Lithium gets a new ground state: For the past 70 years, the lowest-energy crystal structure of lithium was believed to be a relatively complex one called the 9R structure. Ackland et al. show that this is incorrect. The actual lowest-energy structure for lithium is the much simpler closest-packed face-centered cubic form. In addition, ⁶ Li and ⁷ Li isotopes have crystal phase transitions at slightly different pressures and temperatures. This difference is chalked up to large quantum mechanical effects between the isotopes. Lithium is the only metal that shows this type of quantum effect and presents a challenge for theoreticians to explain. Science , this issue p. 1254

We report synchrotron X-ray diffraction, photoconductivity, and photoluminescence investigations of methylammonium-lead-bromide (MAPbBr₃) under various stress conditions, supported by density-functional-theory (DFT) calculations. The properties of MAPbBr₃ show substantial dependence on the hydrostatic conditions. While nonhydrostatic compression of MAPbBr₃ leads to amorphization above 2.4 GPa, under quasi-hydrostatic (Ar) and hydrostatic (He) pressure, the sample remains in crystalline phases. A sequence of phase transitions between two cubic phases and orthorhombic Pnma phase is observed when using Ar, or no pressure-transmitting-medium (PTM). In helium-PTM only transitions between the two cubic structures and a new isostructural phase transition with a large volume collapse to a third cubic-phase at 2.7 GPa was observed. The photoluminescence measurements indicate a pressure-induced band gap-narrowing in the cubic phase I, and a blue-shift in the orthorhombic structure. DFT calculations illustrate that the dynamics of the organic molecules and the inorganic lattice, coupled via the N–H···Br hydrogen-bonding interactions, affect the Pb–Br distance and the bandgap evolution under pressure.

Significance: The emergence of exotic quantum states, such as fluid ground state and two-component superconductivity and superfluidity, in a compressed light metallic system has been entertained theoretically for metallic phases of hydrogen. The difficulty of compressing hydrogen to metallization densities has prevented experimental proof of these effects. Studying lithium, which is isovalent to hydrogen and the lightest metal, is considered as a route to studying the lattice quantum effects in a dense light metallic system. Here, by comparing the superconductivity of lithium isotopes under pressure, we present evidence that properties of lithium at low temperature may significantly be dominated by its lattice quantum dynamics. This study is the first experimental report on superconducting properties of⁶Li, the lightest superconducting material.

Ferroelectricity is typically suppressed under hydrostatic compression because the short-range repulsions, which favor the nonpolar phase, increase more rapidly than the long-range interactions, which prefer the ferroelectric phase. Here, based on single-crystal X-ray diffraction and density-functional theory, we provide evidence of a ferroelectric-like transition from phase I2₁3 to R3 induced by pressure in two isostructural defect antiperovskites Hg₃Te₂Cl₂ (15.5 GPa) and Hg₃Te₂Br₂ (17.5 GPa). First-principles calculations show that this transition is attributed to pressure-induced softening of the infrared phonon mode Γ₄, similar to the archetypal ferroelectric material BaTiO₃ at ambient pressure. Additionally, we observe a gradual band-gap closing from ~2.5 eV to metallic-like state of Hg₃Te₂Br₂ with an unexpectedly stable R3 phase even after semiconductor-to-metal transition. This study demonstrates the possibility of emergence of polar metal under pressure in this class of materials and establishes the possibility of pressure-induced ferroelectric-like transition in perovskite-related systems.

Significance: Metals often take on simple close-packed structures. For what is arguably the simplest metal, lithium, there has been a controversy as to its low-temperature equilibrium structure, with a certain close-packed structure, 9R , long suggested. We propose a way to get further information about the structure when the most useful crystallographic methods are for one or another reason not applicable. This is to measure oscillations of the crystal magnetic moment of a material, the de Haas–van Alphen effect. The resulting spectrum of resonances relates directly to the Fermi surface of the metal and is quite distinctive for different structures. Using this methodology, we reason that the low-temperature crystal structure of lithium is not the heretofore assigned 9R structure.