The dependence of the superconducting transition temperature T(c) on nearly hydrostatic pressure has been determined to 67 GPa in an ac susceptibility measurement for a Li sample embedded in helium pressure medium. With increasing pressure, superconductivity appears at 5.47 K for 20.3 GPa, T(c) rising rapidly to approximately 14 K at 30 GPa. The T(c)(P) dependence to 67 GPa differs significantly from that observed in previous studies where no pressure medium was used. Evidence is given that superconductivity in Li competes with symmetry breaking structural phase transitions which occur near 20, 30, and 62 GPa. In the pressure range 20-30 GPa, T(c) is found to decrease rapidly in a dc magnetic field, the first evidence that Li is a type I superconductor.

The melting curve of lithium between ambient pressure and 64 GPa is measured by detection of an abrupt change in its electrical resistivity at melting and by visual observation. Here we have used a quasi-four-point resistance measurement in a diamond anvil cell and measured the resistance of lithium as it goes through melting. The resistivity near melting exhibits a well documented sharp increase which allowed us to pinpoint the melting transition from ambient pressure to 64 GPa. Our data show that lithium melts clearly above 300 K in all pressure regions and its melting behavior adheres to the classical model. Moreover, we observed an abrupt increase in the slope of the melting curve around 10 GPa. The onset of this increase fits well to the linear extrapolation of the lower temperature bcc-fcc phase boundary.

The crystal structure of elements at zero pressure and temperature is the most fundamental information in condensed matter physics. For decades it has been believed that lithium, the simplest metallic element, has a complicated ground-state crystal structure. Using synchrotron x-ray diffraction in diamond anvil cells and multiscale simulations with density functional theory and molecular dynamics, we show that the previously accepted martensitic ground state is metastable. The actual ground state is face-centered cubic (fcc). We find that isotopes of lithium, under similar thermal paths, exhibit a considerable difference in martensitic transition temperature. Lithium exhibits nuclear quantum mechanical effects, serving as a metallic intermediate between helium, with its quantum effect-dominated structures, and the higher-mass elements. By disentangling the quantum kinetic complexities, we prove that fcc lithium is the ground state, and we synthesize it by decompression.

We report synchrotron X-ray diffraction, photoconductivity, and photoluminescence investigations of methylammonium-lead-bromide (MAPbBr) under various stress conditions, supported by density-functional-theory (DFT) calculations. The properties of MAPbBr show substantial dependence on the hydrostatic conditions. While nonhydrostatic compression of MAPbBr 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.

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