Physical Review Focus, 4/9/ 2009.
A recently developed material is hard enough to scratch diamond but is much easier to make. It's also half as hard along some directions in the crystal as in others. A theory in the Rapid Communications section of the August Physical Review B explains this unusual property and suggests that the hardness depends on the strength of bonds perpendicular to the direction the material is squeezed, not the parallel bonds. The simple model could help researchers find other superhard materials.
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