Intrinsic strength of diamond by shock-compression measurement of large and pure single crystals

Diamond is a symbolic material in nature due to its extremely high hardness. The Hugoniot-elastic limit (HEL) under shock compression can be used as an alternative method to evaluate the strength of a material. We measured the HELs of three types Ia-, Ib-, and IIa-diamond single crystals using large samples (thickness: 1.12--2.50 mm) in the peak stress range up to 119 GPa by a velocity-interferometer system of any reflector (VISAR). A minimum constant (steady-state) HEL value of 20.5±2.0 GPa was obtained for the highly pure type IIa-diamond large single crystals (nitrogen content: $<0.1$ ppm, thickness: 1.52--1.92 mm) along the 100 axis, although the values in the range of 35--53 GPa were obtained for type Ia- and type Ib-diamond large single crystals (nitrogen content:10--2000 ppm, thickness: 1.42--2.50 mm). Considering these and previous reported results for thin diamond samples (thickness: 0.08--0.51 mm), we suggested that our steady-state HEL of type IIa-diamond single crystals was inherent for diamond and that the intrinsic strength was 18.4±2.0 GPa. The slip system was confirmed to be the {111} plane by the transmission electron microscopy observation of the recovered sample. The critical resolved shear stress of the {111}[110] slip system was derived to be 7.5±1.0 GPa, which can be comparable with the Peierls stress of diamond.