which is of importance when the coating is used as a protective coating on metal cutting inserts.
We use metastable NaCl-structure Ti_{0.5}Al_{0.5}N alloys to probe effects of configurational disorder on adatom surface diffusion dynamics which control phase stability and nanostructural evolution during film growth. First-principles calculations were employed to obtain energy potential maps of Ti and Al adsorption on an ordered TiN(001) reference surface and a disordered Ti_{0.5}Al_{0.5}N(001) solid-solution surface. The energetics of adatom migration on these surfaces are determined and compared to isolate effects of configurational disorder. The results show that alloy surface disorder dramatically reduces Ti adatom mobilities. Al adatoms, in sharp contrast, experience only small disorder-induced differences in migration dynamics.
A nitrogen-rich compound, ReN(8)xN(2), was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser-heated diamond anvil cell. Single-crystal X-ray diffraction revealed that the crystal structure, which is based on the ReN8 framework, has rectangular-shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100GPa, ReN(8)xN(2) is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [-N=N-] that constitute the framework have not been previously observed in any compound. Abinitio calculations on ReN(8)xN(2) provide strong support for the experimental results and conclusions.
The lattice parameters of InxAl1-xN in the whole compositional range are studied using first-principle calculations. Deviations from Vegards rule are obtained via the bowing parameters, delta(a)=0.0412 +/- 0.0039 A and delta(c)=-0.060 +/- 0.010 A, which largely differ from previously reported values. Implications of the observed deviations from Vegards rule on the In content extracted from x-ray diffraction are discussed. We also combine these results with x-ray diffraction and Raman scattering studies on InxAl1-xN nanocolumns with 0.627 <= x <= 1 and determine the E-2 phonon frequencies versus In composition in the scarcely studied In-rich compositional range.
We have performed calculations for the hyperfine field in the disordered Fe0.5Ni0.5 alloy using supercells with up to 864 atoms. The computational scheme is based on exact muffin-tin orbitals and the locally self-consistent Green’s function formalism, which scales linearly with the number of atoms in the supercell. This scheme allows local environment effects, such as chemical and magnetic environment, and short-range order, to be explicitly included. Supercell calculations for Fe-Ni show that while the average fields coincides with that obtained using the coherent potential approximation, there is a significant distribution of the hyperfine fields depending on the local environment. The fields of Fe and Ni show qualitatively different behaviour as a function of the on-site magnetic moment, but scale linearly with the average magnetic moment in the first coordination shell.
The elastic properties of alloys between boron suboxide (B_{6}O) and boron carbide (B_{13}C_{2}), denoted by (B_{6}O)_{1−x}(B_{13}C_{2})_{x}, as well as boron carbide with variable carbon content, ranging from B_{13}C_{2} to B_{4}C are calculated from first-principles. Furthermore, the mixing thermodynamics of (B_{6}O)_{1−x}(B_{13}C_{2})x is studied. A superatom-special quasirandom structure approach is used for modeling different atomic configurations, in which effects of configurational disorder between the carbide and suboxide structural units, as well as between boron and carbon atoms within the units, are taken into account. Elastic properties calculations demonstrate that configurational disorder in B_{13}C_{2}, where a part of the C atoms in the CBC chains substitute for B atoms in the B12 icosahedra, drastically increase the Young’s and shear modulus, as compared to an atomically ordered state, B_{12}(CBC). These calculated elastic moduli of the disordered state are in excellent agreement with experiments. Configurational disorder between boron and carbon can also explain the experimentally observed almost constant elastic moduli of boron carbide as the carbon content is changed from B_{4}C to B_{13}C_{2}. The elastic moduli of the (B_{6}O)_{1−x}(B_{13}C_{2})_{x} system are also practically unchanged with composition if boron-carbon disorder is taken into account. By investigating the mixing thermodynamics of the alloys, in which the Gibbs free energy is determined within the mean-field approximation for the configurational entropy, we outline the pseudo-binary phase diagram of (B_{6}O)_{1−x}(B_{13}C_{2})_{x}. The phase diagram reveals the existence of a miscibility gap at all temperatures up to the melting point. Also, the coexistence of B_{6}O-rich as well as ordered or disordered B_{13}C_{2}-rich domains in the material prepared through equilibrium routes is predicted.
