The exceptional mechanical flexibility observed with certain organic crystals defies the common perception of single crystals as brittle objects. Here, we describe the morphostructural consequences of plastic deformation in crystals of hexachlorobenzene that can be bent mechanically at multiple locations to 360° with retention of macroscopic integrity. This extraordinary plasticity proceeds by segregation of the bent section into flexible layers that slide on top of each other, thereby generating domains with slightly different lattice orientations. Microscopic, spectroscopic and diffraction analyses of the bent crystal showed that the preservation of crystal integrity when stress is applied on the (001) face requires sliding of layers by breaking and re-formation of halogen-halogen interactions. Application of stress on the (100) face, in the direction where π···π interactions dominate the packing, leads to immediate crystal disintegration. Within a broader perspective, this study highlights the yet unrecognized extraordinary malleability of molecular crystals with strongly anisotropic supramolecular interactions.

In this paper, we report angle-dispersive X-ray diffraction data of molybdenum melting, measured in a double-sided laser-heated diamond-anvil cell up to a pressure of 119 GPa and temperatures up to 3400 K. The new melting temperatures are in excellent agreement with earlier measurements up to 90 GPa that relied on optical observations of melting and in strong contrast to most theoretical estimates. The X-ray measurements show that the solid melts from the bcc structure throughout the reported pressure range and provide no evidence for a high temperature transition from bcc to a close-packed structure, or to any other crystalline structure. This observation contradicts earlier interpretations of shock data arguing for such a transition. Instead, the values for the Poisson ratios of shock compressed Mo, obtained from the sound speed measurements, and the present X-ray evidence of loss of long-range order suggest that the 210 GPa (approximately 4100 K) transition in the shock experiment is from the bcc structure to a new, highly viscous, structured melt.

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We report for the first time the direct observation of an apparently first-order phase transition in amorphous silica from the low density amorphous phase to the high density amorphous phase with a discontinuous volume change of about 20% from the dilatometric measurements using the piston cylinder apparatus at 36 kbar and 680 degrees C. Our Raman spectroscopy measurements on the retrieved samples quenched from high pressure and high temperature experiments do not show any Raman peaks indicating a large modification in the intermediate range order in the structure of amorphous silica.

Molecular crystals are not known to be as stiff as metals, composites and ceramics. Here we report an exceptional mechanical stiffness and high hardness in a known elastically bendable organic cocrystal [caffeine (CAF), 4-chloro-3-nitrobenzoic acid (CNB) and methanol (1:1:1)] which is comparable to certain low-density metals. Spatially resolved atomic level studies reveal that the mechanically interlocked weak hydrogen bond networks which are separated by dispersive interactions give rise to these mechanical properties. Upon bending, the crystals significantly conserve the overall energy by efficient redistribution of stress while perturbations in hydrogen bonds are compensated by strengthened π -stacking. Furthermore we report a remarkable stiffening and hardening in the elastically bent crystal. Hence, mechanically interlocked architectures provide an unexplored route to reach new mechanical limits and adaptability in organic crystals. This proof of concept inspires the design of light-weight, stiff crystalline organics with potential to rival certain inorganics, which currently seem inconceivable.