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The Open Databases Integration for Materials Design (OPTIMADE) application programming interface (API) empowers users with holistic access to a growing federation of databases, enhancing the accessibility and discoverability of materials and chemical data. Since the first release of the OPTIMADE specification (v1.0), the API has undergone significant development, leading to the upcoming v1.2 release, and has underpinned multiple scientific studies. In this work, we highlight the latest features of the API format, accompanying software tools, and provide an update on the implementation of OPTIMADE in contributing materials databases. We end by providing several use cases that demonstrate the utility of the OPTIMADE API in materials research that continue to drive its ongoing development.

While capable of imaging the atoms constituting thin slabs of material, the achievable resolution of conventional electron imaging techniques in a transmission electron microscope (TEM) is very sensitive to the partial spatial coherence of the electron source, lens aberrations and mechanical instabilities of the microscope. The desire to break free from the limitations of the apparatus spurred the popularity of ptychography, a computational phase retrieval technique that, to some extent, can compensate for the imperfections of the equipment. Recently it was shown that ptychography is capable of resolving specimen features as fine as the blurring due to the vibrations of atoms, a limit defined not by the microscope, but by the investigated sample itself. Here we report on the successful application of a mixed-object formalism in the ptychographic reconstruction that enables the resolution of fluctuations in atomic positions within real space. We show a reconstruction of a symmetric {\Sigma}9 grain boundary in silicon from realistically (molecular dynamics) simulated data. By reconstructing the object as an ensemble of 10 different states we were able to observe movements of atoms in the range of 0.1-0.2 \AA in agreement with the expectation. This is a significant step forward in the field of electron ptychography, as it enables the study of dynamic systems with unprecedented precision and overcomes the resolution limit so far considered to be imposed by the thermal motion of the atoms.

Phonon scattering at grain boundaries (GBs) is significant in controlling nanoscale device thermal conductivity. However, GBs could also act as waveguides for selected modes. To measure localized GB phonon modes, meV energy resolution is needed with sub-nm spatial resolution. Using monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) we have mapped the 60 meV optic mode across GBs in silicon at atomic resolution and compared it to calculated phonon densities of states (DOS). The intensity is strongly reduced at GBs characterised by the presence of five- and seven-fold rings where bond angles differ from the bulk. The excellent agreement between theory and experiment strongly supports the existence of localized phonon modes and thus of GBs acting as waveguides., Comment: Merged article (8 pages, 3 figures, 1 table) and Supplementary Information (8 pages, 8 figures)

Surface strain often controls properties of the material including charge transport and chemical reactivity. Localized surface strain is measured with atomic resolution on (111) ceria nanoparticle surfaces using environmental transmission electron microscopy under different redox conditions. Density Functional Theory (DFT) coupled with TEM image simulations have been used for aid in interpreting the experimental data. Oxygen vacancy creation/annihilation introduces strain at surface and near surface regions on cation sublattice. Static and fluxional strainmaps are generated from images at these different conditions and compared. While fluxional strain is highest at locations associated with unstable vacancies at active sites, highly inhomogeneous static strain fields comprising of alternating tensile/compressing strain is seen at surface and subsurfaces linked to the presence of stable oxygen vacancies. Interestingly, both stable and unstable oxygen vacancies are found within a few atomic spacing of each other on the same surface. The static strain pattern depends on the ambient inside TEM. Oxidizing environments tend to lower vacancy concentrations at the surface whereas a highly reducing environment created using high electron dose creates oxygen vacancies everywhere (bulk and surfaces) in the nanoparticle.

2D materials find promising applications in next-generation devices, however, large-scale, low-defect, and reproducible synthesis of 2D materials remains a challenging task. To assist in the selection of suitable substrates for the synthesis of as-yet hypothetical 2D materials, we have developed an open-source high-throughput workflow package, $Hetero2d$, that searches for low-lattice mismatched substrate surfaces for any 2D material and determines the stability of these 2D-substrate heterostructures using density functional theory (DFT) simulations. $Hetero2d$ automates the generation of 2D-substrate heterostructures, the creation of DFT input files, the submission and monitoring of computational jobs on supercomputing facilities, and the storage of relevant parameters alongside the post-processed results in a MongoDB database. We demonstrate the capability of $Hetero2d$ in identifying stable 2D-substrate heterostructures for four 2D materials, namely $2H$-MoS$_2$, $1T$- and $2H$-NbO$_2$, and hexagonal-ZnTe, considering 50 cubic elemental substrates. We find Cu, Hf, Mn, Nd, Ni, Pd, Re, Rh, Sc, Ta, Ti, V, W, Y, and Zr substrates sufficiently stabilize the formation energies of these 2D materials, with binding energies in the range of ~0.1 - 0.6 eV/atom. Upon examining the $z$-separation, the charge transfer, and the electronic density of states at the 2D-substrate interface, we find a covalent type bonding at the interface which suggests that these substrates can be used as contact materials for the 2D materials. $Hetero2d$ (https://github.com/cmdlab/Hetero2d) is available on GitHub as an open-source package under the GNU license.

Ceria has proven to be an excellent ion-transport and ion-exchange material when used in polycrystalline form and with a high-concentration of aliovalent doped cations. Despite its widespread application, the impact of atomic-scale defects in this material are scarcely studied and poorly understood. In this article, using first-principles simulations, we provide a fundamental understanding of the atomic-structure, thermodynamic stability and electronic properties of undoped grain-boundaries (GBs) and alkaline-earth metal (AEM) doped GBs in ceria. Using density-functional theory simulations, with a GGA+U functional, we find the $\Sigma$3 (111)/[$\bar{1}$01] GB is thermodynamically more stable than the $\Sigma$3 (121)/[$\bar{1}$01] GB due to the larger atomic coherency in the $\Sigma$3 (111)/[$\bar{1}$01] GB plane. We dope the GBs with $\sim$20% [M]$_{GB}$ (M=Be, Mg, Ca, Sr, and Ba) and find that the GB energies have a parabolic dependence on the size of solutes, the interfacial strain and the packing density of the GB. We see a stabilization of the GBs upon Ca, Sr and Ba doping whereas Be and Mg render them thermodynamically unstable. The electronic density of states reveal that no defect states are present in or above the band gap of the AEM doped ceria, which is highly conducive to maintain low electronic mobility in this ionic conductor. The electronic properties, unlike the thermodynamic stability, exhibit complex inter-dependence on the structure and chemistry of the host and the solutes. This work makes advances in the atomic-scale understanding of aliovalent cation doped ceria GBs serving as an anchor to future studies that can focus on understanding and improving ionic-transport., Comment: 23 pages, 4 figures