Lithium, a prototypical simple metal under ambient conditions, has a surprisingly rich phase diagram under pressure, taking up several structures with reduced symmetry, low coordination numbers, and even semiconducting character with increasing density. Using first-principles calculations, we demonstrate that some predicted high-pressure phases of elemental Li also host topological electronic structures. Beginning at 80 GPa and coincident with a transition to the previously predicted Pbca phase, we find Li to be a Dirac nodal line semimetal. We further calculate that Li retains linearly dispersing energy bands near the Fermi energy in subsequent predicted higher-pressure phases and that it exhibits a Lifshitz transition between two Cmca phases at 220 GPa. The Fd[Formula: see text]m phase at 500 GPa forms buckled honeycomb layers that give rise to a Dirac crossing 1 eV below the Fermi energy. The well-isolated topological nodes near the Fermi level in these phases result from increasing p-orbital character with density at the Fermi level, itself a consequence of rising 1s core wavefunction overlap, and a preference for nonsymmorphic symmetries in the crystal structures favored at these pressures. Our results provide evidence that under pressure, bulk 3D materials with light elements, or even pure elemental systems, can undergo phase transitions hosting nontrivial topological phase transitions hosting nontrivial topological properties near the Fermi level with measurable consequences and that, through pressure, we can access these phases in elemental lithium.

Ferroelectrics are a class of polar and switchable functional materials with diverse applications, from microelectronics to energy conversion. Computational searches for new ferroelectric materials have been constrained by accurate prediction of the polarization and switchability with electric field, properties that, in principle, require a comparison with a nonpolar phase whose atomic-scale unit cell is continuously deformable from the polar ground state. For most polar materials, such a higher-symmetry nonpolar phase does not exist or is unknown. Here, we introduce a general high-throughput workflow that screens polar materials as potential ferroelectrics. We demonstrate our workflow on 1978 polar structures in the Materials Project database, for which we automatically generate a nonpolar reference structure using pseudosymmetries, and then compute the polarization difference and energy barrier between polar and nonpolar phases, comparing the predicted values to known ferroelectrics. Focusing on a subset of 182 potential ferroelectrics, we implement a systematic ranking strategy that prioritizes candidates with large polarization and small polar-nonpolar energy differences. To assess stability and synthesizability, we combine information including the computed formation energy above the convex hull, the Inorganic Crystal Structure Database id number, a previously reported machine learning-based synthesizability score, and ab initio phonon band structures. To distinguish between previously reported ferroelectrics, materials known for alternative applications, and lesser-known materials, we combine this ranking with a survey of the existing literature on these candidates through Google Scholar and Scopus databases, revealing ~130 promising materials uninvestigated as ferroelectric. Our workflow and large-scale high-throughput screening lays the groundwork for the discovery of novel ferroelectrics, revealing numerous candidates materials for future experimental and theoretical endeavors.

The promise of lead halide hybrid perovskites for optoelectronic applications makes finding less-toxic alternatives a priority. The double perovskite Cs₂AgBiBr₆ (1) represents one such alternative, offering long carrier lifetimes and greater stability under ambient conditions. However, the large and indirect 1.95 eV bandgap hinders its potential as a solar absorber. Here we report that alloying crystals of 1 with up to 1 atom% Sn results in a bandgap reduction of up to ca. 0.5 eV while maintaining low toxicity. Crystals can be alloyed with up to 1 atom% Sn and the predominant substitution pathway appears to be a ∼2 : 1 substitution of Sn²⁺ and Sn⁴⁺ for Ag⁺ and Bi³⁺, respectively, with Ag⁺ vacancies providing charge compensation. Spincoated films of 1 accommodate a higher Sn loading, up to 4 atom% Sn, where we see mostly Sn²⁺ substitution for both Ag⁺ and Bi³⁺. Density functional theory (DFT) calculations ascribe the bandgap redshift to the introduction of Sn impurity bands below the conduction band minimum of the host lattice. Using optical absorption spectroscopy, photothermal deflection spectroscopy, X-ray absorption spectroscopy, ¹¹⁹Sn NMR, redox titration, single-crystal and powder X-ray diffraction, multiple elemental analysis and imaging techniques, and DFT calculations, we provide a detailed analysis of the Sn content and oxidation state, dominant substitution sites, and charge-compensating defects in Sn-alloyed Cs₂AgBiBr₆ (1:Sn) crystals and films. An understanding of heterovalent alloying in halide double perovskites opens the door to a wider breadth of potential alloying agents for manipulating their band structures in a predictable manner.

Recent advances in high-throughput computational workflows are expanding the realm of materials for a range of applications. Here, we report the experimental evaluation of two such predicted candidate ferroelectric perovskite oxides: BiAlO3 and BiInO3. Attempts were made to synthesize polar BiAlO3 and BiInO3 using pulsed-laser deposition. Despite exploring a wide range of temperatures, pressures, substrates, laser fluences, and so on, attempts to grow BiAlO3 with this approach resulted in no perovskite phase and decomposition to Bi2O3 and Bi24Al2O40. Various orientations of BiInO3 films were synthesized on multiple substrates, with the best crystallinity demonstrated for (200)-oriented films on MgO (001). Density-functional theory predicts two energetically competitive ground-state structures for BiInO3: Pnma (nonpolar) and Pna21 (polar). BiInO3 films were studied by using X-ray diffraction and second-harmonic generation (SHG) and found to exhibit the nonpolar Pnma structure. Temperature-dependent SHG and dielectric measurements revealed no transition to the polar structure. Optical transmission-absorption studies suggest a direct bandgap of ∼4.5 eV for BiInO3. Our study underscores the need for additional descriptors for synthesizability in assessing the potential of ferroelectric candidate materials identified from high-throughput materials databases.