Isostructural stability of B1-NaCl type SiN on (001) and (111) oriented ZrN surfaces is studied theoretically and experimentally. The ZrN/SiNx/ZrN superlattices with modulation wavelength of 3.76 nm (dSiNx similar to 0.4 nm) were grown by dc-magnetron sputtering on MgO(001) and MgO(111). The results indicate that 0.4 nm thin SiNx layers utterly influence the preferred orientation of epitaxial growth: on MgO(001) cube-on-cube epitaxy of ZrN/SiNx superlattices were realized whereas multilayers on MgO(111) surface exhibited an unexpected 002 texture with a complex fourfold 90 degrees-rotated in-plane preferred orientation. Density functional theory calculations confirm stability of a (001) interface with respect to a (111) which explains the anomaly.
We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe. DOI: 10.1103/PhysRevLett.110.117206
Despite the fast development of computational material modeling, the theoretical description of macroscopic elastic properties of textured polycrystalline aggregates starting from basic principles remains a challenging task. In this study we use a supercell-based approach to obtain the elastic properties of a random solid solution cubic Zr1-x Al-x N system as a function of the metallic sublattice composition and texture descriptors. The employed special quasirandom structures are optimized not only with respect to short-range-order parameters, but also to make the three cubic directions [1 0 0], [0 1 0], and [0 0 1] as similar as possible. In this way, only a small spread of elastic constant tensor components is achieved and an optimum trade-off between modeling of chemical disorder and computational limits regarding the supercell size and calculational time is proposed. The single-crystal elastic constants are shown to vary smoothly with composition, yielding x approximate to 0.5 an alloy constitution with an almost isotropic response. Consequently, polycrystals with this composition are suggested to have Youngs modulus independent of the actual microstructure. This is indeed confirmed by explicit calculations of polycrystal elastic properties, both within the isotropic aggregate limit and with fiber textures with various orientations and sharpness. It turns out that for low AlN mole fractions, the spread of the possible Youngs modulus data caused by the texture variation can be larger than 100 GPa. Consequently, our discussion of Youngs modulus data of cubic Zr1-x Al-x N contains also the evaluation of the texture typical for thin films.
We propose a design route for the next generation of nitride alloys via a concept of multicomponent alloying based on self-organization on the nanoscale via a formation of metastable intermediate products during the spinodal decomposition. We predict theoretically and demonstrate experimentally that quasi-ternary (TiCrAl)N alloys decompose spinodally into (TiCr)N and (CrAl)N-rich nanometer sized regions. The spinodal decomposition results in age hardening, while the presence of Cr within the AlN phase delays the formation of a detrimental wurtzite phase leading to a substantial improvement of thermal stability compared to the quasi-binary (TiAl)N or (CrAl)N alloys.
Through a combination of theoretical and experimental observations we study the high temperature decomposition behavior of c-(Ti_{x}Zr_{y}Al_{z}N) alloys. We show that for most concentrations the high formation energy of (ZrAl)N causes a strong tendency for spinodal decomposition between ZrN and AlN while other decompositions tendencies are suppressed. In addition we observe that entropic effects due to configurational disorder favor a formation of a stable Zr-rich (TiZr)N phase with increasing temperature. Our calculations also predict that at high temperatures a Zr rich (TiZrAl)N disordered phase should become more resistant against the spinodal decomposition despite its high and positive formation energy due to the specific topology of the free energy surface at the relevant concentrations. Our experimental observations confirm this prediction by showing strong tendency towards decomposition in a Zr-poor sample while a Zr-rich alloy shows a greatly reduced decomposition rate, which is mostly attributable to binodal decomposition processes. This result highlights the importance of considering the second derivative of the free energy, in addition to its absolute value in predicting decomposition trends of thermodynamically unstable alloys.
