A soft porous material [Zn(L)₂(OH)₂]_n·Guest (where L is 4-(1H-naphtho[2,3-d]imidazol-1-yl)benzoate, and Guest is water or methanol) exhibits the strongest ever observed negative area compressibility (NAC), an extremely rare property, as at hydrostatic pressure most materials shrink in all directions and few expand in one direction. This is the first NAC reported in metal–organic frameworks (MOFs), and its magnitude, clearly visible and by far the highest of all known materials, can be reversibly tuned by exchanging guests adsorbed from hydrostatic fluids. This counterintuitive strong NAC of [Zn(L)₂(OH)₂]_n·Guest arises from the interplay of flexible [−Zn–O(H)−]_n helices with layers of [−Zn–L−]₄ quadrangular puckered rings comprising large channel voids. The compression of helices and flattening of puckered rings combine to give a giant piezo-mechanical response, applicable in ultrasensitive sensors and actuators. The extrinsic NAC response to different hydrostatic fluids is due to varied host–guest interactions affecting the mechanical strain within the range permitted by exceptionally high flexibility of the framework.

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.

At normal conditions, mandelic acid (MA) is exempt from Wallach’s rule, as enantiomers l-MA and d-MA are denser than racemate dl-MA. However, the enantiomers are less compressible than the racemate, dl-MA, which additionally is anomalously compressed at the phase transition at 0.65 GPa and becomes more dense than the enantiomers. Our results exclude the possibility of resolving racemic dl-MA into enantiomers due to their higher density in a spontaneous crystallization induced by pressure up to 2.6 GPa at least. Also, the l-MA crystal undergoes an isostructural phase transition at 1.52 GPa, allowing a tighter packing of molecules mimicking a racemic arrangement. It is a general thermodynamic rule that pressure-induced phase transitions in racemates and enantiomers have the opposite effect for their density relations and act toward or against Wallach’s rule, respectively.