Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure sp(3) bonds and it provides an explanation to several features in experimental x-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to Z-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that Z-carbon is a transparent wide band-gap semiconductor with a hardness comparable to diamond.

The elastic and plastic behaviors of silica glasses densified at various maximum pressure reached (12 GPa, 15 GPa, 19 GPa, and 22 GPa), were analyzed using in situ Raman and Brillouin spectroscopies. The elastic anomaly was observed to progressively vanish up to a maximum pressure reached of 12 GPa, beyond which it is completely suppressed. Above the elastic anomaly the mechanical behavior of silica glass, as derived from Brillouin measurements, is interpreted in terms of pressure induced transformation of low density amorphous silica into high density amorphous silica.

Understanding the mechanism and limits of strain transfer between supported 2D systems and their substrate is a most needed step toward the development of strain engineering at the nanoscale. This includes applications in straintronics, nanoelectromechanical devices, or new nanocomposites. Here, we have studied the limits of biaxial compressive strain transfer among SiO, diamond, and sapphire substrates and graphene. Using high pressure-which allows maximizing the adhesion between graphene and the substrate on which it is deposited-we show that the relevant parameter governing the graphene mechanical response is not the applied pressure but rather the strain that is transmitted from the substrate. Under these experimental conditions, we also show the existence of a critical biaxial stress beyond which strain transfer become partial and introduce a parameter, α, to characterize strain transfer efficiency. The critical stress and α appear to be dependent on the nature of the substrate. Under ideal biaxial strain transfer conditions, the phonon Raman G-band dependence with strain appears to be linear with a slope of -60 ± 3 cm/% down to biaxial strains of -0.9%. This evolution appears to be general for both biaxial compression and tension for different experimental setups, at least in the biaxial strain range -0.9% < ε < 1.8%, thus providing a criterion to validate total biaxial strain transfer hypotheses. These results invite us to cast a new look at mechanical strain experiments on deposited graphene as well as to other 2D layered materials.