State-of-the-art alloys for hard coating applications, such as TiAlN, are known to suffer from decreased hardness during heat treatment in excess of 900 °C due to the formation of detrimental wurtzite AlN phases. Recent research has shown that multicomponent alloying with additional transition metals (TMs) such as Cr can shift the onset of the phase transformations to higher temperatures, but a search for new alloys is generally time-consuming due to the large number of processes that influence material properties along with the large number of alloy compositions that have to be synthesized. To overcome this difficulty we carry out systematic first-principles calculations aimed at finding potential new multicomponent TM aluminum nitride alloys for advanced hard coating applications. We direct our search towards a specific property, the thermal stability of the coating. In particular, we concentrate on the thermodynamic stability of the cubic B1 TM–Al–N phase relative to the wurtzite phase, and choose the enthalpy difference between them as our search descriptor. We perform ab initio calculations for all TMs, considered as impurities in AlN, and identify the most promising candidates that may improve the thermal stability. We present arguments that these elements should be targeted in future in-depth studies, theoretical, as well as experimental.
AlN is a wide band gap semiconductor that is used in many fields, e.g. as electronic material, for piezoelectric applications but also as a component material in high performance hard coating alloys. The stable structure under ambient conditions is a hexagonal wurtzite structure, but it has also been observed in the tetrahedrally bonded cubic zinc-blende structure as well as cubic rock-salt phases that become stable at high pressure. The metastable rock-salt phase of AlN also forms during decomposition processes in hard-coating alloys such as (TiAl)N, (CrAl)N and (TiCrAl)N. Even though thermodynamically unstable, one can expect some amount of Ti and Cr to be present in the c-AlN phase during the decomposition. Still, little study has been done for the dilute (TMAl)N alloys with cubic B1 crystal structure. We study the electronic structure of Ti and Cr impurities in B1 AlN. Because of the limitations of standard local and semi-local approximations within the density functional theory (DFT) in the treatment of wide band gap semiconductors, as well as conventional hybrid functionals for systems consisting of correlated localized and delocalized orbitals, we apply the mHSE+Vw method, which has been developed specifically to dealing with these kind of problems. Simulations are done by means of the supercell technique with single impurities, as well as for the impurity pairs. The effects of different atomic configurations of the TM-impurities on phase stability and magnetic properties of the cubic B1 AlN is studied and compared to the those in hexagonal B4 structures. Our results underline the importance of correlation and magnetic effects for the description of properties of cubic AlN doped with Ti and Cr.
The structure of the SiNx tissue phase in superhard TiN/SiNx nanocomposites has been debated in the literature. We present a theoretical investigation of the possibility of crystalline and coherent ( 001) interfaces that satisfies the two necessary criteria, stability with respect to lattice vibrations as well as to variations in stoichiometry. It is found that one monolayer of Si tetrahedrally coordinated by N in a B3-like geometry embedded between B1-TiN( 001) surfaces is both dynamically stable and thermodynamically stable with respect to vacancy formation. However, with increasing layer thickness the B3-type structure becomes unstable with respect to Si vacancy formation. Instead we suggest that a tetragonal D0(22)-like order of Si vacancies can stabilize the interface. These structures are in line with the experimental findings of the crystalline tissue phase which has coherent interfaces with TiN.
Wehave performed ab initio calculations for the cubic inverse-perovskite Sc_{3}EN(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 Sc_{6}N cluster and the surrounding metallic lattice of eitherAl, Ga, or In atoms. Our calculations show that Sc_{3}GaNhas 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.
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 cubic phases Sc3AlN and Sc3InN, and predict the stability of cubic Sc3GaN and a triclinic phase aP20-Sc3BN. Substantial phonon softening in Sc3AlN and Sc3GaN is observed indicating a possibility that structural defects could form readily. In accord, our experiments show that magnetron sputter deposited films contain regions with high density of nonperiodic stacking faults along the < 111 > growth direction. We suggest that defect-free crystals may exhibit anomalies in the carrier properties, promising for electronic applications.
