We present detailed investigations of the structural, elastic, dielectric, and piezoelectric properties of scandium aluminum nitride (Sc_xAl_1−xN) with the wurtzite crystal structure by means of first-principles calculations based on density functional theory in order to enable a detailed comparison to the corresponding physical properties of GaAlN and InAlN. The goal of our approach is to use atomistic simulations to extract the novel solid state characteristics of Sc_xAl_1−xN crystals by the determination of complete sets of coefficients for the elastic, compliance, and piezoelectric tensor and to confirm the theoretical predictions by experimental measurements of selected tensor coefficients. The calculation of the tensor components is accompanied by a detailed analysis of the crystal structures, e.g., average bond length, bond angles, lattice parameters, and mass density in dependence on alloy composition of Sc_xAl_1−xN. If an increasing number of Al atoms of up to x = 0.5 are replaced by Sc atoms, we observe a nonlinear change of the ratio of lattice parameter $\frac{c(x)}{a(x)}$ and average bond angles of about 10% and 5%, respectively, which give an indication of an increasing deviation of the crystal structure of Sc_xAl_1−xN from an ideal hexagonal lattice. As a consequence of the deformed crystal structure and the iconicity of the Sc–N bond, we predict a change in value of the elastic coefficient $C_{33}^{ScAlN}(x)$ , the piezoelectric coefficient $e_{33}^{ScAlN}(x)$ , and the value of spontaneous polarization $P_{SP}^{ScAlN}(x)$ of up to 65%, 150%, and 230%, respectively. Based, on these simulation results, physical features of practical use are derived, like the average bulk, shear, and the Young modulus as well as the reciprocal Young's modulus and Poisson ratio. Furthermore, the spontaneous polarization of Sc_xAl_1−xN is approximated, taking nonlinear effects into account as well as the piezoelectric polarization caused by uniaxial, biaxial, and hydrostatic stresses in dependency on alloy composition and strain. A detailed comparison of the structural and polarization related properties of GaAlN and InAlN allows us to point out the peculiarity of wurtzite ScAlN crystals within the group III-nitrides.

In this work, all electroacoustic material parameters, i.e., the elastic, piezoelectric, and dielectric coefficients, as well as the mass density, were determined experimentally for wurtzite aluminum scandium nitride (Al${}_{1-\mathrm{x}}$ Sc${}_{\mathrm{x}}$ N) for a wide range of Sc concentrations of up to x = 0.32 from the same material source for the first time. Additionally, the mass density and piezoelectric coefficient were determined even up to $\mathrm{x}=0.42$ . Two sets of 1 $\mu$ m-thick AlScN(0001) thin films were deposited on Si(001) using reactive pulsed-DC magnetron cosputtering. One set of thin films was used to determine the $a$ - and $c$ - lattice parameters and the effective relative dielectric coefficient ${\epsilon }_{33,f}$ , using X-ray diffraction and capacitive measurements, respectively. Lattice parameters were then used to extract average internal parameter $u$ , bond length, and bond angle, as well as mass density, as a function of Sc concentration. Density functional theory calculations were performed to provide the equilibrium lattice parameters $a$ , $c$ , and $u$ , as well as the bond angle and the bond lengths for wurtzite-AlN and layered hexagonal-ScN. The second set of films was used to fabricate surface acoustic wave (SAW) resonators with wavelengths $\lambda$ from 2 up to 24 $\mu$ m. The SAW dispersion in conjunction with finite element modeling fitting was used to extract the elastic stiffness as well as the piezoelectric coefficients. The overall evolution of the material parameters and the change of the crystal structure as a function of Sc concentration is discussed in order to provide a possible explanation of the observed behavior.

We study the electroacoustic properties of aluminum scandium nitride crystals ${\mathrm{Al}}_{1-x}{\mathrm{Sc}}_x\mathrm{N}$ with the metastable wurtzite structure by means of first-principles calculations based on density functional theory. We extract the material property data relevant for electroacoustic device design, namely, the full tensors of elastic and piezoelectric constants. Atomistic models were constructed and analyzed for a variety of Sc concentrations $0\le x\le 50$%. The functional dependence of the material properties on the scandium concentration was extracted by fitting the data obtained from an averaging procedure for different disordered atomic configurations. We give an explanation of the observed elastic softening and the extraordinary increase in piezoelectric response as a function of Sc content in terms of an element-specific analysis of bond lengths and bond angles.

Knowledge about the formation energies of compounds is essential to derive phase diagrams of multicomponent phases with respect to elemental reservoirs. The determination of formation energies using common (semi-)local exchange-correlation approximations of the density functional theory (DFT) exhibits well-known systematic errors if applied to oxide compounds containing transition metal elements. In this work, we generalize, reevaluate, and discuss a set of approaches proposed and widely applied in the literature to correct for errors arising from the over-binding of the O₂ molecule and from correlation effects of electrons in localized transition-metal orbitals. The DFT+U method is exemplarily applied to iron oxide compounds, and a procedure is presented to obtain the U values, which lead to formation energies and electronic band gaps comparable to the experimental values. Using such corrected formation energies, we derive the phase diagrams for LaFeO₃, Li₅FeO₄, and NaFeO₂, which are promising materials for energy conversion and storage devices. A scheme is presented to transform the variables of the phase diagrams from the chemical potentials of elemental phases to those of precursor compounds of a solid-state reaction, which represents the experimental synthesis process more appropriately. The discussed workflow of the methods can directly be applied to other transition metal oxides.

We present a density functional theory analysis of the negatively charged nitrogen-vacancy (NV) defect complex located at or close to the core of ${30}^{\circ }$ and ${90}^{\circ }$ partial glide dislocations in diamond. Formation energies, electronic densities of states, structural deformations, hyperfine structure, and zero-field splitting parameters of NV centers in such structurally distorted environments are analyzed. The formation energies of the NV centers are up to 3 eV lower at the dislocation cores compared to the bulk values of crystalline diamond. We found that the lowest energy configuration of the NV center at the core of a ${30}^{\circ }$ partial glide dislocation is realized when the axis of the NV center is oriented parallel to the dislocation line. This special configuration has a stable triplet ground state. Its hyperfine constants and zero-field splitting parameters deviate by only $3\%$ from values of the bulk NV center. Hence, this is an interesting candidate for a self-assembly of a linear array of NV centers along the dislocation line.

We investigate the spin-coherence decay of NV − spins interacting with the strongly coupled bath of nitrogen defects in diamond layers. For thin diamond layers, we demonstrate that the spin-coherence times exceed those of bulk diamond, thus allowing to surpass the limit imposed by high-defect concentrations in bulk. We show that the stretched-exponential parameter for the short-time spin-coherence decay is governed by the hyperfine interaction in the bath, thereby constraining random-noise models. We introduce a method based on the cluster-correlation expansion applied to strongly interacting bath partitions instead of individual spins. Our results facilitate material development for quantum-technology devices. Published by the American Physical Society 2024