The lattice vibrations (phonon modes) of crystals underpin a large number of material properties. The harmonic phonon spectrum of a solid is the simplest description of its structural dynamics and can be straightforwardly derived from the Hellman-Feynman forces obtained in a ground-state electronic structure calculation. The presence of imaginary harmonic modes in the spectrum indicates that a structure is not a local minimum on the structural potential-energy surface and is instead a saddle point or a hilltop, for example. This can in turn yield important insight into the fundamental nature and physical properties of a material. In this review article, we discuss the physical significance of imaginary harmonic modes and distinguish between cases where imaginary modes are indicative of such phenomena, and those where they reflect technical problems in the calculations. We outline basic approaches for exploring and renormalising imaginary modes, and demonstrate their utility through a set of three case studies in the materials sciences.

Combinatorial introduction of heteroatoms in the two-dimensional framework of aromatic hydrocarbons opens up possibilities to design compound libraries exhibiting desirable photovoltaic and photochemical properties. Exhaustive enumeration and first-principles characterization of this chemical space provide indispensable insights for rational compound design strategies. Here, for the smallest seventy-seven Kekulean-benzenoid polycyclic systems, we reveal combinatorial substitution of C atom pairs with the isosteric and isoelectronic B, N pairs to result in 7,453,041,547,842 (7.4 tera) unique molecules. We present comprehensive frequency distributions of this chemical space, analyze trends and discuss a symmetry-controlled selectivity manifestable in synthesis product yield. Furthermore, by performing high-throughput ab initio density functional theory calculations of over thirty-three thousand (33k) representative molecules, we discuss quantitative trends in the structural stability and inter-property relationships across heteroarenes. Our results indicate a significant fraction of the 33k molecules to be electronically active in the 1.5-2.5 eV region, encompassing the most intense region of the solar spectrum, indicating their suitability as potential light-harvesting molecular components in photo-catalyzed solar cells.

Chalcogenide perovskites have been recently proposed as novel absorber materials for photovoltaic applications. BaZrS3, the most investigated compound of this family, shows a high absorption coefficient, a bandgap of around 1.8 eV, and excellent stability. In addition to the 3D perovskite BaZrS3, the Ba–Zr–S compositional space contains various 2D Ruddlesden–Popper phases Ban + 1ZrnS3n + 1 (with n = 1, 2, 3) which have recently been reported. Herein, it is shown that at high temperature the Gibbs free energies of 3D and 2D perovskites are very close, suggesting that 2D phases can be easily formed at high temperatures. The product of the BaS and ZrS2 solid‐state reaction, in different stoichiometric conditions, presents a mixture of BaZrS3 and Ba4Zr3S10. To carefully resolve the composition, X‐ray diffraction, scanning electron microscopy, and energy‐dispersive X‐ray spectroscopy analysis are complemented with Raman spectroscopy. For this purpose, the phonon dispersions, and the consequent Raman spectra, are calculated for the 3D and 2D chalcogenide perovskites, as well as for the binary precursors. This thorough characterization demonstrates the thermodynamic limitations and experimental difficulties in forming phase‐pure chalcogenide perovskites through solid‐state synthesis and the importance of using multiple techniques to soundly resolve the composition of these materials.

In this study, we explore the potential of machine learning for modeling molecular electronic spectral intensities as a continuous function in a given wavelength range. Since presently available chemical space...

Soft-phonon modes of an undistorted phase encode a material’s preference for symmetry lowering. However, the evidence is sparse for the relationship between an unstable phonon wavevector’s reciprocal and the number of formula units in the stable distorted phase. This “1/q*-criterion” holds great potential for the first-principles design of materials, especially in low-dimension. We validate the approach on the Q1D organometallic materials space containing 1199 ring–metal units and identify candidates that are stable in undistorted (1 unit), Peierls (2 units), charge density wave (3–5 units), or long wave (>5 units) phases. We highlight materials exhibiting gap-opening as well as an uncommon gap-closing Peierls transition and discuss an example case stabilized as a charge density wave insulator. We present the data generated for this study through an interactive publicly accessible Big Data analytics platform (https://moldis.tifrh.res.in/data/rmq1d) facilitating limitless and seamless data-mining explorations.