In the present work, the decomposition of unstable arc evaporated Ti_{0.6}Al_{0.4}N at elevated temperatures and quasihydrostatic pressures has been studied both experimentally and by first-principles calculations. High pressure and high temperature (HPHT) treatment of the samples was realized using the multi anvil press and diamond anvil cell techniques. The products of the HPHT treatment of Ti_{0.6}Al_{0.4}N were investigated using x-ray diffractometry and transmission electron microscopy. Complimentary calculations show that both hydrostatic pressure and high temperature stabilize the cubic phase of AlN, which is one of the decomposition products of Ti_{0.6}Al_{0.4}N. This is in agreement with the experimental results which in addition suggest that the presence of Ti in the system serves to increase the stability region of the cubic c-AlN phase. The results are industrially important as they show that Ti_{0.6}Al_{0.4}N coatings on cutting inserts do not deteriorate faster under pressure due to the cubic AlN to hexagonal AlN transformation.
We use molecular dynamics (MD) based on the modified embedded atom method (MEAM) to determine diffusion coefficients and migration pathways for Ti and N adatoms (Ti-ad and N-ad) on TiN(111). The reliability of the classical model-potential is verified by comparison with density functional theory (DFT) results at 0 K. MD simulations carried out at temperatures between 600 and 1800 K show that both Ti-ad and N-ad favor fcc surface sites and migrate among them by passing through metastable hcp positions. We find that N-ad species are considerably more mobile than Ti-ad on TiN(111); contrary to our previous results on TiN(001). In addition, we show that lattice vibrations at finite temperatures strongly modify the potential energy landscape and result in smaller adatom migration energies, E-a = 1.03 for Ti-ad and 0.61 eV for N-ad, compared to 0 K values E-aOK = 1.55 (Ti-ad) and 0.79 eV (N-ad). We also demonstrate that the inclusion of dipole corrections, neglected in previous DFT calculations, is necessary in order to obtain the correct formation energies for polar surfaces such as TiN(111). (C) 2016 Elsevier B.V. All rights reserved.
Ti1_{−x}Al_{x}N is a technologically important alloy that undergoes a process of high temperature age-hardening that is strongly influenced by its elastic properties. We have performed first principles calculations of the elastic constants and anisotropy using the newly developed symmetry imposed force constant temperature dependent effective potential method, that include lattice vibrations and therefore the effects of temperature, including thermal expansion and intrinsic anharmonicity. These are compared with in situ high temperature x-ray diffraction measurements of the lattice parameter. We show that anharmonic effects are crucial to the recovery of finite temperature elasticity. The effects of thermal expansion and intrinsic anharmonicity on the elastic constants are of the same order, and cannot be considered separately. Furthermore, the effect of thermal expansion on elastic constants is such that the volume change induced by zero point motion has a significant effect. For TiAlN, the elastic constants soften non-uniformly with temperature: C_{11} decreases substantially when the temperature increases for all compositions, resulting in an increased anisotropy. These findings suggest that an increased Al content and annealing at higher temperatures will result in a harder alloy.
Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants C_{11}, C_{12}, and C_{44} are calculated. A strong dependence on the temperature is predicted, with C_{11} decreasing by more than 29% at 1800 K as compared to its value obtained at T=0 K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications.
Currently, AlN is the preferred material in electroacoustic applications but modern applications necessitate the synthesis of materials with a range of electroacoustic properties. Among the promising candidates are the wurtzite Boron-containing AlN alloys. In here we study theoretically some of the material properties of wurtzite B0.125Al0.875N. The results indicate that wurtzite B0.125Al0.875N exhibits a strong configurational dependence of the electromechanical coupling constant. It is further shown that the lattice parameters as well as the stiffness constants are less sensitive of the atomic configuration and comply well with the Vegards rule.
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 strong change in the response of the internal atomic coordinates to strain and pronounced softening of C-33 elastic constant. The underlying mechanism is the flattening of the energy landscape due to a competition between the parent wurtzite and the so far experimentally unknown hexagonal phases of the alloy. Our observation provides a route for the design of materials with high piezoelectric response.
