Knoop hardness anisotropy in anthracene and phenanthrene single crystals

Microhardness anisotropy in single crystals of anthracene and phenanthrene has been observed by the Knoop indentation technique. The Knoop hardness variation with respect to the [010] direction shows opposite trends. Both these crystals have the same crystal structure and space group. The present observations are explained in terms of orientation and disposition of molecules in the unit cells.