In this study we discuss the performance of the special quasirandom structure (SQS) method in predicting the elastic properties of B1 (rocksalt) Ti0.5Al0.5N 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 Zeners type elastic anisotropy (A) of Ti0.5Al0.5N. 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 special quasirandom structure (SQS) approach is a successful technique for modelling of alloys, however it breaks inherently the point symmetry of the underlying crystal lattice. We demonstrate that monocrystalline and polycrystalline elastic moduli can scatter significantly depending on the chosen SQS model and even on the supercell orientation in space. Also, we demonstrate that local disturbances, such as vacancies or interfaces change the SQS configuration in a way, that significantly affects the values of the calculated physical properties. Moreover, the diversity of local environments in random alloys results in a large variation of the calculated local properties. We underline that improperly chosen, generated or handled SQS may result in erroneous theoretical findings. The challenges of the SQS method are discussed using bulk Ti0.5Al0.5N alloy and TiN/Ti0.5Al0.5N multilayer as model systems. We present methodological corrections for the mindful application of this approach in studies of advanced properties of alloys.
Strong compositional-dependent elastic properties have been observed theoretically and experimentally in Ti_{1−x}Al_{x}N alloys. The elastic constant, C_{11}, changes by more than 50% depending on the Al-content. Increasing the Al-content weakens the average bond strength in the local octahedral arrangements resulting in a more compliant material. On the other hand, it enhances the directional (covalent) nature of the nearest neighbor bonds that results in greater elastic anisotropy and higher sound velocities. The strong dependence of the elastic properties on the Al-content offers new insight into the detailed understanding of the spinodal decomposition and age hardening in Ti_{1−x}Al_{x}N alloys.
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 ScN or YN is investigated using ab initio calculations. We confirm that the energy flattening phenomenon gives rise to the simultaneous appearance of elastic softening and local structural instability, and explains the enhanced piezoelectricity of the alloys. Furthermore, we present a volume matching condition for an efficient search of new piezoelectric materials. It states that alloys in which the parent components show close volume matching exhibit a flatter potential-energy landscape and higher increase of piezoelectric moduli. We suggest YInN, beyond ScAlN, as a promising material for piezoelectric energy harvesting with its enhanced ≈400% piezoelectric moduli.
Substituting Al for Ti in TiN(001), TiN(011), and N- and Ti-terminated TiN(111) surfaces has significant effects on adatom surface energetics which vary strongly with the adatom species and surface orientation. Here, we investigate Ti, Al, and N adatom surface dynamics using density functional methods. We calculate adatom binding and diffusion energies with both a nudged elastic band and grid-probing techniques. The adatom diffusivities are analyzed within a transition-state theory approximation. We determine the stable and metastable Ti, Al, and N binding sites on all three surfaces as well as the lowest energy migration paths. In general, adatom mobilities are fastest on TiN(001), slower on TiN(111), and slowest on TiN(011). The introduction of Al has two major effects on the surface diffusivity of Ti and Al adatoms. First, Ti adatom diffusivity on TiN(001) is significantly reduced near substituted Al surface atoms; we observe a 200% increase in Ti adatom diffusion barriers out of fourfold hollow sites adjacent to Al surface atoms, while Al adatom diffusivity between bulk sites is largely unaffected. Secondly, on TiN(111), the effect is opposite; Al adatoms are slowed near the substituted Al surface atom, while Ti adatom diffusivity is largely unaffected. In addition, we note the importance of magnetic spin polarization on Ti adatom binding energies and diffusion path. These results are of relevance for the atomistic understanding of Ti_{1-x}Al_{x}N alloy and Ti_{1-x}Al_{x}N/TiN multilayer thin-film growth processes.
By combining theoretical prediction and experimental verification we investigate the piezoelectric properties of yttrium indium nitride (Y_{x}In_{1-x}N). Ab initio calculations show that the Y_{x}In_{1-x}N wurtzite phase is lowest in energy among relevant alloy structures for 0≤x≤0.5. Reactive magnetron sputter epitaxy was used to prepare thin films with Y content up to x=0.51. The composition dependence of the lattice parameters observed in the grown films is in agreement with that predicted by the theoretical calculations confirming the possibility to synthesize a wurtzite solid solution. An AlN buffer layer greatly improves the crystalline quality and surface morphology of subsequently grown Y_{x}In_{1-x}N films. The piezoelectric response in films with x=0.09 and x=0.14 is observed using piezoresponse force microscopy. Theoretical calculations of the piezoelectric properties predict YxIn1âxN to have comparable piezoelectric properties to Sc_{x}Al_{1-x}N.
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.
In this work the phase field method has been applied to model the spinodal decomposition of TiAlN. Here we have used thermodynamic data from ab initio calculations that takes into account clustering effects, and experimental diffusivity data of TiAlN as an input to the model. The effect of alloy composition on microstructure and stresses, is studied in time and space. In addition, Young’s modulus evolution of the decomposing microstructure is reported. It was found that the microstructure changes from round AlN rich domains in a TiN matrix, to outstretched TiN rich domains in the {100} crystallographic directions in an AlN matrix, as the composition was changed from x=0.3 to x=0.75 in Ti_{1-x}Al_{x}N. The microstructure evolution was observed to undergo different stages. In short; first elongated structures enriched of the majority element in random directions evolve. Thereafter round AlN rich domains evolve, independent of composition studied, and a completely segregated microstructure forms that finally coarsens. The initiation, decomposition, and coarsening rate was found to increase with Al content due to the increase in driving force with Al content. Al rich domains purify fastest, independent of composition studied. The evolving compositional wavelength decreases with Al content resulting in a finer microstructure for alloys rich in Al. During decomposition high local strains and stresses develop, which reach maximum values of 6·10^{-3} and 12 GPa respectively.
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 strongest stabilization occurs for compositions in which the mixing enthalpy of bulk Ti1-xAlxN has its maximum. The effect is split into a strain and an interfacial (or chemical) contribution, and we show that both contributions are significant. An analysis of the local atomic structure reveals that the Ti atoms located in the interfacial layers relax significantly different from those in the other atomic layers of the multilayer. Considering the electronic structure of the studied system, we demonstrate that the lower Ti-site projected density of states at epsilon(F) in the Ti1-xAlxN/TiN multilayers compared to the corresponding monolithic bulk explains a decreased tendency toward decomposition.
AlN is challenged as the material choice in important thin film electroacoustic devices for modern wireless communication applications. We present the promise of superior electromechanical coupling (k_{t}^{2}), in w−Sc_{x}Al_{1−x}N by studying its dielectric properties. w−Sc_{x}Al_{1−x}N (0≤x≤0.3) thin films grown by dual reactive magnetron sputtering exhibited low dielectric losses along with minor increased dielectric constant (ε). Ellipsometry measurements of the high frequency ε showed good agreement with density function perturbation calculations. Our data show that k_{t}^{2} will improve from 7% to 10% by alloying AlN with up to 20 mol % ScN.
A comprehensive density-functional theory (DFT)-based investigation of rhombohedral (ABC)-type graphene stacks with finite and infinite layer numbers and zero or finite static electric fields applied perpendicular to the surface is presented. Electronic band structures and field-induced charge densities are critically compared with related literature data including tight-binding and DFT approaches as well as with our own results on (AB) stacks. It is found that the undoped AB bilayer has a tiny Fermi line consisting of one electron pocket around the K point and one hole pocket on the line K-Gamma. In contrast to (AB) stacks, the breaking of translational symmetry by the surface of finite (ABC) stacks produces a gap in the bulklike states for slabs up to a yet unknown critical thickness N(semimet) andgt;andgt; 10, while ideal (ABC) bulk (beta graphite) is a semimetal. Unlike in (AB) stacks, the ground state of (ABC) stacks is shown to be topologically nontrivial in the absence of an external electric field. Consequently, surface states crossing the Fermi level must unavoidably exist in the case of (ABC)-type stacking, which is not the case in (AB)-type stacks. These surface states in conjunction with the mentioned gap in the bulklike states have two major implications. First, electronic transport parallel to the slab is confined to a surface region up to the critical layer number N(semimet). Related implications are expected for stacking domain walls and grain boundaries. Second, the electronic properties of (ABC) stacks are highly tunable by an external electric field. In particular, the dielectric response is found to be strongly nonlinear and can, e. g., be used to discriminate slabs with different layer numbers. Thus, (ABC) stacks rather than (AB) stacks with more than two layers should be of potential interest for applications relying on the tunability by an electric field.
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 constants and biaxial coefficients, as well as the respective deviations from linearity (Vegards rule) by using ab initio calculations. The stress-strain relationships to extract composition from the lattice parameters are derived in different coordinate systems for InxAl1-xN with an arbitrary surface orientation. The error made in the composition extracted from the lattice parameters if the deviations from linearity are not taken into account is discussed for different surface orientations, compositions and degrees of strain in the InxAl1-xN films. The strain induced piezoelectric polarization is analyzed for InxAl1-xN alloys grown pseudomorphically on GaN. The polarization values are compared with those obtained from our experimental data for the lattice parameters. We establish the importance of the deviation from linearity to correctly determine the piezoelectric polarization and also a smooth, not particular piezoelectric response at GaN lattice matched conditions.
We study the effect of the most common impurities and dopants on the lattice parameters of InN by using ab-initio calculations. We have considered both the size and deformation potential effect and report results for H, O, Si andMg. The incorporation of H on interstitial site and substitutional O leads to expansion of the lattice. On the other hand, incorporation of Si or Mg leads to contraction of the lattice. The most pronounced effect is observed for Si. Our results indicate that the increase of the in-plane lattice parameter of Mg doped InN cannot be explained neither by the size nor by the deformation potential effect and suggest that the growth strain is changed in this case.a)Electronic mail: vanya@ifm.liu.se.
In a recent experiment, Weisheit et al ( 2007 Science 315 349) demonstrated that the coercivity of thin L1(0) FePt and FePd films can be modified by the external electric field in an electrochemical environment. Here, this observation is confirmed by density functional calculations for the intrinsic magnetic anisotropy. The origin of the effect is clarified by means of a general and simple method to simulate charged metal surfaces. It is predicted that the coercivity of thin CoPt films is much more susceptible to electric field than that of FePt films.
Reactive magnetron sputtering was used to deposit YxAl1-xN thin films, 0≤x≤0.22, onto Al2O3(0001) and Si(100) substrates. X-ray diffraction and analytical electron microscopy show that the films are solid solutions. Lattice constants are increasing with Y concentration, in agreement with ab initio calculations. Spectroscopic ellipsometry measurements reveal a band gap decrease from 6.2 eV (x=0) down to 4.9 eV (x=0.22). Theoretical investigations within the special quasirandom structure approach show that the wurtzite structure has the lowest mixingenthalpy for 0≤x≤0.75.
Pseudomorphic stabilization in wurtzite Sc_{x}Al_{1-x}N/AlN and Sc_{x}Al_{1-x}N/In_{y}Al_{1-y}N superlattices (x=0.2, 0.3, and 0.4; y=0.2-0.72), grown by reactive magnetron sputter epitaxy was investigated. X-ray diffraction and transmission electron microscopy show that in Sc_{x}Al_{1-x}N/AlN superlattices the compressive biaxial stresses due to positive lattice mismatch in Sc_{0.3}Al_{0.7}N and Sc_{0.4}Al_{0.6}N lead to loss of epitaxy, although the structure remains layered. For the negative lattice mismatched In-rich Sc_{x}Al_{1-x}N/In_{y}Al_{1-y}N superlattices a tensile biaxial stress promotes the stabilization of wurtzite Sc_{x}Al_{1-x}N even for the highest investigated concentration x=0.4. Ab initio calculations with fixed in-plane lattice parameters show a reduction in mixing energy for wurtzite Sc_{x}Al_{1-x}N under tensile stress when x≥0.375 and support the experimental results.